Epsilon Eridani
Encyclopedia
Epsilon Eridani (ε Eri, ε Eridani) is a star in the southern constellation Eridanus
, along a declination
9.46° south of the celestial equator
. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years (ly), it has an apparent magnitude
of 3.73. It is the third closest of the individual stars or star system
s visible to the naked eye and is the closest star known to host a planet. Its age is estimated at less than a billion years. Because of its youth, Epsilon Eridani has a higher level of magnetic activity
than the present-day Sun, with a stellar wind
30 times as strong. Its rotation period is 11.2 days at the equator. Epsilon Eridani is smaller and less massive than the Sun, and has a comparatively lower level of elements heavier than helium
. Astronomers categorize it as a main-sequence
star of spectral class K2, which means that energy generated at the core through nuclear fusion
of hydrogen
is emitted from the surface at a temperature of about 5,000 K
, giving the star an orange hue.
The motion of this star along the line of sight to the Earth, known as the radial velocity
, has been regularly observed for more than twenty years. Periodic changes in this data yielded evidence of a giant planet orbiting Epsilon Eridani, making it the nearest extrasolar system with a candidate exoplanet. This object, Epsilon Eridani b
, was formally announced in 2000 by a team of astronomers led by Artie Hatzes
. Current data indicate that this planet orbits with a period of about 7 years at a mean separation of 3.4 astronomical units (AU), where 1 AU is the mean distance between the Earth and the Sun. Although this discovery has been controversial because of the amount of background noise in the radial velocity data, many astronomers now regard the planet as confirmed.
The system includes two belts of rocky asteroid
s: one at about 3 AU and a second at about 20 AU, whose structure may be maintained by a hypothetical second planet, Epsilon Eridani c. Epsilon Eridani harbors an extensive outer debris disk
of remnant planetesimal
s left over from the system's formation.
The designation for this star was established in 1603 by Johann Bayer
. It may be a member of the Ursa Major Moving Group
of stars that share a similar motion through the Milky Way
, implying these stars shared a common origin in an open cluster
. Its nearest neighbor, the binary star system Luyten 726-8
, will have a close encounter with Epsilon Eridani in approximately 31,500 years when they will be separated by about 0.93 ly. As one of the nearest Sun-like stars
with the potential for a planet that may harbor life, Epsilon Eridani has been the target of SETI
searches. The star appears in science fiction
stories and has been suggested as a destination for interstellar travel
.
for this star, was established in 1603 as part of the Uranometria
, a star catalogue produced by German celestial cartographer Johann Bayer
. His catalogue assigned letters from the Greek alphabet
to groups of stars belonging to the same visual magnitude class in each constellation, beginning with alpha (α) for a star in the brightest class. However, Bayer made no attempt to arrange stars by relative brightness within each class. Thus, although Epsilon is the fifth letter in the Greek alphabet, the star is the tenth brightest star in Eridanus. The star catalogue of English astronomer John Flamsteed
, published in 1712, gave this star the Flamsteed designation
18 Eridani as it was the eighteenth catalogued star in the constellation of Eridanus by order of increasing right ascension. In 1918 this star appeared in the Henry Draper Catalogue
with the designation HD 22049 and a preliminary spectral classification of K0.
Based on observations between 1800 and 1880, Epsilon Eridani was found to have a large proper motion
across the celestial sphere
, which was estimated at an angular velocity
of three arcseconds annually. This movement implied it was relatively close to the Sun, making it a star of interest for the purpose of trigonometric parallax
measurements. This process involves recording the position of the star as the Earth moves around the Sun, which allows the star's distance to be estimated. From 1881 to 1883, American astronomer William L. Elkin
used a heliometer
at the Royal Observatory at the Cape of Good Hope, South Africa
to compare the position of Epsilon Eridani with two nearby stars. From these observations, a parallax of was calculated. By 1917, observers had refined their parallax estimate to 0.317 arcseconds. The modern value of 0.3109 arcseconds is equivalent to a distance of about 10.5 ly.
proposed that an unseen companion with an orbital period of 25 years was causing gravitational perturbation
s in the star's position. This claim was refuted in 1993 by German astronomer Wulff-Dieter Heintz and the false detection was blamed on a systematic error in the photographic plates.
Launched in 1983, the space telescope IRAS
detected infrared
emissions from stars near to the Sun. Two years later, the presence of an excess infrared emission close to Epsilon Eridani was announced, which indicated a disk of fine-grained cosmic dust
was orbiting the star. This debris disk
has been extensively studied since that time. Evidence for a planetary system was discovered in 1998 by the observation of asymmetries in this dust ring. These clumps of dust could be explained by gravitational interaction with a planet orbiting just inside the ring of dust.
From 1980 to 2000, a team of astronomers led by American Artie P. Hatzes
made radial velocity
observations of Epsilon Eridani, measuring changes in motion of the star along the line of sight to the Earth, which provided evidence of the gravitational effect of a planet orbiting the star with a period of about seven years. Although there is a high level of noise in the radial velocity data due to magnetic activity in the star's photosphere
, any periodicity caused by this magnetic activity is expected to show a strong correlation with variations in emission lines of ionized calcium (the Ca II H and K lines). Because no such correlation was found, a planetary companion was deemed the most likely cause. This discovery was supported by astrometric measurements of Epsilon Eridani made between 2001 and 2003 with the Hubble Space Telescope
, which showed evidence for gravitational perturbation of the star by a planet.
American astrophysicist Alice C. Quillen and her student Stephen Thorndike
performed computer simulations of the structure of the dust disk around the star. Their model suggested that the clumping of the dust particles could be explained by the presence of a second planet in an eccentric orbit. They announced this finding in 2002.
and Italian physicist Giuseppe Cocconi
proposed that extraterrestrial civilizations might be using radio signals for communication. Project Ozma
, headed by American astronomer Frank Drake
, used the Tatel Telescope to search for such signals from the nearby Sun-like stars Epsilon Eridani and Tau Ceti
. They were observed at the emission frequency of neutral hydrogen, 1,420 MHz. No signals of intelligent extraterrestrial origin were detected. The experiment was repeated by Drake in 2010, with the same negative result. Despite this lack of success, Epsilon Eridani made its way into science fiction literature and television shows
for many years following news of Drake's initial experiment.
In Habitable Planets for Man, a 1964 RAND Corporation study by American space scientist Stephen H. Dole, the odds of a habitable planet being in orbit around Epsilon Eridani were estimated at 3.3%. Among the known stars within 22 ly, it was listed with the 14 stars that were thought most likely to have a habitable planet.
A new strategy in the search for extraterrestrial intelligence (SETI
) was proposed by American space scientist William I. McLaughlin
in 1977. He suggested that widely observable events such as nova
explosions might be used by intelligent extraterrestrials to synchronize the transmission and reception of their signals. This idea was tested from the National Radio Astronomy Observatory
in 1988, which used outbursts of Nova Cygni 1975 as the timer. Fifteen days of observation showed no anomalous radio signals coming from Epsilon Eridani.
