Methods of detecting extrasolar planets
Encyclopedia
Any planet
is an extremely faint light source compared to its parent star
. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those
reasons, only a very few extrasolar planet
s have been observed directly.
Instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. At the present time, several different indirect methods have yielded success.
s due to the Doppler effect
. The radial velocity method measures these variations in order to confirm the presence of the planet.
The velocity of the star around the center of mass
is much smaller than that of the planet, because the radius of its orbit around the center of mass is so small. Velocity variations down to 1 m/s can be detected with modern spectrometer
s, such as the HARPS (High Accuracy Radial Velocity Planet Searcher
) spectrometer at the ESO
3.6 meter telescope in La Silla Observatory
, Chile, or the HIRES spectrometer at the Keck telescopes
.
An especially simple and inexpensive method for measuring radial velocity is "externally dispersed interferometry".
This has been by far the most productive technique used by planet hunters. It is also known as Doppler spectroscopy. The method is distance independent, but requires high signal-to-noise ratio
s to achieve high precision, and so is generally only used for relatively nearby stars out to about 160 light-years from Earth. It easily finds massive planets that are close to stars, but detection of those orbiting at great distances requires many years of observation. Planets with orbits highly inclined to the line of sight from Earth produce smaller wobbles, and are thus more difficult to detect. One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's minimum mass. The posterior distribution of the inclination angle depends on the true mass distribution of the planets.
The radial-velocity method can be used to confirm findings made by using the transit method. When both methods are used in combination, then the planet's true mass can be estimated.
A pulsar
is a neutron star: the small, ultradense remnant of a star that has exploded as a supernova
. Pulsars emit radio waves extremely regularly as they rotate. Because the intrinsic rotation of a pulsar is so regular, slight anomalies in the timing of its observed radio pulses can be used to track the pulsar's motion. Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulse-timing observations can then reveal the parameters of that orbit.
This method was not originally designed for the detection of planets, but is so sensitive that it is capable of detecting planets far smaller than any other method can, down to less than a tenth the mass of Earth. It is also capable of detecting mutual gravitational perturbations between the various members of a planetary system, thereby revealing further information about those planets and their orbital parameters.
The main drawback of the pulsar-timing method is that pulsars are relatively rare, so it is unlikely that a large number of planets will be found this way. Also, life as we know it could not survive on planets orbiting pulsars since high-energy radiation there is extremely intense.
In 1992 Aleksander Wolszczan
and Dale Frail
used this method to discover planets around the pulsar PSR 1257+12. Their discovery was quickly confirmed, making it the first confirmation of planets outside our Solar System
.
While the above methods provide information about a planet's mass, this photometric
method can determine the radius of a planet. If a planet crosses (transits
) in front of its parent star's disk, then the observed visual brightness of the star drops a small amount. The amount the star dims depends on the relative sizes of the star and the planet. For example, in the case of HD 209458
, the star dims 1.7%.
This method has two major disadvantages. First of all, planetary transits are only observable for planets whose orbits happen to be perfectly aligned from the astronomers' vantage point. The probability of a planetary orbital plane being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit. About 10% of planets with small orbits have such alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a sun-sized star at 1 AU
, the probability of a random alignment producing a transit is 0.47% However, by scanning large areas of the sky containing thousands or even hundreds of thousands of stars at once, transit surveys can in principle find extrasolar planets at a rate that could potentially exceed that of the radial-velocity method, although it would not answer the question of whether any particular star is host to planets.
Secondly, the method suffers from a high rate of false detections. A transit detection requires additional confirmation, typically from the radial-velocity method.
The main advantage of the transit method is that the size of the planet can be determined from the lightcurve. When combined with the radial velocity method (which determines the planet's mass) one can determine the density of the planet, and hence learn something about the planet's physical structure. The nine planets that have been studied by both methods are by far the best-characterized of all known exoplanets.
The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet. By studying the high-resolution stellar spectrum carefully, one can detect elements present in the planet's atmosphere. A planetary atmosphere (and planet for that matter) could also be detected by measuring the polarisation of the starlight as it passed through or is reflected off the planet's atmosphere.
Additionally, the secondary eclipse (when the planet is blocked by its star) allows direct measurement of the planet's radiation. If the star's photometric
intensity during the secondary eclipse is subtracted from its intensity before or after, only the signal caused by the planet remains. It is then possible to measure the planet's temperature and even to detect possible signs of cloud formations on it. In March 2005, two groups of scientists carried out measurements using this technique with the Spitzer Space Telescope
. The two teams, from the Harvard-Smithsonian Center for Astrophysics
, led by David Charbonneau
, and the Goddard Space Flight Center
, led by L. D. Deming, studied the planets TrES-1
and HD 209458b respectively. The measurements revealed the planets' temperatures: 1,060 K
(790°C
) for TrES-1 and about 1,130 K (860°C) for HD 209458b.
In addition the hot Neptune Gliese 436 b
enters secondary eclipse. However some transiting planets orbit such that they do not enter secondary eclipse relative to Earth; HD 17156 b
is over 90% likely to be one of the latter.
A French Space Agency
mission, COROT
, began in 2006 to search for planetary transits from orbit, where the absence of atmospheric scintillation
allows improved accuracy. This mission was designed to be able to detect planets "a few times to several times larger than Earth" and is currently performing "better than expected", with two exoplanet discoveries (both "hot jupiter" type) as of early 2008. The 17th CoRoT exoplanet was announced in 2010.
In March 2009, NASA
mission Kepler
was launched to scan a large number of stars in the constellation Cygnus
with a measurement precision expected to detect and characterize Earth-sized planets. The NASA Kepler Mission
uses the transit method to scan a hundred thousand stars in the constellation Cygnus for planets. Kepler will be sensitive enough to detect planets even smaller than Earth. By scanning a hundred thousand stars simultaneously, it will not only be able to detect Earth-sized planets, it will be able to collect statistics on the numbers of such planets around sunlike stars.
Kepler has already been able to detect the light from a known transiting extrasolar gas giant, HAT-P-7b
. It is expected that Kepler will even be able to detect light from non-transiting gas giants on close orbits, though it will not be able to resolve that light into an image. Instead, the brightness of the host star seems to change gradually over time in a periodic manner, because like the Moon
, the planet goes through phases
from full to new and back again. The variation, although small, will be the signature of a planet. In addition to the reflected light from the star, some of the light from the planet will be thermally emitted by the planet itself. Thus the shape of the phase curve constrains the composition of the atmosphere based on the reflectivity of the planet, and also gives indications of the transport of heat on the planet from the day side to the night. This planet phase variation method may actually provide the greatest number of planets to be discovered by the Kepler satellite, since it does not require the planet to pass in front of the disk of the star.
