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55 Cancri c

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Background Information

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55 Cancri c
Extrasolar planet List of extrasolar planets
Parent star
Star 55 Cancri A
Constellation Cancer
Right ascension (α) 08h 52m 35.8s
Declination (δ) +28° 19′ 51″
Distance 40.9  ly
(12.5  pc)
Spectral type G8V
Orbital elements
Semimajor axis (a) 0.240 ± 0.005 AU
Eccentricity (e) 0.44 ± 0.08
Orbital period (P) 43.93 ± 0.021 d
Time of periastron (T0) 2,453,028.63 ± 0.25 JD
Semi-amplitude (K) 9.60 ± 0.86 m/s
Physical characteristics
Mass (m) >0.217 ± 0.04 MJ
Discovery information
Discovery date June 13, 2002
Discoverer(s) Marcy et al.
Detection method Radial velocity
Discovery site California, USA
Discovery status Published
Other designations
Rho1 Cancri c, HD 75732 c

55 Cancri c is an extrasolar planet in an eccentric orbit around the Sun-like star 55 Cancri A, making one revolution every 43.93 days. 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 comparable to that of Saturn.


Like the majority of known extrasolar planets, 55 Cancri c was detected by observing changes in its star's radial velocity. 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, labelled 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 hypotheseis, though 55 Cancri c remains the least certain of the system's three Jovian-mass planets.

Orbit and mass

The orbit of 55 Cancri c is highly eccentric: at apoapsis the planet is almost twice as far from its star as it is at periapsis. It is located closer to 55 Cancri A than Mercury is to our sun, though it has a longer orbital period than the hot Jupiters. The planet is in a 3:1 resonance with the inner planet 55 Cancri b, which may help to keep the system stable.

A limitation of the radial velocity method used to discover the planet is that only a lower limit on the mass can be obtained. However astrometric observations with the Hubble Space Telescope suggest that the outer planet 55 Cancri d is inclined at 53° to the plane of the sky. If this measurement is confirmed, and assuming the planetary system is coplanar, this would mean the true mass of 55 Cancri c is about 25% greater than this lower limit, at around 90% of the mass of Saturn.


Since the planet was detected indirectly through observations of its star, properties such as its radius, 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.

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