Because of the proximity and Sun-like properties of this star, it was considered as one of the targets for interstellar travel
by American physicist Robert L. Forward in 1985. The following year, Epsilon Eridani was suggested as one of several targets in the Project Daedalus
paper study by the British Interplanetary Society
. It has continued to be among the targets of such proposals, as with Project Icarus
in 2011.
Based on its location within 23.5 ly, Epsilon Eridani was among the target stars of Project Phoenix
, a 1995 microwave
survey for signals from extraterrestrial intelligence. The project had checked about 800 stars by 2004, but had not yet detected an unimpeachable signal.
At a distance of 10.5 ly, Epsilon Eridani is the 13th nearest known star (and ninth nearest solitary star or stellar system) to the Sun as of 2011. The proximity of this star makes it one of the most studied stars of its stellar classification
. This star is located in the northern part of the constellation Eridanus, about 3° east of the slightly brighter star Delta Eridani
. With a declination of −9.46°, Epsilon Eridani can be viewed from much of the Earth's surface. Only to the north of latitude 80° N
is it permanently hidden below the horizon. The apparent magnitude
of 3.73 can make this star difficult to observe from an urban area with the unaided eye, as the night skies over cities are obscured by light pollution
.
Epsilon Eridani has an estimated 82% of the Sun's mass
and 74% of the Sun's radius, but only 34% of its luminosity. The estimated surface temperature is 5,084 K. With a stellar classification of K2V, it is the second-nearest K-type main sequence star after Alpha Centauri B
. Its metallicity
, or enrichment in elements heavier than helium
, is slightly lower than the Sun's. In the star's chromosphere
, a region of the outer atmosphere just above the light emitting photosphere, the proportion of iron is estimated at 74% of the Sun's abundance.
The K-type classification of this star indicates that the spectrum displays relatively weak absorption lines from energy absorbed by hydrogen, plus strong lines of neutral atoms and singly ion
ized calcium (Ca II). The luminosity class V is assigned to stars that are undergoing thermonuclear fusion of hydrogen at their core. For a K-type main sequence star, this fusion is dominated by the proton–proton chain reaction, wherein a series of mergers of four hydrogen nuclei results in a helium nucleus. In the inner region of this star, energy is transported outward from the core by means of radiation
, which results in no net motion of the surrounding plasma. Outside of this region, in the star's envelope, energy is carried to the photosphere by plasma convection
, where it radiates into space.
Epsilon Eridani has a higher level of magnetic activity
than the Sun, and hence demonstrates increased activity in the outer parts of the star's atmosphere: the chromosphere
and corona
. The average magnetic field strength of this star across the entire surface is , which is more than forty times greater than the magnetic field strength in the Sun's photosphere. The magnetic properties can be modeled by assuming that regions with a magnetic flux
of about 0.14 T randomly cover approximately 9% of the photosphere, while the remainder of the surface is free of magnetic fields. The overall magnetic activity of this star is irregular, but it may vary with a 4.9-year period. Assuming that the radius of the star does not change over this interval, the long term variation in activity level appears to produce a temperature variation of 15 K, which corresponds to a variation in visual magnitude
(V) of 0.014.
The magnetic field on the surface of Epsilon Eridani causes variations in the hydrodynamic behavior of the photosphere. This results in greater jitter during measurements of the star's radial velocity Doppler shift. Variations of −1 were measured over a 20 year period, which is much higher than the measurement error rate of −1. This makes interpretation of periodicities in the radial velocity of Epsilon Eridani, such as those caused by the gravitational perturbations of an orbiting planet, more difficult.
Epsilon Eridani is classified as a BY Draconis variable
because it has regions of higher magnetic activity that move into and out of the line of sight as the star rotates. Measurement of this rotational modulation suggests that the equatorial region of the star rotates with an average period of 11.2 days, which is less than half of the rotation period of the Sun. Observations have shown this star to vary as much as 0.050 in V magnitude due to starspot
s and other short-term magnetic activity. Photometry
has also shown that the surface of Epsilon Eridani, like the Sun, is undergoing differential rotation
, which means that the rotation period at the surface varies by latitude
. The measured periods range from 10.8 to 12.3 days. The axial tilt
of Epsilon Eridani toward the line of sight from Earth is uncertain. Estimates range from 24° to 72°.
The high levels of chromospheric activity, strong magnetic field, and relatively fast rotation rate of Epsilon Eridani are characteristic of a young star. The age of Epsilon Eridani is about , but this remains subject to debate. Most age estimation methods place it in the range from 200 million to 800 million years. However, the low abundance of heavy elements in the chromosphere of Epsilon Eridani is indicative of an older star, because the medium
out of which stars form is steadily enriched by heavier elements produced by older generations of stars. This anomaly might be caused by a diffusion
process that has transported some of the helium and heavier elements out of the photosphere and into a region below the star's convection zone
.
The X-ray
luminosity of Epsilon Eridani is about . It is brighter in X-ray emission than the Sun at peak activity
. The source for this strong X-ray emission is the star's hot corona. Epsilon Eridani's corona appears larger and hotter than the Sun's, with a temperature of as measured from observation of the corona's ultraviolet and X-ray emission.
The stellar wind
emitted by Epsilon Eridani expands until it collides with the surrounding interstellar medium
of sparse gas and dust, resulting in a bubble of heated hydrogen gas. The absorption spectrum from this gas has been measured with the Hubble Space Telescope
, allowing the properties of the stellar wind to be estimated. Epsilon Eridani's hot corona results in a mass loss rate from the star's stellar wind that is 30 times higher than the Sun's. This wind is generating an astrosphere
(the equivalent of the heliosphere
that surrounds the Sun) that spans about 8,000 AU and contains a bow shock
that lies 1,600 AU from the star. At its estimated distance from Earth, this astrosphere spans 42 arcminutes, which is wider than the apparent size of the full Moon.
, moving −0.976 arcseconds per year in right ascension (the celestial longitude) and 0.018 arcseconds per year in declination (the celestial latitude), for a total proper motion of 0.962 arcseconds per year. It has a radial velocity of +15.5 km/s away from the Sun. The space velocity components of Epsilon Eridani in the Galactic coordinate system
are = , which means that it is traveling within the Milky Way
at a mean galactocentric distance
of 28.7 kly (8.79 kiloparsecs) from the core along an elliptical orbit that has an eccentricity
of 0.09. The velocity and heading
of this star indicates that it may be a member of the Ursa Major Moving Group
of stars that share a common motion through space. This behavior suggests that the members originated in an open cluster
of stars that has since diffused. The estimated age of this group is years, which lies within the range of the age estimates for this star.
During the past million years, three stars are believed to have come within 7 ly (2 parsecs) of Epsilon Eridani. The most recent and closest of these encounters was with Kapteyn's Star
, which approached to a distance of about 3 ly (0.9 parsecs) roughly 12,500 years ago. The other two stars were Sirius
and Ross 614
. None of these encounters are thought to have affected the circumstellar disk orbiting Epsilon Eridani.