On February 2, 2011, the Kepler team released a list of 1,235 extrasolar planet candidates, including 54 that may be in the habitable zone
.
shows TTV with an amplitude of 5 minutes and a period of about 300 days, indicating the presence of a second planet, Kepler-19c
, which has a period which is a near rational multiple of the period of the transiting planet
"Timing variation" asks whether the transit occurs with strict periodicity or if there's a variation. "Duration variation" asks how long the transit takes. Duration variations may be caused by an exomoon.
If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing contributions at a reasonable rate. This method is most fruitful for planets between Earth and the center of the galaxy, as the galactic center provides a large number of background stars.
In 1991, astronomers Shude Mao and Bohdan Paczyński
of Princeton University
first proposed using gravitational microlensing to look for exoplanets. Successes with the method date back to 2002, when a group of Polish astronomers (Andrzej Udalski
, Marcin Kubiak
and Michał Szymański from Warsaw
, and Bohdan Paczyński
) during project OGLE (the Optical Gravitational Lensing Experiment
) developed a workable technique. During one month they found several possible planets, though limitations in the observations prevented clear confirmation. Since then, four confirmed extrasolar planets have been detected using microlensing. this was the only method capable of detecting planets of Earthlike mass around ordinary main-sequence stars.
A notable disadvantage of the method is that the lensing cannot be repeated because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations with other methods are usually impossible. However, if enough background stars can be observed with enough accuracy then the method should eventually reveal how common earth-like planets are in the galaxy.
Observations are usually performed using networks of robotic telescope
s. In addition to the NASA
/National Science Foundation
-funded OGLE, the Microlensing Observations in Astrophysics
(MOA) group is working to perfect this approach.
The PLANET (Probing Lensing Anomalies NETwork
)/RoboNet project is even more ambitious. It allows nearly continuous round-the-clock coverage by a world-spanning telescope network, providing the opportunity to pick up microlensing contributions from planets with masses as low as Earth. This strategy was successful in detecting the first low-mass planet on a wide orbit, designated OGLE-2005-BLG-390Lb
.
Some projects to equip telescopes with planet-imaging-capable instruments include: Gemini telescope (GPI
), the VLT
(SPHERE
), and the Subaru telescope (HiCiao).
Up until the year 2010, telescopes could only directly image exoplanets under exceptional circumstances. Specifically, it is easier to obtain images when the planet is especially large (considerably larger than Jupiter
), widely separated from its parent star, and hot so that it emits intense infrared radiation. However in 2010 a team from NASA
s Jet Propulsion Laboratory
demonstrated that a vortex
coronagraph
could enable small scopes to directly image planets. They did this by imaging the previously imaged HR 8799
planets using just a 1.5 m portion of the Hale Telescope
.
Another promising approach is nulling interferometry.
Images taken in 2003 and reanalysed in 2008, revealed a planet orbiting Beta Pictoris
which in 2009 was observed to have moved to the other side of the star.
In July 2004, a group of astronomers used the European Southern Observatory
's Very Large Telescope
array in Chile to produce an image of 2M1207b
, a companion to the brown dwarf
2M1207. In December 2005, the planetary status of the companion was confirmed.
The planet is believed to be several times more massive than Jupiter
and to have an orbital radius greater than 40 AU.
In September 2008, an object was imaged at a separation of 330AU from the star 1RXS J160929.1−210524, but it was not until 2010 that it was confirmed to be a companion planet to the star and not just a chance alignment.
The first multiplanet system, announced on 13 November 2008, was imaged in 2007 using telescopes at both Keck Observatory and Gemini Observatory
. Three planets were directly observed orbiting HR 8799
, whose masses are approximately 10, 10 and 7 times that of Jupiter
. On the same day, 13 November 2008, it was announced that the Hubble Space Telescope directly observed an exoplanet orbiting Fomalhaut
with mass no more than 3MJ. Both systems are surrounded by disks not unlike the Kuiper belt
. An additional system, GJ 758
, was imaged in November 2009, by a team using the HiCIAO instrument of the Subaru Telescope
but it was a brown dwarf.
Other possible exoplanets to have been directly imaged: GQ Lupi b
, AB Pictoris b, and SCR 1845 b. As of March 2006 none have been confirmed as planets; instead, they might themselves be small brown dwarfs.
This method consists of precisely measuring a star's position in the sky and observing how that position changes over time. Originally this was done visually with hand-written records. By the end of the 19th century this method used photographic plates, greatly improving the accuracy of the measurements as well as creating a data archive. If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit. Effectively, star and planet each orbit around their mutual center of mass (barycenter
), as explained by solutions to the two-body problem
. Since the star is much more massive, its orbit will be much smaller. Frequently, the mutual center of mass will lie within the radius of the larger body.
Astrometry is the oldest search method for extrasolar planets and originally popular because of its success in characterizing astrometric binary star systems. It dates back at least to statements made by William Herschel
in the late 18th century. He claimed that an unseen companion was affecting the position of the star he cataloged as 70 Ophiuchi
. The first known formal astrometric calculation for an extrasolar planet was made by W. S. Jacob in 1855 for this star. Similar calculations were repeated by others for another half-century until finally refuted in the early 20th century.
For two centuries claims circulated of the discovery of unseen companions in orbit around nearby star systems that all were reportedly found using this method, culminating in the prominent 1996 announcement of multiple planets orbiting the nearby star Lalande 21185
by George Gatewood. None of these claims survived scrutiny by other astronomers, and the technique fell into disrepute. Unfortunately, the changes in stellar position are so small and atmospheric and systematic distortions so large that even the best ground-based telescopes cannot produce precise enough measurements. All claims of a planetary companion of less than 0.1 solar mass, as the mass of the planet, made before 1996 using this method are likely spurious. In 2002, the Hubble Space Telescope
did succeed in using astrometry to characterize a previously discovered planet around the star Gliese 876
.
Future space-based observatories such as NASA's Space Interferometry Mission
may succeed in uncovering new planets via astrometry, but for the time being no planet detected by astrometry has been confirmed.
