55 Cancri c
Encyclopedia
55 Cancri c is an extrasolar planet
in an eccentric orbit around the Sun
-like star
55 Cancri A
, making one revolution every 44.34 day
s. It is the third known planet in order of distance from its star. 55 Cancri c was discovered on June 13, 2002 and has a mass
roughly half of Saturn
.
. This was achieved by making sensitive measurements of the Doppler shift of the star's spectrum
. At the time of discovery, 55 Cancri A was already known to possess one planet (55 Cancri b
), however there was still a drift in the radial velocity measurements which was unaccounted-for.
In 2002, further measurements revealed the presence of a long-period planet in an orbit at around 5 AU
from the star. Even when both of the two planets were accounted for, there was still a periodicity at around 43 days. However this period is close to the rotation period of 55 Cancri A, which led to the possibility that the 43-day period was caused by stellar rotation rather than a planet. Both the 43-day planet (designated 55 Cancri c) and the 5 AU planet (designated 55 Cancri d
) were announced in the same paper, labeled in order of increasing distance from the star.
Further measurements which led to the discovery of the inner planet 55 Cancri e
in 2004 lent support to the planet hypothesis. Photometric
measurements of the star over 11 years show no activity with the same period as 55 Cancri c's radial velocity variations, and furthermore the period is stable over long timescales, which is inconsistent with the hypothesis of stellar activity causing the radial velocity variations. The amplitude of the radial velocity signal is inconsistent with stellar variations on stars with 55 Cancri A's low level of chromospheric
activity.
is to our sun, though it has a longer orbital period than the hot Jupiter
s. The planet is located close to a 3:1 resonance
with the inner planet 55 Cancri b; however, simulations indicate that the two planets are not actually in this resonance.
A limitation of the radial velocity method used to discover the planet is that only a lower limit on the mass
can be obtained. Further astrometric observations with the Hubble Space Telescope
on the outer planet 55 Cancri d, on the other hand, suggest that planet is inclined at 53° to the plane of the sky. The system is further expected to be coplanar. If all this is confirmed, this would mean the true mass
of 55 Cancri c is about 0.21 times that of Jupiter, or about two-thirds that of Saturn.
, composition, and temperature
are unknown. With a mass similar to that of Saturn, 55 Cancri c is likely to be a gas giant
with no solid surface.
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...
in an eccentric orbit around the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
-like 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...
55 Cancri A
55 Cancri
55 Cancri , also cataloged Rho1 Cancri or abbreviated 55 Cnc, is a binary star approximately 41 light-years away from Earth in the constellation of Cancer...
, making one revolution every 44.34 day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
s. It is the third known planet in order of distance from its star. 55 Cancri c was discovered on June 13, 2002 and has a mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
roughly half of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
.
Discovery
Like the majority of known extrasolar planets, 55 Cancri c was detected by observing changes in its star's radial velocityRadial 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...
. This was achieved by making sensitive measurements of the Doppler shift of the star's spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
. At the time of discovery, 55 Cancri A was already known to possess one planet (55 Cancri b
55 Cancri b
55 Cancri b is an extrasolar planet orbiting the Sun-like star 55 Cancri A every 14.65 days. It is the second planet in order of distance from its star, and is an example of a hot Jupiter. Discovered in 1996 by Geoffrey Marcy and R...
), however there was still a drift in the radial velocity measurements which was unaccounted-for.
In 2002, further measurements revealed the presence of a long-period planet in an orbit at around 5 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
from the star. Even when both of the two planets were accounted for, there was still a periodicity at around 43 days. However this period is close to the rotation period of 55 Cancri A, which led to the possibility that the 43-day period was caused by stellar rotation rather than a planet. Both the 43-day planet (designated 55 Cancri c) and the 5 AU planet (designated 55 Cancri d
55 Cancri d
55 Cancri d is an extrasolar planet in a long-period orbit around the Sun-like star 55 Cancri A. Located at a similar distance from its star as Jupiter is from our Sun, it is the fifth and outermost known planet in its planetary system...
) were announced in the same paper, labeled in order of increasing distance from the star.
Further measurements which led to the discovery of the inner planet 55 Cancri e
55 Cancri e
55 Cancri e is an extrasolar planet with half the mass of Neptune orbiting the Sun-like star 55 Cancri A. Its mass is about 8.3 Earth masses, thus classifying it as the first Super-Earth discovered around a main sequence star, predating Gliese 876 d by a year...
in 2004 lent support to the planet hypothesis. Photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
measurements of the star over 11 years show no activity with the same period as 55 Cancri c's radial velocity variations, and furthermore the period is stable over long timescales, which is inconsistent with the hypothesis of stellar activity causing the radial velocity variations. The amplitude of the radial velocity signal is inconsistent with stellar variations on stars with 55 Cancri A's low level of chromospheric
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
activity.
Orbit and mass
In the 5-planet solution for the 55 Cancri system, the orbit of 55 Cancri c is mildly eccentric: at apoastron the planet is about 19% further from the star as it is at periastron. It is located closer to 55 Cancri A than MercuryMercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
is to our sun, though it has a longer orbital period than the hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s. The planet is located close to a 3:1 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...
with the inner planet 55 Cancri b; however, simulations indicate that the two planets are not actually in this resonance.
A limitation of the radial velocity method used to discover the planet is that only a lower limit on the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
can be obtained. Further astrometric observations 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...
on the outer planet 55 Cancri d, on the other hand, suggest that planet is inclined at 53° to the plane of the sky. The system is further expected to be coplanar. If all this is confirmed, this would mean the true mass
True mass
The term true mass is synonymous with the term mass, but is used in astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained from radial velocity techniques...
of 55 Cancri c is about 0.21 times that of Jupiter, or about two-thirds that of Saturn.
Characteristics
Since the planet was detected indirectly through observations of its star, properties such as its radiusRadius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
, composition, and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
are unknown. With a mass similar to that of Saturn, 55 Cancri c is likely to be a gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
with no solid surface.