Epsilon Eridani made its closest approach to the Sun about 105,000 years ago, when the two stars were separated by 7 ly. Based upon a simulation of close encounters by nearby stars, in approximately 31,500 years, the binary star system Luyten 726-8
, which includes the variable star
UV Ceti, will encounter Epsilon Eridani at a minimum distance of about 0.9 ly (0.29 parsecs). They will be less than 1 ly (0.3 parsecs) apart for about 4,600 years. If Epsilon Eridani has an Oort cloud
, Luyten 726-8 could gravitationally perturb
some of the comet
s with long orbital period
s.
at a wavelength
of 850 μm show an extended flux of radiation out to an angular radius
of 35 arcseconds around the star. The peak emission occurs at an angular radius of 18 arcseconds, which at the distance of the star corresponds to a radius of about 60 AU. The highest level of emission occurs over the radius 35–75 AU from the star and is substantially reduced inside 30 AU. This emission is interpreted as coming from a young analogue of the Solar System's Kuiper Belt
: a compact dusty disk structure surrounding the star. From the Earth, this belt is viewed at an inclination of roughly 25° to the line of sight.
Dust and possibly water ice from this belt migrates inward because of drag from the stellar wind and a process by which stellar radiation causes dust grains to slowly spiral toward the star, known as the Poynting–Robertson effect. At the same time, these dust particles can be destroyed through mutual collisions. The time scale for all of the dust in the disk to be cleared away by these processes is less than the estimated age of the star. Hence, the current dust disk must have been created by collisions or other effects of larger parent bodies, and the disk represents a late stage in the planet formation process of this star. It would have required collisions between 11 Earth masses' worth of parent bodies to have maintained the disk in its current state over the estimated age of the star.
The disk contains an estimated mass of dust equal to a sixth of the mass of the Moon, with individual dust grains exceeding 3.5 μm in size at a temperature of about 55 K. This dust is being generated by the collision of comets, which range up to 10 to 30 km in diameter and have a combined mass of 5 to 9 times the mass of the Earth. This is similar to the estimated 10 Earth masses in the primordial Kuiper Belt. However, the disk around Epsilon Eridani contains less than of carbon monoxide
. This low level suggests a paucity of volatile-bearing comets and icy planetesimal
s compared to the Kuiper belt.
The clumpy structure of the dust belt may be explained by gravitational perturbation from a planet, dubbed Epsilon Eridani b. The clumps in the dust occur at orbits that have an integer resonance with the orbit of the suspected planet. For example, the region of the disk that completes two orbits for every three orbits of a planet is in a 3:2 orbital resonance
. In computer simulations the ring morphology can be reproduced by the capture of dust particles in 5:3 and 3:2 orbital resonances with a planet that has an orbital eccentricity
of about 0.3. Alternatively, the clumpiness may have been caused by collisions between minor planet
s known as plutino
s.
Observations from NASA's Spitzer Space Telescope
suggest that Epsilon Eridani actually has two asteroid belts and a cloud of exozodiacal dust
. The latter is an analog of the zodiacal dust
that occupies the plane of the Solar System
. One belt sits at approximately the same position as the one in our solar system, orbiting at a distance of from the star, and consists of silicate
grains with a diameter of 3 μm
and a combined mass of about 1018 kg. If the planet Epsilon Eridani b exists then this belt is unlikely to have had a source outside the orbit of the planet, so the dust may have been created by fragmentation and cratering of larger bodies such as asteroid
s. The second, denser belt, most likely also populated by asteroids, lies between the first belt and the outer comet disk. The structure of the belts and the dust disk suggests that more than two planets in the Epsilon Eridani system are needed to maintain this configuration.
In an alternative scenario, the exozodiacal dust may be generated in an outer belt, which is orbiting between 55 and 90 AU from the host star and has an assumed mass of 10−3 times the mass of the Earth. This dust is then transported inward past the orbit of Epsilon Eridani b. When collisions between the dust grains are taken into account, the dust will reproduce the observed infrared spectrum and brightness. Outside the radius of ice sublimation, located beyond 10 AU from the star where the temperatures fall below 100 K, the best fit to the observations occurs when a mix of ice and silicate
dust is assumed. Inside this radius, the dust must consist of silicate grains that lack volatiles
.
The inner region around the star, from a radius of 2.5 AU inward, appears to be clear of dust down to the detection limit of the 6.5 m MMT telescope
. Grains of dust in this region are efficiently removed by drag from the stellar wind, while the presence of a planetary system may also help keep this area clear of debris. Still, this does not preclude the possibility that an inner asteroid belt may be present with a combined mass no greater than the asteroid belt in the Solar System.
es in this system.
, this planet was announced in 2000, but the discovery has remained controversial. A comprehensive study in 2008 called the detection "tentative" and described the proposed planet as "long suspected but still unconfirmed." However, many astronomers believe the evidence is sufficiently compelling that they regard the discovery as confirmed.
Published sources remain in disagreement as to the proposed planet's basic parameters. Values for its orbital period range from 6.85 to 7.2 years. Estimates of the maximum radius of its elliptical orbit—the semimajor axis—range from 3.38 AU to 3.50 AU and approximations of its orbital eccentricity
range from to .
The true mass of this planet remains unknown, but it can be estimated based on the displacement effect of the planet's gravity upon the star. Only the component of the displacement along the line of sight to the Earth is known, which yields a value for the formula m sin i, where m is the mass of the planet and i is the orbital inclination. Estimates for the value of range from 0.60 Jupiter mass
es to 1.06 Jupiter masses, which sets the lower limit for the mass of the planet (since the sine
function has a maximum value of 1). By choosing a mass of 0.78 and an estimated inclination of 30°, this yields the frequently cited value of Jupiter masses for the planet's mass.
Of all the measured parameters for this planet, the value for orbital eccentricity is the most uncertain. The frequently cited value of 0.7 for Epsilon Eridani b's eccentricity is inconsistent with the presence of the proposed asteroid belt at a distance of 3 AU from the star. If the eccentricity was actually this high, the planet would pass through the asteroid belt and clear it out within about ten thousand years. If the belt has existed for longer than this period, which appears likely, it imposes an upper limit on Epsilon Eridani b's eccentricity of about 0.10–0.15. If the dust disk is instead being generated from the outer debris disk, rather than from collisions in an asteroid belt, then no constraints on the planet's orbital eccentricity are needed to explain the dust distribution.
Computer simulations of the dusty disk orbiting Epsilon Eridani suggest that the disk shape may be explained by the presence of a second planet, tentatively dubbed Epsilon Eridani c. Clumping in the dust disk may occur because dust particles are being trapped in orbits that have resonant
orbital periods with a planet in an eccentric orbit. The postulated Epsilon Eridani c would orbit at a distance of 40 AU, with an eccentricity of 0.3 and a period of 280 years. The inner cavity of the disk may be explained by the presence of additional planets. Current models of planet formation cannot easily explain how a planet could have been created at this distance from the star. The disk is expected to have dissipated long before a gas giant could have formed. Instead, the planet may have formed at an orbital distance of about 10 AU then migrated outward because of gravitational interaction with the disc or with other planets in the system.