One potential advantage of the astrometric method is that it is most sensitive to planets with large orbits. This makes it complementary to other methods that are most sensitive to planets with small orbits. However, very long observation times will be required — years, and possibly decades, as planets far enough from their star to allow detection via astrometry also take a long time to complete an orbit.
In 2009 the discovery of VB 10b by astrometry was announced. This planetary object was reported to have a mass 7 times that of Jupiter
and orbiting the nearby low mass red dwarf
star VB 10
. If confirmed, this would be the first exoplanet discovered by astrometry of the many that have been claimed through the years. However recent radial velocity
independent studies rule out the existence of the claimed planet.
system is aligned such that - from the Earth's point of view - the stars pass in front of each other in their orbits, the system is called an "eclipsing binary" star system. The time of minimum light, when the star with the brighter surface area is at least partially obscured by the disc of the other star, is called the primary eclipse
, and approximately half an orbit later, the secondary eclipse occurs when the brighter surface area star obscures some portion of the other star. These times of minimum light, or central eclipse, constitute a time stamp on the system, much like the pulses from a pulsar
(except that rather than a flash, they are a dip in the brightness). If there is a planet in circum-binary orbit around the binary stars, the stars will be offset around a binary-planet center of mass
. As the stars in the binary are displaced by the planet back and forth, the times of the eclipse minima will vary; they will be too late, on time, too early, on time, too late, etc.. The periodicity of this offset may be the most reliable way to detect extrasolar planets around close binary systems.
, they will go through phases
from full to new and back again. Although the effect is small — the photometric precision required is about the same as to detect an Earth-sized planet in transit across a solar-type star — such Jupiter-sized planets should be detectable by space telescopes such as the Kepler Space Observatory. This method may actually constitute the most planets that will be discovered by that mission because the reflected light variation with orbital phase is largely independent of orbital inclination of the planet's orbit. In addition, the phase function of the giant planet may be constrained which will, in turn, lead to constraints on the actual particle size distribution of the atmospheric particles.
By analyzing the polarization in the combined light of the planet and star (about one part in a million), these measurements can in principle be made with very high sensitivity, as polarimetry is not limited by the stability of the Earth's atmosphere.
Astronomical devices used for polarimetry, called polarimeters, are capable of detecting the polarized light and rejecting the unpolarized beams (starlight). Groups such as ZIMPOL/CHEOPS
and PlanetPol
are currently using polarimeters to search for extra-solar planets, though no planets have yet been detected using this method.
l radio
emissions from giant planets with plasma
sources such as Jupiter
's volcanic moon Io
could be detected with future radio telescopes such as LOFAR
.
Disks of space dust (debris disk
s) surround many stars. The dust can be detected because it absorbs ordinary starlight and re-emits it as infrared
radiation. Even if the dust particles have a total mass well less than that of Earth, they can still have a large enough total surface area that they outshine their parent star in infrared wavelengths.
The Hubble Space Telescope
is capable of observing dust disks with its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument. Even better images have now been taken by its sister instrument, the Spitzer Space Telescope
, and by the European Space Agency
's Herschel Space Observatory
, which can see far deeper into infrared
wavelengths than the Hubble can. Dust disks have now been found around more than 15% of nearby sunlike stars.
The dust is believed to be generated by collisions among comets and asteroids. Radiation pressure from the star will push the dust particles away into interstellar space over a relatively short timescale. Therefore, the detection of dust indicates continual replenishment by new collisions, and provides strong indirect evidence of the presence of small bodies like comets and asteroid
s that orbit the parent star. For example, the dust disk around the star tau Ceti
indicates that that star has a population of objects analogous to our own Solar System's Kuiper Belt
, but at least ten times thicker.
More speculatively, features in dust disks sometimes suggest the presence of full-sized planets. Some disks have a central cavity, meaning that they are really ring-shaped. The central cavity may be caused by a planet "clearing out" the dust inside its orbit. Other disks contain clumps that may be caused by the gravitational influence of a planet. Both these kinds of features are present in the dust disk around epsilon Eridani
, hinting at the presence of a planet with an orbital radius of around 40 AU
(in addition to the inner planet detected through the radial-velocity method). These kinds of planet-disk interactions can be modeled numerically using collisional grooming techniques.
s' atmospheres
by Spitzer Space Telescope
found contamination of heavier elements like magnesium
and calcium
. These elements cannot originate from the stars' core and it is probable that the contamination comes from asteroid
s that got too close (within the Roche limit
) to these stars by gravitational interaction with larger planets and were torn apart by star's tidal forces. Spitzer data suggests that 1-3% of the white dwarfs has similar contamination.
On February 2, 2006 NASA announced an indefinite suspension of work on the Terrestrial Planet Finder due to budget problems. Then in June 2006, the Appropriations Committee of the U.S. House of Representatives partially restored funding, permitting development work on the project to continue at least through 2007. COROT was launched on December 27, 2006 and Kepler's launch was performed on March 7, 2009.)
NASA's Space Interferometry Mission
, now cancelled, will use astrometry. It may be able to detect Earth-like planets around several nearby stars. The European Space Agency
's Darwin
probe and NASA
's Terrestrial Planet Finder
probes will attempt to image planets directly. A recently proposed idea is the New Worlds Mission, which will use an occulter to block a star's light, allowing astronomers to directly observe the dimmer orbiting planets.
Huge proposed ground telescopes may also be able to directly image extrasolar planets. ESO is planning to build the European Extremely Large Telescope, with a mirror diameter of 39.3 meters. Using a coronagraph together with extreme adaptive optics it's EPICS instrument will most likley be able to image earth sized planet's around nearby stars.
If it goes ahead sometime between 2025–2035, the proposed ATLAST
telescope would be able to image small and rocky extrasolar planets either with it's internal coronagraph or with an external occulter.
The Transiting Exoplanet Survey Satellite
(TESS) is a space mission that will monitor the brightest and closest 2.5 million stars to Earth in order to detect rocky and water dominated planets using the transit method. TESS
will have the capability to identify the closest transiting rocky planets to Earth that orbit in the habitable zone
of their stellar host. TESS is a collaboration between MIT and Harvard-Smithsonian Center for Astrophysics
as the primary contributors. The mission was not selected in NASA
's most recent Small Explorer program
.
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
is an extremely faint light source compared to its parent star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those
reasons, only a very few extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
s have been observed directly.
Instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. At the present time, several different indirect methods have yielded success.
