55 Cancri d

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

55 Cancri d
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) 5.257 ± 0.9 AU
Eccentricity (e) 0.327 ± 0.28
Orbital period (P) 4517.4 ± 77.8 d
Inclination (i)  ?°
Longitude of
(ω) 234.73 ± 7°
Time of periastron (τ) 2,452,837.69 ± 68.87 JD
Physical characteristics
Mass (m) >3.92 ± 0.5 MJ
Radius (r)  ? RJ
Density (ρ)  ? kg/ m3
Temperature (T)  ? K
Discovery information
Discovery date 2002
Discoverer(s) Marcy et al.
Detection method Radial velocity
Discovery status Confirmed
Other designations
Rho1 Cancri d, HD 75732 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 fourth and outermost known planet in its planetary system. 55 Cancri d was discovered in 2002.


Like the majority of known extrasolar planets, 55 Cancri d 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. The same measurements also indicated the presence of another inner planet, designated 55 Cancri c.

Orbit and mass

When 55 Cancri d was discovered, it was thought to be on a fairly low eccentricity orbit similar to Jupiter in our solar system, though the orbital elements were not well determined. Further observations of the star refined the estimates, revealing that the planet's orbit was in fact highly eccentric, more so than any of the planets in our solar system (including Pluto). The planet is thus an example of an eccentric Jupiter.

A limitation of the radial velocity method used to discover 55 Cancri d is that only a lower limit on the planet's mass can be obtained. In the case of 55 Cancri d, this lower limit was around 4 times the mass of Jupiter. In 2004, astrometric measurements with the Fine Guidance Sensors on the Hubble Space Telescope suggest that the planet's orbit is inclined by around 53° with respect to the plane of the sky. If this measurement is confirmed, it implies that the planet's true mass is 25% greater than the lower limit, at around 4.9 Jupiter masses.


Given the planet's high mass, at least 3.9 times that of Jupiter, it is likely that the planet is a gas giant with no solid surface. Since the planet has only been detected indirectly, parameters such as its radius, composition and temperature are unknown. Assuming a composition similar to that of Jupiter and that the planet's atmosphere is close to chemical equilibrium, it is predicted that 55 Cancri d is covered in a layer of water clouds: the planet's internal heat probably keeps it too warm to form the ammonia-based clouds that are typical of Jupiter.

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