, it was a target star for NASA's proposed Space Interferometry Mission
to search for Earth-sized planets. The proximity, Sun-like properties and suspected planets of this star have also made it the subject of multiple studies on whether an interstellar probe
can be sent to Epsilon Eridani.
The orbital radius at which the stellar flux from Epsilon Eridani matches the solar constant
—where the emission matches the Sun's output at the orbital distance of the Earth—is 0.61 astronomical units (AU). That is within the maximum habitable zone
of a conjectured Earth-like planet orbiting Epsilon Eridani, which currently stretches from about 0.5 to 1.0 AU. As the star ages over a period of 20 billion years, the net luminosity will increase, causing this zone to slowly expand outward to about 0.6–1.4 AU. However, the presence of a large planet with a highly elliptical orbit in proximity to the habitable zone of the star reduces the likelihood of a terrestrial planet
having a stable orbit within the habitable zone.
A young star such as Epsilon Eridani can produce large amounts of ultraviolet
radiation that may be harmful to life. The orbital radius where the UV flux matches that on the early Earth lies at just under 0.5 AU. The proximity, Sun-like properties and suspected planets of this star have made it a destination for interstellar travel
in science fiction
stories.
Eridanus (constellation)
Eridanus is a constellation. It is represented as a river; its name is the Ancient Greek name for the Po River. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations. It is the sixth largest of the modern...
, along a declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
9.46° south of the celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...
. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years (ly), it has an apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of 3.73. It is the third closest of the individual stars or star system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...
s visible to the naked eye and is the closest star known to host a planet. Its age is estimated at less than a billion years. Because of its youth, Epsilon Eridani has a higher level of magnetic activity
Stellar magnetic field
A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively...
than the present-day Sun, with a stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
30 times as strong. Its rotation period is 11.2 days at the equator. Epsilon Eridani is smaller and less massive than the Sun, and has a comparatively lower level of elements heavier than helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
. Astronomers categorize it as a main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star of spectral class K2, which means that energy generated at the core through nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
is emitted from the surface at a temperature of about 5,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, giving the star an orange hue.
The motion of this star along the line of sight to the Earth, known as the radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
, has been regularly observed for more than twenty years. Periodic changes in this data yielded evidence of a giant planet orbiting Epsilon Eridani, making it the nearest extrasolar system with a candidate exoplanet. This object, Epsilon Eridani b
Epsilon Eridani b
Epsilon Eridani b is an unconfirmed extrasolar planet approximately 10 light-years away orbiting the star Epsilon Eridani, in the constellation of Eridanus .- Discovery :...
, was formally announced in 2000 by a team of astronomers led by Artie Hatzes
Artie P. Hatzes
Artie P. Hatzes is an American astronomer. He is a professor at the Friedrich Schiller University of Jena and director of the Karl Schwarzschild Observatory ....
. Current data indicate that this planet orbits with a period of about 7 years at a mean separation of 3.4 astronomical units (AU), where 1 AU is the mean distance between the Earth and the Sun. Although this discovery has been controversial because of the amount of background noise in the radial velocity data, many astronomers now regard the planet as confirmed.
The system includes two belts of rocky asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s: one at about 3 AU and a second at about 20 AU, whose structure may be maintained by a hypothetical second planet, Epsilon Eridani c. Epsilon Eridani harbors an extensive outer debris disk
Debris disk
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...
of remnant planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
s left over from the system's formation.
The designation for this star was established in 1603 by Johann Bayer
Johann Bayer
Johann Bayer was a German lawyer and uranographer . He was born in Rain, Bavaria, in 1572. He began his study of philosophy in Ingolstadt in 1592, and moved later to Augsburg to begin work as a lawyer. He grew interested in astronomy during his time in Augsburg...
. It may be a member of the Ursa Major Moving Group
Ursa Major Moving Group
The Ursa Major Moving Group, also known as Collinder 285 or Ursa Major association, is a nearby stellar moving group, a set of stars with common velocities in space and thought to have a common origin. Its core is located roughly 80 light years away...
of stars that share a similar motion through the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, implying these stars shared a common origin in an open cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...
. Its nearest neighbor, the binary star system Luyten 726-8
Luyten 726-8
Luyten 726-8 is a binary star system that is one of Earth's nearest neighbors, at about 8.7 light years from Earth in the constellation Cetus...
, will have a close encounter with Epsilon Eridani in approximately 31,500 years when they will be separated by about 0.93 ly. As one of the nearest Sun-like stars
Solar analog
Solar-type, solar analog, and solar twin stars are those stars that are particularly similar to the Sun. The classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type...
with the potential for a planet that may harbor life, Epsilon Eridani has been the target of SETI
SETI
The search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
searches. The star appears in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
stories and has been suggested as a destination for interstellar travel
Interstellar travel
Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
.
Observation history
Epsilon Eridani, the Bayer designationBayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name...
for this star, was established in 1603 as part of the Uranometria
Uranometria
Uranometria is the short title of a star atlas produced by Johann Bayer.It was published in Augsburg, Germany, in 1603 by Christophorus Mangus under the full title Uranometria : omnium asterismorum continens schemata, nova methodo delineata, aereis laminis expressa. This translates to...
, a star catalogue produced by German celestial cartographer Johann Bayer
Johann Bayer
Johann Bayer was a German lawyer and uranographer . He was born in Rain, Bavaria, in 1572. He began his study of philosophy in Ingolstadt in 1592, and moved later to Augsburg to begin work as a lawyer. He grew interested in astronomy during his time in Augsburg...
. His catalogue assigned letters from the Greek alphabet
Greek alphabet
The Greek alphabet is the script that has been used to write the Greek language since at least 730 BC . The alphabet in its classical and modern form consists of 24 letters ordered in sequence from alpha to omega...
to groups of stars belonging to the same visual magnitude class in each constellation, beginning with alpha (α) for a star in the brightest class. However, Bayer made no attempt to arrange stars by relative brightness within each class. Thus, although Epsilon is the fifth letter in the Greek alphabet, the star is the tenth brightest star in Eridanus. The star catalogue of English astronomer John Flamsteed
John Flamsteed
Sir John Flamsteed FRS was an English astronomer and the first Astronomer Royal. He catalogued over 3000 stars.- Life :Flamsteed was born in Denby, Derbyshire, England, the only son of Stephen Flamsteed...
, published in 1712, gave this star the Flamsteed designation
Flamsteed designation
Flamsteed designations for stars are similar to Bayer designations, except that they use numbers instead of Greek letters. Each star is assigned a number and the Latin genitive of the constellation it lies in...
18 Eridani as it was the eighteenth catalogued star in the constellation of Eridanus by order of increasing right ascension. In 1918 this star appeared in the Henry Draper Catalogue
Henry Draper Catalogue
The Henry Draper Catalogue is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the Henry Draper Extension , published between 1925 and 1936, which gave classifications for 46,850 more stars, and by the...
with the designation HD 22049 and a preliminary spectral classification of K0.