Radial velocity
A star with a planet will move in its own small orbit in response to the planet's gravity. This leads to variations in the speed with which the star moves toward or away from Earth, i.e. the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's spectral lineSpectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s due to the Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
. The radial velocity method measures these variations in order to confirm the presence of the planet.
The velocity of the star around the center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
is much smaller than that of the planet, because the radius of its orbit around the center of mass is so small. Velocity variations down to 1 m/s can be detected with modern spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
s, such as the HARPS (High Accuracy Radial Velocity Planet Searcher
High Accuracy Radial Velocity Planet Searcher
The High Accuracy Radial velocity Planet Searcher is a high-precision echelle spectrograph installed in 2002 on ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003...
) spectrometer at the ESO
ESO
ESO, as a three-letter abbreviation, may stand for:* European Southern Observatory* Ensemble Studios Online* English Symphony Orchestra* Edmonton Symphony Orchestra* Executive Stock Options...
3.6 meter telescope in La Silla Observatory
La Silla Observatory
La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory organisation, and several others are partly maintained by ESO...
, Chile, or the HIRES spectrometer at the Keck telescopes
Keck telescopes
The W. M. Keck Observatory is a two-telescope astronomical observatory at an elevation of near the summit of Mauna Kea in Hawai'i. The primary mirrors of each of the two telescopes are in diameter, making them the second largest optical telescopes in the world, slightly behind the Gran Telescopio...
.
An especially simple and inexpensive method for measuring radial velocity is "externally dispersed interferometry".
This has been by far the most productive technique used by planet hunters. It is also known as Doppler spectroscopy. The method is distance independent, but requires high signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
s to achieve high precision, and so is generally only used for relatively nearby stars out to about 160 light-years from Earth. It easily finds massive planets that are close to stars, but detection of those orbiting at great distances requires many years of observation. Planets with orbits highly inclined to the line of sight from Earth produce smaller wobbles, and are thus more difficult to detect. One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's minimum mass. The posterior distribution of the inclination angle depends on the true mass distribution of the planets.
The radial-velocity method can be used to confirm findings made by using the transit method. When both methods are used in combination, then the planet's true mass can be estimated.
Pulsar timing
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
is a neutron star: the small, ultradense remnant of a star that has exploded as a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
. Pulsars emit radio waves extremely regularly as they rotate. Because the intrinsic rotation of a pulsar is so regular, slight anomalies in the timing of its observed radio pulses can be used to track the pulsar's motion. Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulse-timing observations can then reveal the parameters of that orbit.
This method was not originally designed for the detection of planets, but is so sensitive that it is capable of detecting planets far smaller than any other method can, down to less than a tenth the mass of Earth. It is also capable of detecting mutual gravitational perturbations between the various members of a planetary system, thereby revealing further information about those planets and their orbital parameters.
The main drawback of the pulsar-timing method is that pulsars are relatively rare, so it is unlikely that a large number of planets will be found this way. Also, life as we know it could not survive on planets orbiting pulsars since high-energy radiation there is extremely intense.
In 1992 Aleksander Wolszczan
Aleksander Wolszczan
Aleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...
and Dale Frail
Dale Frail
Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...
used this method to discover planets around the pulsar PSR 1257+12. Their discovery was quickly confirmed, making it the first confirmation of planets outside our 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...
.
Transit method
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
method can determine the radius of a planet. If a planet crosses (transits
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
) in front of its parent star's disk, then the observed visual brightness of the star drops a small amount. The amount the star dims depends on the relative sizes of the star and the planet. For example, in the case of HD 209458
HD 209458
HD 209458 is an 8th magnitude star in the constellation Pegasus. It is very similar to our Sun, and it is classified as a yellow dwarf . Because it is located at a distance of about 150 light years, it is not visible to the unaided eye...
, the star dims 1.7%.
This method has two major disadvantages. First of all, planetary transits are only observable for planets whose orbits happen to be perfectly aligned from the astronomers' vantage point. The probability of a planetary orbital plane being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit. About 10% of planets with small orbits have such alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a sun-sized star at 1 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, the probability of a random alignment producing a transit is 0.47% However, by scanning large areas of the sky containing thousands or even hundreds of thousands of stars at once, transit surveys can in principle find extrasolar planets at a rate that could potentially exceed that of the radial-velocity method, although it would not answer the question of whether any particular star is host to planets.
Secondly, the method suffers from a high rate of false detections. A transit detection requires additional confirmation, typically from the radial-velocity method.
The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet. By studying the high-resolution stellar spectrum carefully, one can detect elements present in the planet's atmosphere. A planetary atmosphere (and planet for that matter) could also be detected by measuring the polarisation of the starlight as it passed through or is reflected off the planet's atmosphere.
Additionally, the secondary eclipse (when the planet is blocked by its star) allows direct measurement of the planet's radiation. If the star's photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
intensity during the secondary eclipse is subtracted from its intensity before or after, only the signal caused by the planet remains. It is then possible to measure the planet's temperature and even to detect possible signs of cloud formations on it. In March 2005, two groups of scientists carried out measurements using this technique with the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
. The two teams, from the Harvard-Smithsonian Center for Astrophysics
Harvard-Smithsonian Center for Astrophysics
The Harvard–Smithsonian Center for Astrophysics is one of the largest and most diverse astrophysical institutions in the world, where scientists carry out a broad program of research in astronomy, astrophysics, earth and space sciences, and science education...
, led by David Charbonneau
David Charbonneau
David Charbonneau is a Professor of Astronomy at Harvard University. His research focuses on the development of novel techniques for the detection and characterization of planets orbiting nearby, Sun-like stars...
, and the Goddard Space Flight Center
Goddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...
, led by L. D. Deming, studied the planets TrES-1
TrES-1
TrES-1b is an extrasolar planet approximately 512 light-years away in the constellation of Lyra . The planet's mass and radius indicate that it is a Jovian planet with a similar bulk composition to Jupiter...
and HD 209458b respectively. The measurements revealed the planets' temperatures: 1,060 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...
(790°C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
) for TrES-1 and about 1,130 K (860°C) for HD 209458b.
In addition the hot Neptune Gliese 436 b
Gliese 436 b
Gliese 436 b is a Neptune-sized extrasolar planet orbiting the red dwarf star Gliese 436. It was among the smallest known transiting planets in mass and radius until the much smaller Kepler discoveries started coming in 2010.-Discovery:...
enters secondary eclipse. However some transiting planets orbit such that they do not enter secondary eclipse relative to Earth; HD 17156 b
HD 17156 b
HD 17156 b is an extrasolar planet approximately 255 light-years away in the constellation of Cassiopeia. The planet was discovered orbiting the yellow subgiant star HD 17156 in April 2007. The planet is classified as a relatively cool hot Jupiter planet slightly smaller than Jupiter but slightly...
is over 90% likely to be one of the latter.
