55 Cancri b

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

55 Cancri b
Extrasolar planet Lists of extrasolar planets
Parent star
Star 55 Cancri A
Constellation Cancer
Right ascension (α) 08h 52m 35.8s
Declination (δ) +28° 19′ 51″
Spectral type G8V
Orbital elements
Semimajor axis (a) 0.115 ± 0.003 AU
Eccentricity (e) 0.0197 ± 0.01
Orbital period (P) 14.67 ± 0.0006 d
Inclination (i)  ?°
Longitude of
(ω) 131.49 ± 33°
Time of periastron (τ) 2,453,021.08 ± 0.01 JD
Physical characteristics
Mass (m) >0.784 ± 0.09 MJ
Radius (r)  ? RJ
Density (ρ)  ? kg/ m3
Temperature (T)  ? K
Discovery information
Discovery date 1996
Discoverer(s) Butler, Marcy
Detection method Radial velocity
Discovery status Confirmed
Other designations
55 Cancri Ab, Rho1 Cancri b, HD 75732 b

55 Cancri b (occasionally referred to as 55 Cancri Ab in order to distinguish it from the star 55 Cancri B) is an extrasolar planet orbiting the Sun-like star 55 Cancri A every 14.7 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. Paul Butler, 55 Cancri b was the fourth known extrasolar planet, excluding pulsar planets.


Like the majority of known extrasolar planets, 55 Cancri b was discovered by detecting variations in its star's radial velocity caused by the planet's gravity. By making sensitive measurements of the Doppler shift of the spectrum of 55 Cancri A, a 15-day periodicity was detected. The planet was announced in 1996, together with the planet of Tau Boötis and the innermost planet of Upsilon Andromedae.

Even when this inner planet, with a mass at least 78% times that of Jupiter was accounted for, the star still showed a drift in its radial velocity. This eventually led to the discovery of the outer planet 55 Cancri d in 2002.

Orbit and mass

55 Cancri b is in a short-period orbit, though not so extreme as that of the previously-detected hot Jupiter 51 Pegasi b. The planet is in a 1:3 orbital resonance with the nearby planet 55 Cancri c.

A limitation of the radial velocity method used to discover the planet is that only a lower limit on the mass can be determined. Astrometric measurements made with the Hubble Space Telescope suggest that the outer planet is inclined at around 53° with respect to the plane of the sky. Assuming this measurement is correct and the planetary system is coplanar, the planet's true mass is 25% greater than the lower limit, at around 1 Jupiter mass.


Given the planet's high mass, it is likely that 55 Cancri b is a gas giant with no solid surface. Since the planet has only been detected indirectly, properties such as its radius, composition and temperature are unknown. Assuming a composition similar to that of Jupiter and that its environment is close to chemical equilibrium, 55 Cancri b is predicted to have a cloudless upper atmosphere with a spectrum dominated by alkali metal absorption.

The planet is unlikely to have large moons, since tidal forces would either eject them from orbit or destroy them on short timescales relative to the age of the system.

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