Based on observations between 1800 and 1880, Epsilon Eridani was found to have a large proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
across the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
, which was estimated at an angular velocity
Angular velocity
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...
of three arcseconds annually. This movement implied it was relatively close to the Sun, making it a star of interest for the purpose of trigonometric parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
measurements. This process involves recording the position of the star as the Earth moves around the Sun, which allows the star's distance to be estimated. From 1881 to 1883, American astronomer William L. Elkin
William Lewis Elkin
William Louis Elkin was an American astronomer.He was born in New Orleans to Jane and Lewis Elkin, one of five children but the only one to survive to adulthood. Following the death of her husband in 1867, Jane travelled abroad for the following seventeen years, taking along William...
used a heliometer
Heliometer
Heliometer is an instrument originally designed for measuring the variation of the sun's diameter at different seasons of the year, but applied now to the modern form of the instrument which is capable of much wider use....
at the Royal Observatory at the Cape of Good Hope, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
to compare the position of Epsilon Eridani with two nearby stars. From these observations, a parallax of was calculated. By 1917, observers had refined their parallax estimate to 0.317 arcseconds. The modern value of 0.3109 arcseconds is equivalent to a distance of about 10.5 ly.
Circumstellar discoveries
Based on unexplained changes in the position of Epsilon Eridani between 1938 and 1972, Dutch-American astronomer Peter van de KampPeter van de Kamp
Piet van de Kamp , known as Peter van de Kamp in the United States, was a Dutch astronomer who lived most of his life in the United States. He was professor of astronomy at Swarthmore College and director of the college's Sproul Observatory from 1937 until 1972...
proposed that an unseen companion with an orbital period of 25 years was causing gravitational perturbation
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....
s in the star's position. This claim was refuted in 1993 by German astronomer Wulff-Dieter Heintz and the false detection was blamed on a systematic error in the photographic plates.
Launched in 1983, the space telescope IRAS
IRAS
The Infrared Astronomical Satellite was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths....
detected infrared
Infrared astronomy
Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers...
emissions from stars near to the Sun. Two years later, the presence of an excess infrared emission close to Epsilon Eridani was announced, which indicated a disk of fine-grained cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...
was orbiting the star. This debris disk
Debris disk
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...
has been extensively studied since that time. Evidence for a planetary system was discovered in 1998 by the observation of asymmetries in this dust ring. These clumps of dust could be explained by gravitational interaction with a planet orbiting just inside the ring of dust.
From 1980 to 2000, a team of astronomers led by American Artie P. Hatzes
Artie P. Hatzes
Artie P. Hatzes is an American astronomer. He is a professor at the Friedrich Schiller University of Jena and director of the Karl Schwarzschild Observatory ....
made radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
observations of Epsilon Eridani, measuring changes in motion of the star along the line of sight to the Earth, which provided evidence of the gravitational effect of a planet orbiting the star with a period of about seven years. Although there is a high level of noise in the radial velocity data due to magnetic activity in the star's photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
, any periodicity caused by this magnetic activity is expected to show a strong correlation with variations in emission lines of ionized calcium (the Ca II H and K lines). Because no such correlation was found, a planetary companion was deemed the most likely cause. This discovery was supported by astrometric measurements of Epsilon Eridani made between 2001 and 2003 with the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, which showed evidence for gravitational perturbation of the star by a planet.
American astrophysicist Alice C. Quillen and her student Stephen Thorndike
Stephen Thorndike
Stephen Thorndike is a high school teacher. He is known for discovering, along with astrophysicist Alice C. Quillen, Epsilon Eridani c, a hypothetical planet orbiting the star Epsilon Eridani. After working as a professor at the University of Rochester, he now works as a high school physics and...
performed computer simulations of the structure of the dust disk around the star. Their model suggested that the clumping of the dust particles could be explained by the presence of a second planet in an eccentric orbit. They announced this finding in 2002.
SETI and proposed exploration
In 1960, American physicist Philip MorrisonPhilip Morrison
Philip Morrison, was Institute Professor Emeritus and Professor of Physics Emeritus at the Massachusetts Institute of Technology .-Early life and education:...
and Italian physicist Giuseppe Cocconi
Giuseppe Cocconi
Giuseppe Cocconi was a former director of the Proton Synchrotron at CERN in Geneva. He is known for is work in particle physics and for his involvement with SETI.-Life:...
proposed that extraterrestrial civilizations might be using radio signals for communication. Project Ozma
Project Ozma
Project Ozma was a pioneering SETI experiment started in 1960 by Cornell University astronomer Frank Drake, at the National Radio Astronomy Observatory at Green Bank, West Virginia. The object of the experiment was to search for signs of life in distant solar systems through interstellar radio waves...
, headed by American astronomer Frank Drake
Frank Drake
Frank Donald Drake PhD is an American astronomer and astrophysicist. He is most notable as one of the pioneers in the search for extraterrestrial intelligence, including the founding of SETI, mounting the first observational attempts at detecting extraterrestrial communications in 1961 in Project...
, used the Tatel Telescope to search for such signals from the nearby Sun-like stars Epsilon Eridani and Tau Ceti
Tau Ceti
Tau Ceti is a star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under 12 light-years from the Solar System, it is a relatively close star. Tau Ceti is metal-deficient and so is thought to be less likely to...
. They were observed at the emission frequency of neutral hydrogen, 1,420 MHz. No signals of intelligent extraterrestrial origin were detected. The experiment was repeated by Drake in 2010, with the same negative result. Despite this lack of success, Epsilon Eridani made its way into science fiction literature and television shows
Epsilon Eridani in fiction
The fifth brightest star in the constellation Eridanus. As a Sun-like star relatively close to the Solar System, Epsilon Eridani regularly appears in science fiction....
for many years following news of Drake's initial experiment.
In Habitable Planets for Man, a 1964 RAND Corporation study by American space scientist Stephen H. Dole, the odds of a habitable planet being in orbit around Epsilon Eridani were estimated at 3.3%. Among the known stars within 22 ly, it was listed with the 14 stars that were thought most likely to have a habitable planet.
A new strategy in the search for extraterrestrial intelligence (SETI
SETI
The search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
) was proposed by American space scientist William I. McLaughlin
William I. McLaughlin
William I. McLaughlin is a retired American space scientist.After being award his PhD in mathematics at the University of California at Berkeley in 1968 , he worked at Bellcomm, Inc. in Washington D.C. on the Apollo lunar-landing program. In 1971 he joined the Jet Propulsion Laboratory, where he...
in 1977. He suggested that widely observable events such as nova
Nova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...
explosions might be used by intelligent extraterrestrials to synchronize the transmission and reception of their signals. This idea was tested from the National Radio Astronomy Observatory
National Radio Astronomy Observatory
The National Radio Astronomy Observatory is a Federally Funded Research and Development Center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy...
in 1988, which used outbursts of Nova Cygni 1975 as the timer. Fifteen days of observation showed no anomalous radio signals coming from Epsilon Eridani.