A French Space Agency
CNES
The is the French government space agency . Established under President Charles de Gaulle in 1961, its headquarters are located in central Paris and it is under the supervision of the French Ministries of Defence and Research...
mission, COROT
Corot
Corot may refer to:* Jean-Baptiste-Camille Corot, French landscape painter * COROT, a space mission with the dual aims of finding extrasolar planets and performing asteroseismology* COROT-7, a dwarf star in the Monoceros constellation...
, began in 2006 to search for planetary transits from orbit, where the absence of atmospheric scintillation
Scintillation (astronomy)
Scintillation or twinkling are generic terms for rapid variations in apparent brightness or color of a distant luminous object viewed through a medium, most commonly the atmosphere ....
allows improved accuracy. This mission was designed to be able to detect planets "a few times to several times larger than Earth" and is currently performing "better than expected", with two exoplanet discoveries (both "hot jupiter" type) as of early 2008. The 17th CoRoT exoplanet was announced in 2010.
In March 2009, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
mission Kepler
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
was launched to scan a large number of stars in the constellation Cygnus
Cygnus (constellation)
Cygnus is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...
with a measurement precision expected to detect and characterize Earth-sized planets. The NASA Kepler Mission
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
uses the transit method to scan a hundred thousand stars in the constellation Cygnus for planets. Kepler will be sensitive enough to detect planets even smaller than Earth. By scanning a hundred thousand stars simultaneously, it will not only be able to detect Earth-sized planets, it will be able to collect statistics on the numbers of such planets around sunlike stars.
Kepler has already been able to detect the light from a known transiting extrasolar gas giant, HAT-P-7b
HAT-P-7b
HAT-P-7b is an extrasolar planet discovered in 2008. It orbits very close to its parent star GSC 03547-01402, and is both larger and more massive than Jupiter. Due to the extreme heat it receives from its star, the dayside temperature is predicted to be 2730 K...
. It is expected that Kepler will even be able to detect light from non-transiting gas giants on close orbits, though it will not be able to resolve that light into an image. Instead, the brightness of the host star seems to change gradually over time in a periodic manner, because like the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, the planet goes through phases
Planetary phase
Planetary phase is the term used to describe the appearance of the illuminated section of a planet. Like lunar phases, the planetary phase depends on the relative position of the sun, the planet and the observer....
from full to new and back again. The variation, although small, will be the signature of a planet. In addition to the reflected light from the star, some of the light from the planet will be thermally emitted by the planet itself. Thus the shape of the phase curve constrains the composition of the atmosphere based on the reflectivity of the planet, and also gives indications of the transport of heat on the planet from the day side to the night. This planet phase variation method may actually provide the greatest number of planets to be discovered by the Kepler satellite, since it does not require the planet to pass in front of the disk of the star.
On February 2, 2011, the Kepler team released a list of 1,235 extrasolar planet candidates, including 54 that may be in the 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...
.
Transit timing variation method (TTV) and transit duration variation method (TDV)
If a planet has been detected by the transit method, then variations in the timing of the transit provide an extremely sensitive method which is capable of detecting additional planets in the system with sizes potentially as small as Earth-sized planets. The first significant detection of a non-transiting planet using TTV was carried out with NASA's Kepler Space Telescope. The transiting planet Kepler-19bKepler-19b
Kepler-19b is a planet in the orbit of star Kepler-19. The planet has an orbital period of 9.3 days, with an estimated radius of roughly 2.2 Earth radii, and mass less than 20.3 Earth masses .-References:*http://arxiv.org/pdf/1109.1561v1...
shows TTV with an amplitude of 5 minutes and a period of about 300 days, indicating the presence of a second planet, Kepler-19c
Kepler-19c
-Discovery:The planet was discovered as a result of examinations of data from the previously discovered exoplanet : Kepler-19b. Timing variations in the orbital period of the first planet necessitated gravitational forces to be acting upon the planet, resulting from an additional body in the...
, which has a period which is a near rational multiple of the period of the transiting planet
"Timing variation" asks whether the transit occurs with strict periodicity or if there's a variation. "Duration variation" asks how long the transit takes. Duration variations may be caused by an exomoon.
Gravitational microlensing
Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned. Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years.If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing contributions at a reasonable rate. This method is most fruitful for planets between Earth and the center of the galaxy, as the galactic center provides a large number of background stars.
In 1991, astronomers Shude Mao and Bohdan Paczyński
Bohdan Paczynski
Bohdan Paczyński or Bohdan Paczynski was a Polish astronomer, a leading scientist in theory of the evolution of stars, accretion discs and gamma ray bursts....
of Princeton University
Princeton University
Princeton University is a private research university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League, and is one of the nine Colonial Colleges founded before the American Revolution....
first proposed using gravitational microlensing to look for exoplanets. Successes with the method date back to 2002, when a group of Polish astronomers (Andrzej Udalski
Andrzej Udalski
Professor Andrzej Udalski, PhD is a Polish astronomer and director of the Astronomical Observatory of the University of Warsaw...
, Marcin Kubiak
Marcin Kubiak
Prof Marcin Antoni Kubiak, PhD - an Polish astrophysicist, obtained his professorship title in 25 April 1994. Member of Committee of Astronomy of the Polish Academy of Sciences, a member of the Optical Gravitational Lensing Experiment , co-discoverer of many extrasolar planetary systems...
and Michał Szymański from Warsaw
Warsaw
Warsaw is the capital and largest city of Poland. It is located on the Vistula River, roughly from the Baltic Sea and from the Carpathian Mountains. Its population in 2010 was estimated at 1,716,855 residents with a greater metropolitan area of 2,631,902 residents, making Warsaw the 10th most...
, and Bohdan Paczyński
Bohdan Paczynski
Bohdan Paczyński or Bohdan Paczynski was a Polish astronomer, a leading scientist in theory of the evolution of stars, accretion discs and gamma ray bursts....
) during project OGLE (the Optical Gravitational Lensing Experiment
Optical Gravitational Lensing Experiment
The Optical Gravitational Lensing Experiment or OGLE is a Polish astronomical project based at the University of Warsaw that is chiefly concerned with discovering dark matter using the microlensing technique. Since the project began in 1992, it has discovered several extrasolar planets as a side...
) developed a workable technique. During one month they found several possible planets, though limitations in the observations prevented clear confirmation. Since then, four confirmed extrasolar planets have been detected using microlensing. this was the only method capable of detecting planets of Earthlike mass around ordinary main-sequence stars.
A notable disadvantage of the method is that the lensing cannot be repeated because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations with other methods are usually impossible. However, if enough background stars can be observed with enough accuracy then the method should eventually reveal how common earth-like planets are in the galaxy.
Observations are usually performed using networks of robotic telescope
Robotic telescope
A robotic telescope is an astronomical telescope and detector system that makes observations without the intervention of a human. In astronomical disciplines, a telescope qualifies as robotic if it makes those observations without being operated by a human, even if a human has to initiate the...
s. In addition to the NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
/National Science Foundation
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...
-funded OGLE, the Microlensing Observations in Astrophysics
Microlensing Observations in Astrophysics
Microlensing Observations in Astrophysics is a collaborative project between researchers in New Zealand and Japan, led by Professor Yasushi Muraki of Nagoya University. They use microlensing to observe dark matter, extra-solar planets, and stellar atmospheres from the Southern Hemisphere...
(MOA) group is working to perfect this approach.
The PLANET (Probing Lensing Anomalies NETwork
Probing Lensing Anomalies Network
The Probing Lensing Anomalies NETwork collaboration coordinates a network of telescopes to rapidly sample photometric measurements of the magnification of stars in the galactic bulge undergoing gravitational microlensing by intervening foreground stars...
)/RoboNet project is even more ambitious. It allows nearly continuous round-the-clock coverage by a world-spanning telescope network, providing the opportunity to pick up microlensing contributions from planets with masses as low as Earth. This strategy was successful in detecting the first low-mass planet on a wide orbit, designated OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb is a 'super-Earth' extrasolar planet orbiting the star OGLE-2005-BLG-390L, which is situated 21,500 ± 3,300 light years away from Earth, near the center of the Milky Way galaxy...
.
Direct imaging
As mentioned previously, planets are extremely faint light sources compared to stars and what little light comes from them tends to be lost in the glare from their parent star. So in general, it is very difficult to detect them directly.Some projects to equip telescopes with planet-imaging-capable instruments include: Gemini telescope (GPI
Gemini Planet Imager
The Gemini Planet Imager is a high contrast imaging instrument being built for the Gemini South Telescope in Chile. The instrument will achieve high contrast at small angular separations, allowing for the direct imaging and integral field spectroscopy of extrasolar planets around nearby stars...
), the VLT
VLT
VLT may stand for:* Very Large Telescope, a system of four large optical telescopes organized in an array formation, located in northern Chile...
(SPHERE
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
), and the Subaru telescope (HiCiao).
Up until the year 2010, telescopes could only directly image exoplanets under exceptional circumstances. Specifically, it is easier to obtain images when the planet is especially large (considerably larger than Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
), widely separated from its parent star, and hot so that it emits intense infrared radiation. However in 2010 a team from NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
s Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
demonstrated that a vortex
Optical vortex
An optical vortex is a zero of an optical field, a point of zero intensity. Research into the properties of vortices has thrived since a comprehensive paper by Nye and Berry, in 1974, described the basic properties of "dislocations in wave trains"...
coronagraph
Coronagraph
A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved...
could enable small scopes to directly image planets. They did this by imaging the previously imaged HR 8799
HR 8799
HR 8799 is a young main sequence star located 129 light years away from Earth in the constellation of Pegasus, with roughly 1.5 times the Sun's mass and 4.9 times its luminosity. It is part of a system that also contains a debris disk and at least four massive planets...
planets using just a 1.5 m portion of the Hale Telescope
Hale telescope
The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
.
Another promising approach is nulling interferometry.
Beta Pictoris
Beta Pictoris is the second brightest star in the constellation Pictor. It is located 63.4 light years from our solar system, and is 1.75 times as massive and 8.7 times as luminous as the Sun. The Beta Pictoris system is very young, only 8–20 million years old, although it is already in the main...
which in 2009 was observed to have moved to the other side of the star.
In July 2004, a group of astronomers used the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
's Very Large Telescope
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...
array in Chile to produce an image of 2M1207b
2M1207b
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth...
, a companion to the brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
2M1207. In December 2005, the planetary status of the companion was confirmed.
The planet is believed to be several times more massive than Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and to have an orbital radius greater than 40 AU.
In September 2008, an object was imaged at a separation of 330AU from the star 1RXS J160929.1−210524, but it was not until 2010 that it was confirmed to be a companion planet to the star and not just a chance alignment.
The first multiplanet system, announced on 13 November 2008, was imaged in 2007 using telescopes at both Keck Observatory and Gemini Observatory
Gemini Observatory
The Gemini Observatory is an astronomical observatory consisting of two telescopes at sites in Hawai‘i and Chile. Together, the twin Gemini telescopes provide almost complete coverage of both the northern and southern skies...
. Three planets were directly observed orbiting HR 8799
HR 8799
HR 8799 is a young main sequence star located 129 light years away from Earth in the constellation of Pegasus, with roughly 1.5 times the Sun's mass and 4.9 times its luminosity. It is part of a system that also contains a debris disk and at least four massive planets...
, whose masses are approximately 10, 10 and 7 times that of Jupiter
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....
. On the same day, 13 November 2008, it was announced that the Hubble Space Telescope directly observed an exoplanet orbiting Fomalhaut
Fomalhaut
Fomalhaut is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. Fomalhaut can be seen low in the southern sky in the northern hemisphere in fall and early winter evenings. Near latitude 50˚N, it sets around the time Sirius rises, and does not...
with mass no more than 3MJ. Both systems are surrounded by disks not unlike the 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...
. An additional system, GJ 758
GJ 758
GJ 758 is a G-type main sequence star located approximately 50 light years away from the Earth, in the constellation Lyra. At about magnitude 6 it is a little too faint to be seen with the naked eye but can be easily seen through a small telescope or binoculars.- System :In November 2009, a team...
, was imaged in November 2009, by a team using the HiCIAO instrument of the Subaru Telescope
Subaru (telescope)
Subaru Telescope is the 8.2 metre flagship telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades...
but it was a brown dwarf.