Because of the proximity and Sun-like properties of this star, it was considered as one of the targets for interstellar travel
Interstellar travel
Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
by American physicist Robert L. Forward in 1985. The following year, Epsilon Eridani was suggested as one of several targets in the Project Daedalus
Project Daedalus
Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use current or near-future technology and had to...
paper study by the British Interplanetary Society
British Interplanetary Society
The British Interplanetary Society founded in 1933 by Philip E. Cleator, is the oldest space advocacy organisation in the world whose aim is exclusively to support and promote astronautics and space exploration.-Structure:...
. It has continued to be among the targets of such proposals, as with Project Icarus
Project Icarus (Interstellar Probe Design Study)
Project Icarus is a theoretical design study aimed at designing a credible nuclear fusion-based interstellar spacecraft that will stand as a blueprint for a possible unmanned mission...
in 2011.
Based on its location within 23.5 ly, Epsilon Eridani was among the target stars of Project Phoenix
Project Phoenix (SETI)
Project Phoenix is a SETI project: a search for extraterrestrial intelligence by analyzing patterns in radio signals. It is run by the independently funded SETI Institute of Mountain View, California, USA....
, a 1995 microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
survey for signals from extraterrestrial intelligence. The project had checked about 800 stars by 2004, but had not yet detected an unimpeachable signal.
Properties
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
. This star is located in the northern part of the constellation Eridanus, about 3° east of the slightly brighter star Delta Eridani
Delta Eridani
Delta Eridani is a 3.54 magnitude star in the constellation of Eridanus. It is also called Rana. The name Rana means "the frog" in Latin...
. With a declination of −9.46°, Epsilon Eridani can be viewed from much of the Earth's surface. Only to the north of latitude 80° N
80th parallel north
The 80th parallel north is a circle of latitude that is 80 degrees north of the Earth's equatorial plane, in the Arctic. It crosses the Atlantic Ocean, Europe, Asia, the Arctic Ocean and North America....
is it permanently hidden below the horizon. The apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of 3.73 can make this star difficult to observe from an urban area with the unaided eye, as the night skies over cities are obscured by light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
.
Epsilon Eridani has an estimated 82% of the Sun's mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
and 74% of the Sun's radius, but only 34% of its luminosity. The estimated surface temperature is 5,084 K. With a stellar classification of K2V, it is the second-nearest K-type main sequence star after Alpha Centauri B
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
. Its metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
, or enrichment in elements heavier than helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, is slightly lower than the Sun's. In the star's chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
, a region of the outer atmosphere just above the light emitting photosphere, the proportion of iron is estimated at 74% of the Sun's abundance.
The K-type classification of this star indicates that the spectrum displays relatively weak absorption lines from energy absorbed by hydrogen, plus strong lines of neutral atoms and singly ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ized calcium (Ca II). The luminosity class V is assigned to stars that are undergoing thermonuclear fusion of hydrogen at their core. For a K-type main sequence star, this fusion is dominated by the proton–proton chain reaction, wherein a series of mergers of four hydrogen nuclei results in a helium nucleus. In the inner region of this star, energy is transported outward from the core by means of radiation
Radiative transfer
Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission and scattering processes. The equation of radiative transfer describes these interactions mathematically...
, which results in no net motion of the surrounding plasma. Outside of this region, in the star's envelope, energy is carried to the photosphere by plasma convection
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...
, where it radiates into space.
Magnetic activity
Stellar magnetic field
A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively...
than the Sun, and hence demonstrates increased activity in the outer parts of the star's atmosphere: the chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
and corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
. The average magnetic field strength of this star across the entire surface is , which is more than forty times greater than the magnetic field strength in the Sun's photosphere. The magnetic properties can be modeled by assuming that regions with a magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
of about 0.14 T randomly cover approximately 9% of the photosphere, while the remainder of the surface is free of magnetic fields. The overall magnetic activity of this star is irregular, but it may vary with a 4.9-year period. Assuming that the radius of the star does not change over this interval, the long term variation in activity level appears to produce a temperature variation of 15 K, which corresponds to a variation in visual magnitude
UBV photometric system
UBV photometric system, also called the Johnson system , is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are...
(V) of 0.014.
The magnetic field on the surface of Epsilon Eridani causes variations in the hydrodynamic behavior of the photosphere. This results in greater jitter during measurements of the star's radial velocity Doppler shift. Variations of −1 were measured over a 20 year period, which is much higher than the measurement error rate of −1. This makes interpretation of periodicities in the radial velocity of Epsilon Eridani, such as those caused by the gravitational perturbations of an orbiting planet, more difficult.
Epsilon Eridani is classified as a BY Draconis variable
BY Draconis variable
BY Draconis variables are main sequence variable stars of late spectral types, usually K or M. The name comes from the archetype for this category of variable star system, BY Draconis. They exhibit variations in their luminosity due to rotation of the star coupled with star spots, and other...
because it has regions of higher magnetic activity that move into and out of the line of sight as the star rotates. Measurement of this rotational modulation suggests that the equatorial region of the star rotates with an average period of 11.2 days, which is less than half of the rotation period of the Sun. Observations have shown this star to vary as much as 0.050 in V magnitude due to starspot
Starspot
Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...
s and other short-term magnetic activity. Photometry
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
has also shown that the surface of Epsilon Eridani, like the Sun, is undergoing differential rotation
Differential rotation
Differential rotation is seen when different parts of a rotating object move with different angular velocities at different latitudes and/or depths of the body and/or in time. This indicates that the object is not solid. In fluid objects, such as accretion disks, this leads to shearing...
, which means that the rotation period at the surface varies by latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
. The measured periods range from 10.8 to 12.3 days. The axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
of Epsilon Eridani toward the line of sight from Earth is uncertain. Estimates range from 24° to 72°.
The high levels of chromospheric activity, strong magnetic field, and relatively fast rotation rate of Epsilon Eridani are characteristic of a young star. The age of Epsilon Eridani is about , but this remains subject to debate. Most age estimation methods place it in the range from 200 million to 800 million years. However, the low abundance of heavy elements in the chromosphere of Epsilon Eridani is indicative of an older star, because the medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
out of which stars form is steadily enriched by heavier elements produced by older generations of stars. This anomaly might be caused by a diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
process that has transported some of the helium and heavier elements out of the photosphere and into a region below the star's convection zone
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...
.
The X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
luminosity of Epsilon Eridani is about . It is brighter in X-ray emission than the Sun at peak activity
Solar cycle
The solar cycle, or the solar magnetic activity cycle, is a periodic change in the amount of irradiation from the Sun that is experienced on Earth. It has a period of about 11 years, and is one component of solar variation, the other being aperiodic fluctuations. Solar variation causes changes in...
. The source for this strong X-ray emission is the star's hot corona. Epsilon Eridani's corona appears larger and hotter than the Sun's, with a temperature of as measured from observation of the corona's ultraviolet and X-ray emission.
The stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
emitted by Epsilon Eridani expands until it collides with the surrounding interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
of sparse gas and dust, resulting in a bubble of heated hydrogen gas. The absorption spectrum from this gas has been measured with the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, allowing the properties of the stellar wind to be estimated. Epsilon Eridani's hot corona results in a mass loss rate from the star's stellar wind that is 30 times higher than the Sun's. This wind is generating an astrosphere
Stellar wind bubble
Stellar-wind bubble is the astronomical term usually used to describe a cavity light years across filled with hot gas blown into the interstellar medium by the high-velocity stellar wind from a single massive star of type O or B. Weaker stellar winds still blow bubble structures though, and these...
(the equivalent of the heliosphere
Heliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...
that surrounds the Sun) that spans about 8,000 AU and contains a bow shock
Bow shock
A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
that lies 1,600 AU from the star. At its estimated distance from Earth, this astrosphere spans 42 arcminutes, which is wider than the apparent size of the full Moon.
Kinematics
This star has a high proper motionProper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
, moving −0.976 arcseconds per year in right ascension (the celestial longitude) and 0.018 arcseconds per year in declination (the celestial latitude), for a total proper motion of 0.962 arcseconds per year. It has a radial velocity of +15.5 km/s away from the Sun. The space velocity components of Epsilon Eridani in the Galactic coordinate system
Galactic coordinate system
The galactic coordinate system is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane...
are = , which means that it is traveling within the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
at a mean galactocentric distance
Galactocentric distance
Galactocentric distance is a star's distance from the center of a galaxy. For example, our Sun is about 27 kly from the center of the Milky Way. Galactocentric distance may also refer to a galaxy's distance from another galaxy....
of 28.7 kly (8.79 kiloparsecs) from the core along an elliptical orbit that has an eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
of 0.09. The velocity and heading
Stellar kinematics
Stellar kinematics is the study of the movement of stars without needing to understand how they acquired their motion. This differs from stellar dynamics, which takes into account gravitational effects...
of this star indicates that it may be a member of the Ursa Major Moving Group
Ursa Major Moving Group
The Ursa Major Moving Group, also known as Collinder 285 or Ursa Major association, is a nearby stellar moving group, a set of stars with common velocities in space and thought to have a common origin. Its core is located roughly 80 light years away...
of stars that share a common motion through space. This behavior suggests that the members originated in an open cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...
of stars that has since diffused. The estimated age of this group is years, which lies within the range of the age estimates for this star.
During the past million years, three stars are believed to have come within 7 ly (2 parsecs) of Epsilon Eridani. The most recent and closest of these encounters was with Kapteyn's Star
Kapteyn's Star
Kapteyn's Star is a class M1 red dwarf star about 13 light years from Earth in the southern constellation of Pictor. With a magnitude of nearly 9 it is visible through binoculars or a telescope.-History:...
, which approached to a distance of about 3 ly (0.9 parsecs) roughly 12,500 years ago. The other two stars were Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
and Ross 614
Ross 614
Ross 614 or Gliese 234A is a red dwarf UV Cetiflare star and it is the primary member of a nearby binary star system in the constellation of Monoceros. This star has an magnitude of about 11, making it invisible to the unaided eye even though it is one of the stars nearest to the sun...
. None of these encounters are thought to have affected the circumstellar disk orbiting Epsilon Eridani.
Epsilon Eridani made its closest approach to the Sun about 105,000 years ago, when the two stars were separated by 7 ly. Based upon a simulation of close encounters by nearby stars, in approximately 31,500 years, the binary star system Luyten 726-8
Luyten 726-8
Luyten 726-8 is a binary star system that is one of Earth's nearest neighbors, at about 8.7 light years from Earth in the constellation Cetus...
, which includes the variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
UV Ceti, will encounter Epsilon Eridani at a minimum distance of about 0.9 ly (0.29 parsecs). They will be less than 1 ly (0.3 parsecs) apart for about 4,600 years. If Epsilon Eridani has an Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
, Luyten 726-8 could gravitationally perturb
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....
some of the comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s with long orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
s.
Planetary system
Dust disk
Observations with the James Clerk Maxwell TelescopeJames Clerk Maxwell Telescope
The James Clerk Maxwell Telescope is a submillimetre-wavelength telescope at Mauna Kea Observatory in Hawaii. Its primary mirror is 15 metres across: it is the largest astronomical telescope that operates in submillimetre wavelengths of the electromagnetic spectrum...
at a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of 850 μm show an extended flux of radiation out to an angular radius
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
of 35 arcseconds around the star. The peak emission occurs at an angular radius of 18 arcseconds, which at the distance of the star corresponds to a radius of about 60 AU. The highest level of emission occurs over the radius 35–75 AU from the star and is substantially reduced inside 30 AU. This emission is interpreted as coming from a young analogue of the Solar System's Kuiper Belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
: a compact dusty disk structure surrounding the star. From the Earth, this belt is viewed at an inclination of roughly 25° to the line of sight.
Dust and possibly water ice from this belt migrates inward because of drag from the stellar wind and a process by which stellar radiation causes dust grains to slowly spiral toward the star, known as the Poynting–Robertson effect. At the same time, these dust particles can be destroyed through mutual collisions. The time scale for all of the dust in the disk to be cleared away by these processes is less than the estimated age of the star. Hence, the current dust disk must have been created by collisions or other effects of larger parent bodies, and the disk represents a late stage in the planet formation process of this star. It would have required collisions between 11 Earth masses' worth of parent bodies to have maintained the disk in its current state over the estimated age of the star.
The disk contains an estimated mass of dust equal to a sixth of the mass of the Moon, with individual dust grains exceeding 3.5 μm in size at a temperature of about 55 K. This dust is being generated by the collision of comets, which range up to 10 to 30 km in diameter and have a combined mass of 5 to 9 times the mass of the Earth. This is similar to the estimated 10 Earth masses in the primordial Kuiper Belt. However, the disk around Epsilon Eridani contains less than of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
. This low level suggests a paucity of volatile-bearing comets and icy planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
s compared to the Kuiper belt.
The clumpy structure of the dust belt may be explained by gravitational perturbation from a planet, dubbed Epsilon Eridani b. The clumps in the dust occur at orbits that have an integer resonance with the orbit of the suspected planet. For example, the region of the disk that completes two orbits for every three orbits of a planet is in a 3:2 orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
. In computer simulations the ring morphology can be reproduced by the capture of dust particles in 5:3 and 3:2 orbital resonances with a planet that has an orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
of about 0.3. Alternatively, the clumpiness may have been caused by collisions between minor planet
Minor planet
An asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...
s known as plutino
Plutino
In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...
s.
Observations from NASA's Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
suggest that Epsilon Eridani actually has two asteroid belts and a cloud of exozodiacal dust
Exozodiacal dust
Exozodiacal dust is the exoplanetary analog of zodiacal dust, the 1-100 micrometre-sized grains of silicate dust that fill the plane of the solar system, especially interior to the asteroid belt. As for the zodiacal dust, these grains are probably produced by outgasing comets as well as by...