Other possible exoplanets to have been directly imaged: GQ Lupi b
GQ Lupi b
GQ Lupi b is a possible extrasolar planet orbiting the star GQ Lupi. Its discovery was announced in April 2005. Along with 2M1207b, this was one of the first extrasolar planet candidates to be directly imaged...
, AB Pictoris b, and SCR 1845 b. As of March 2006 none have been confirmed as planets; instead, they might themselves be small brown dwarfs.
Astrometry
Barycentric coordinates (astronomy)
In astronomy, barycentric coordinates are non-rotating coordinates with origin at the center of mass of two or more bodies.The barycenter is the point between two objects where they balance each other. For example, it is the center of mass where two or more celestial bodies orbit each other...
), as explained by solutions to the two-body problem
Two-body problem
In classical mechanics, the two-body problem is to determine the motion of two point particles that interact only with each other. Common examples include a satellite orbiting a planet, a planet orbiting a star, two stars orbiting each other , and a classical electron orbiting an atomic nucleus In...
. Since the star is much more massive, its orbit will be much smaller. Frequently, the mutual center of mass will lie within the radius of the larger body.
Astrometry is the oldest search method for extrasolar planets and originally popular because of its success in characterizing astrometric binary star systems. It dates back at least to statements made by William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...
in the late 18th century. He claimed that an unseen companion was affecting the position of the star he cataloged as 70 Ophiuchi
70 Ophiuchi
70 Ophiuchi a binary star system located 16.6 light years away from the Earth. It is in the constellation Ophiuchus. At magnitude 4 it is a typical less bright star usually visible to the unaided eye away from city lights.-Binary star:...
. The first known formal astrometric calculation for an extrasolar planet was made by W. S. Jacob in 1855 for this star. Similar calculations were repeated by others for another half-century until finally refuted in the early 20th century.
For two centuries claims circulated of the discovery of unseen companions in orbit around nearby star systems that all were reportedly found using this method, culminating in the prominent 1996 announcement of multiple planets orbiting the nearby star Lalande 21185
Lalande 21185
Lalande 21185 is a red dwarf star in the constellation of Ursa Major. Although relatively close by, it is only magnitude 7 in visible light and thus is too dim to see with the unaided eye...
by George Gatewood. None of these claims survived scrutiny by other astronomers, and the technique fell into disrepute. Unfortunately, the changes in stellar position are so small and atmospheric and systematic distortions so large that even the best ground-based telescopes cannot produce precise enough measurements. All claims of a planetary companion of less than 0.1 solar mass, as the mass of the planet, made before 1996 using this method are likely spurious. In 2002, 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...
did succeed in using astrometry to characterize a previously discovered planet around the star Gliese 876
Gliese 876
Gliese 876 is a red dwarf star approximately 15 light-years away from Earth in the constellation of Aquarius. As of 2011, it has been confirmed that four extrasolar planets orbit the star...
.
Future space-based observatories such as NASA's 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...
may succeed in uncovering new planets via astrometry, but for the time being no planet detected by astrometry has been confirmed.
One potential advantage of the astrometric method is that it is most sensitive to planets with large orbits. This makes it complementary to other methods that are most sensitive to planets with small orbits. However, very long observation times will be required — years, and possibly decades, as planets far enough from their star to allow detection via astrometry also take a long time to complete an orbit.
In 2009 the discovery of VB 10b by astrometry was announced. This planetary object was reported to have a mass 7 times that of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and orbiting the nearby low mass red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
star VB 10
VB 10
VB 10, also referred to as Van Biesbroeck's star, is a very small and very dim M-type red dwarf star located in the constellation Aquila...
. If confirmed, this would be the first exoplanet discovered by astrometry of the many that have been claimed through the years. However recent 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...
independent studies rule out the existence of the claimed planet.
Eclipsing binary minima timing
When a double starDouble star
In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e...
system is aligned such that - from the Earth's point of view - the stars pass in front of each other in their orbits, the system is called an "eclipsing binary" star system. The time of minimum light, when the star with the brighter surface area is at least partially obscured by the disc of the other star, is called the primary eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
, and approximately half an orbit later, the secondary eclipse occurs when the brighter surface area star obscures some portion of the other star. These times of minimum light, or central eclipse, constitute a time stamp on the system, much like the pulses from a pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
(except that rather than a flash, they are a dip in the brightness). If there is a planet in circum-binary orbit around the binary stars, the stars will be offset around a binary-planet center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
. As the stars in the binary are displaced by the planet back and forth, the times of the eclipse minima will vary; they will be too late, on time, too early, on time, too late, etc.. The periodicity of this offset may be the most reliable way to detect extrasolar planets around close binary systems.
Orbital phase reflected light variations
Short period giant planets in close orbits around their stars will undergo reflected light variations changes because, like the MoonMoon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, they will go through phases
Planetary phase
Planetary phase is the term used to describe the appearance of the illuminated section of a planet. Like lunar phases, the planetary phase depends on the relative position of the sun, the planet and the observer....
from full to new and back again. Although the effect is small — the photometric precision required is about the same as to detect an Earth-sized planet in transit across a solar-type star — such Jupiter-sized planets should be detectable by space telescopes such as the Kepler Space Observatory. This method may actually constitute the most planets that will be discovered by that mission because the reflected light variation with orbital phase is largely independent of orbital inclination of the planet's orbit. In addition, the phase function of the giant planet may be constrained which will, in turn, lead to constraints on the actual particle size distribution of the atmospheric particles.
Polarimetry
Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random. However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and they are polarized.By analyzing the polarization in the combined light of the planet and star (about one part in a million), these measurements can in principle be made with very high sensitivity, as polarimetry is not limited by the stability of the Earth's atmosphere.
Astronomical devices used for polarimetry, called polarimeters, are capable of detecting the polarized light and rejecting the unpolarized beams (starlight). Groups such as ZIMPOL/CHEOPS
ZIMPOL/CHEOPS
ZIMPOL/CHEOPS is a polarimetric imager for the detecting extrasolar planets. Based at the Very Large Telescope....
and PlanetPol
PlanetPol
PlanetPol is a ground based extrasolar planet search device. Based at William Herschel Telescope....
are currently using polarimeters to search for extra-solar planets, though no planets have yet been detected using this method.