. The latter is an analog of the zodiacal dust
Zodiacal dust
The Zodiacal dust cloud is visible as a diffuse glow, known as the zodiacal light, that stretches along the zodiac, and is best seen just after sunset and before sunrise in spring and autumn when the zodiac is at a steep angle to the horizon....
that occupies the plane of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. One belt sits at approximately the same position as the one in our solar system, orbiting at a distance of from the star, and consists of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
grains with a diameter of 3 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
and a combined mass of about 1018 kg. If the planet Epsilon Eridani b exists then this belt is unlikely to have had a source outside the orbit of the planet, so the dust may have been created by fragmentation and cratering of larger bodies such as asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s. The second, denser belt, most likely also populated by asteroids, lies between the first belt and the outer comet disk. The structure of the belts and the dust disk suggests that more than two planets in the Epsilon Eridani system are needed to maintain this configuration.
In an alternative scenario, the exozodiacal dust may be generated in an outer belt, which is orbiting between 55 and 90 AU from the host star and has an assumed mass of 10−3 times the mass of the Earth. This dust is then transported inward past the orbit of Epsilon Eridani b. When collisions between the dust grains are taken into account, the dust will reproduce the observed infrared spectrum and brightness. Outside the radius of ice sublimation, located beyond 10 AU from the star where the temperatures fall below 100 K, the best fit to the observations occurs when a mix of ice and silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
dust is assumed. Inside this radius, the dust must consist of silicate grains that lack volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
.
The inner region around the star, from a radius of 2.5 AU inward, appears to be clear of dust down to the detection limit of the 6.5 m MMT telescope
MMT Observatory
The MMT Observatory is an astronomical observatory on the site of Fred Lawrence Whipple Observatory . The Whipple observatory complex is located on Mount Hopkins, Arizona, USA in the Santa Rita Mountains...
. Grains of dust in this region are efficiently removed by drag from the stellar wind, while the presence of a planetary system may also help keep this area clear of debris. Still, this does not preclude the possibility that an inner asteroid belt may be present with a combined mass no greater than the asteroid belt in the Solar System.
Possible planets
As one of the nearest Sun-like stars, Epsilon Eridani has been the target of many attempts to search for planetary companions. However, its chromospheric activity and variability means that finding planets with the radial velocity method is difficult, because the stellar activity may create signals that mimic the presence of planets. Attempts at direct imaging of potential exoplanets have proven unsuccessful to date. Infrared observation has shown there are no bodies of three or more Jupiter massJupiter mass
Jupiter mass , is the unit of mass equal to the total mass of the planet Jupiter . Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs....
es in this system.
Planet b
Referred to as Epsilon Eridani bEpsilon Eridani b
Epsilon Eridani b is an unconfirmed extrasolar planet approximately 10 light-years away orbiting the star Epsilon Eridani, in the constellation of Eridanus .- Discovery :...
, this planet was announced in 2000, but the discovery has remained controversial. A comprehensive study in 2008 called the detection "tentative" and described the proposed planet as "long suspected but still unconfirmed." However, many astronomers believe the evidence is sufficiently compelling that they regard the discovery as confirmed.
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
range from to .
The true mass of this planet remains unknown, but it can be estimated based on the displacement effect of the planet's gravity upon the star. Only the component of the displacement along the line of sight to the Earth is known, which yields a value for the formula m sin i, where m is the mass of the planet and i is the orbital inclination. Estimates for the value of range from 0.60 Jupiter mass
Jupiter mass
Jupiter mass , is the unit of mass equal to the total mass of the planet Jupiter . Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs....
es to 1.06 Jupiter masses, which sets the lower limit for the mass of the planet (since the sine
Sine
In mathematics, the sine function is a function of an angle. In a right triangle, sine gives the ratio of the length of the side opposite to an angle to the length of the hypotenuse.Sine is usually listed first amongst the trigonometric functions....
function has a maximum value of 1). By choosing a mass of 0.78 and an estimated inclination of 30°, this yields the frequently cited value of Jupiter masses for the planet's mass.
Of all the measured parameters for this planet, the value for orbital eccentricity is the most uncertain. The frequently cited value of 0.7 for Epsilon Eridani b's eccentricity is inconsistent with the presence of the proposed asteroid belt at a distance of 3 AU from the star. If the eccentricity was actually this high, the planet would pass through the asteroid belt and clear it out within about ten thousand years. If the belt has existed for longer than this period, which appears likely, it imposes an upper limit on Epsilon Eridani b's eccentricity of about 0.10–0.15. If the dust disk is instead being generated from the outer debris disk, rather than from collisions in an asteroid belt, then no constraints on the planet's orbital eccentricity are needed to explain the dust distribution.
Planet c
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
orbital periods with a planet in an eccentric orbit. The postulated Epsilon Eridani c would orbit at a distance of 40 AU, with an eccentricity of 0.3 and a period of 280 years. The inner cavity of the disk may be explained by the presence of additional planets. Current models of planet formation cannot easily explain how a planet could have been created at this distance from the star. The disk is expected to have dissipated long before a gas giant could have formed. Instead, the planet may have formed at an orbital distance of about 10 AU then migrated outward because of gravitational interaction with the disc or with other planets in the system.
Potential habitability
Epsilon Eridani is a target for planet finding programs because it has properties that allow an Earth-like planet to form. Although this system was not chosen as a primary candidate for the now-canceled Terrestrial Planet FinderTerrestrial Planet Finder
The Terrestrial Planet Finder was a proposed project by NASA to construct a system of telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled...
, it was a target star for NASA's proposed Space Interferometry Mission
Space Interferometry Mission
The Space Interferometry Mission, or SIM, also known as SIM Lite , was a planned space telescope developed by the U.S. National Aeronautics and Space Administration , in conjunction with contractor Northrop Grumman...
to search for Earth-sized planets. The proximity, Sun-like properties and suspected planets of this star have also made it the subject of multiple studies on whether an interstellar probe
Interstellar probe
An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar space, typically defined as the region beyond the heliopause...
can be sent to Epsilon Eridani.
The orbital radius at which the stellar flux from Epsilon Eridani matches the solar constant
Solar constant
The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit...
—where the emission matches the Sun's output at the orbital distance of the Earth—is 0.61 astronomical units (AU). That is within the maximum habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...
of a conjectured Earth-like planet orbiting Epsilon Eridani, which currently stretches from about 0.5 to 1.0 AU. As the star ages over a period of 20 billion years, the net luminosity will increase, causing this zone to slowly expand outward to about 0.6–1.4 AU. However, the presence of a large planet with a highly elliptical orbit in proximity to the habitable zone of the star reduces the likelihood of a terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
having a stable orbit within the habitable zone.
A young star such as Epsilon Eridani can produce large amounts of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation that may be harmful to life. The orbital radius where the UV flux matches that on the early Earth lies at just under 0.5 AU. The proximity, Sun-like properties and suspected planets of this star have made it a destination for interstellar travel
Interstellar travel
Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
stories.
See also
- Epsilon Eridani in fictionEpsilon Eridani in fictionThe fifth brightest star in the constellation Eridanus. As a Sun-like star relatively close to the Solar System, Epsilon Eridani regularly appears in science fiction....
- List of nearest stars
- List of extrasolar planets