Auroral radio emissions
AuroraAurora
Aurora most commonly refers to:*Aurora , a glow in the sky seen at polar latitudes*Aurora , the goddess of the dawn in Roman mythologyAurora may also refer to:-Literature:*Aurora , a superheroine in the Marvel Universe...
l radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
emissions from giant planets with plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
sources such as Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
's volcanic moon Io
IO
Io, IO, I/O, i/o, or i.o. may refer to:-An abbreviation:* I.O., a theater in Chicago, Illinois dedicated to improvisational comedy* i.o., "in illo ordine", Latin phrase meaning "respectively"...
could be detected with future radio telescopes such as LOFAR
LOFAR
LOFAR is the Low Frequency Array for radio astronomy, built by the Netherlands astronomical foundation ASTRON and operated by ASTRON's radio observatory....
.
Circumstellar disks
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...
s) surround many stars. The dust can be detected because it absorbs ordinary starlight and re-emits it as infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
radiation. Even if the dust particles have a total mass well less than that of Earth, they can still have a large enough total surface area that they outshine their parent star in infrared wavelengths.
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...
is capable of observing dust disks with its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument. Even better images have now been taken by its sister instrument, the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
, and by the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
's Herschel Space Observatory
Herschel Space Observatory
The Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of in diameter....
, which can see far deeper into infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
wavelengths than the Hubble can. Dust disks have now been found around more than 15% of nearby sunlike stars.
The dust is believed to be generated by collisions among comets and asteroids. Radiation pressure from the star will push the dust particles away into interstellar space over a relatively short timescale. Therefore, the detection of dust indicates continual replenishment by new collisions, and provides strong indirect evidence of the presence of small bodies like comets and 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 that orbit the parent star. For example, the dust disk around the star 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...
indicates that that star has a population of objects analogous to our own 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...
, but at least ten times thicker.
More speculatively, features in dust disks sometimes suggest the presence of full-sized planets. Some disks have a central cavity, meaning that they are really ring-shaped. The central cavity may be caused by a planet "clearing out" the dust inside its orbit. Other disks contain clumps that may be caused by the gravitational influence of a planet. Both these kinds of features are present in the dust disk around epsilon Eridani
Epsilon Eridani
Epsilon 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 , it has an apparent magnitude of 3.73...
, hinting at the presence of a planet with an orbital radius of around 40 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
(in addition to the inner planet detected through the radial-velocity method). These kinds of planet-disk interactions can be modeled numerically using collisional grooming techniques.
Contamination of stellar atmospheres
Recent spectral analysis of white dwarfWhite dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
s' atmospheres
Stellar atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. It is divided into several regions of distinct character:...
by Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
found contamination of heavier elements like magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
. These elements cannot originate from the stars' core and it is probable that the contamination comes from 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 that got too close (within the Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...
) to these stars by gravitational interaction with larger planets and were torn apart by star's tidal forces. Spitzer data suggests that 1-3% of the white dwarfs has similar contamination.
Future missions
Several space missions are planned that will employ already proven planet-detection methods. Astronomical measurements done from space can be more sensitive than measurements done from the ground, since the distorting effect of the Earth's atmosphere is removed, and the instruments can view in infrared wavelengths that do not penetrate the atmosphere. Some of these space probes should be capable of detecting planets similar to our own Earth.NASA's 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...
, now cancelled, will use astrometry. It may be able to detect Earth-like planets around several nearby stars. The European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
's Darwin
Darwin (ESA)
Darwin was a suggested ESA Cornerstone mission which would have involved a constellation of four to nine spacecraft designed to directly detect Earth-like planets orbiting nearby stars and search for evidence of life on these planets...
probe and NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Terrestrial Planet Finder
Terrestrial 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...
probes will attempt to image planets directly. A recently proposed idea is the New Worlds Mission, which will use an occulter to block a star's light, allowing astronomers to directly observe the dimmer orbiting planets.
Huge proposed ground telescopes may also be able to directly image extrasolar planets. ESO is planning to build the European Extremely Large Telescope, with a mirror diameter of 39.3 meters. Using a coronagraph together with extreme adaptive optics it's EPICS instrument will most likley be able to image earth sized planet's around nearby stars.
If it goes ahead sometime between 2025–2035, the proposed ATLAST
Advanced Technology Large-Aperture Space Telescope
The Advanced Technology Large-Aperture Space Telescope is an 8 to 16.8-meter UV-optical-NIR space telescope proposed by Space Telescope Science Institute, the science operations center for the Hubble Space Telescope...
telescope would be able to image small and rocky extrasolar planets either with it's internal coronagraph or with an external occulter.
The Transiting Exoplanet Survey Satellite
Transiting Exoplanet Survey Satellite
Transiting Exoplanet Survey Satellite is a proposed space telescope by MIT for NASA's Small Explorer program. It was not selected for development in the most recent Small Explorer phase A study round...
(TESS) is a space mission that will monitor the brightest and closest 2.5 million stars to Earth in order to detect rocky and water dominated planets using the transit method. TESS
Transiting Exoplanet Survey Satellite
Transiting Exoplanet Survey Satellite is a proposed space telescope by MIT for NASA's Small Explorer program. It was not selected for development in the most recent Small Explorer phase A study round...
will have the capability to identify the closest transiting rocky planets to Earth that orbit in the 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 their stellar host. TESS is a collaboration between MIT and Harvard-Smithsonian Center for Astrophysics
Harvard-Smithsonian Center for Astrophysics
The Harvard–Smithsonian Center for Astrophysics is one of the largest and most diverse astrophysical institutions in the world, where scientists carry out a broad program of research in astronomy, astrophysics, earth and space sciences, and science education...
as the primary contributors. The mission was not selected in NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's most recent Small Explorer program
Small Explorer program
The Small Explorer program is an effort within NASA to fund space exploration missions that cost no more than $120 million.- Program history :...
.
See also
- AstronomyAstronomyAstronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
- COROTCorotCorot may refer to:* Jean-Baptiste-Camille Corot, French landscape painter * COROT, a space mission with the dual aims of finding extrasolar planets and performing asteroseismology* COROT-7, a dwarf star in the Monoceros constellation...
- Extrasolar PlanetExtrasolar planetAn extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
- Extremely Large TelescopeExtremely Large TelescopeThe European Extremely Large Telescope is a ground-based extremely large telescope featuring an optical/near-infrared telescope of large size and advanced features that is being planned by the European Southern Observatory...
- Kepler MissionKepler MissionThe Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
- StarStarA star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
- Terrestrial Planet FinderTerrestrial Planet FinderThe 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...
- Solar SystemSolar SystemThe 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...