|Extrasolar planet||List of extrasolar planets|
The stars around its sun are the three innermost planets.
|Star||55 Cancri A|
|Right ascension||(α)||08h 52m 35.8s|
|Declination||(δ)||+28° 19′ 51″|
|Semimajor axis||(a)||0.781 ± 0.007
|Eccentricity||(e)||0.2 ± 0.2|
|Orbital period||(P)||260.00 ± 1.1
|(ω)||181.1 ± 60°|
|Time of periastron||(T0)||2,450,080.9108 ± 1.1 JD|
|Semi-amplitude||(K)||4.879 ± 0.6 m/s|
|Minimum mass||(m sin i)||0.144 ± 0.04
(45.7 ± 12.7 M⊕)
|Discovery date||11 April 2005 (announced)
6 November 2007 (published)
|Discoverer(s)||announced by J. Wisdom
published by D. Fischer
|Detection method||Doppler spectroscopy|
|Discovery site||United States|
55 Cancri Af, Rho1 Cancri f, HD 75732 f
55 Cancri f, also referred to as Rho1 Cancri f, is an extrasolar planet approximately 41 light-years away from Earth in the constellation of Cancer (the Crab). 55 Cancri f is the fourth known planet (in order of distance) from the star 55 Cancri and the first planet to have been given the designation of "f". As of July 2009, this makes 55 Cancri the only planetary system outside the Solar System that is known to contain at least five planets.
The initial presentation of this planet occurred at a meeting of the American Astronomical Society in April 2005, however it was another two and a half years before the planet was to be published in a peer-reviewed journal. It is the first known planet outside our solar system to spend its entire orbit within what astronomers call the "habitable zone". Furthermore, its discovery made 55 Cancri the first star other than the Sun known to have five planets. As of September 2008, this is still the only planetary system outside the Solar System that is known to contain at least five planets.
55 Cancri f is located about 0.781 AU away from the star and takes 260 days to complete a full orbit. A limitation of the radial velocity method used to detect 55 Cancri f is that only a minimum mass can be obtained, in this case around 0.144 times that of Jupiter, or half the mass of Saturn. A Keplerian fit to the radial velocity data of 55 Cancri A indicates that the orbit is consistent with being circular, however changing the value in a range between 0 and 0.4 does not significantly alter the chi-square statistic of the fit, thus a representative eccentricity of 0.2±0.2 was assumed. In a Newtonian model which takes interactions between the planets into account, the eccentricity comes out as 0.0002, almost circular.
Astrometric observations made with the Hubble Space Telescope suggest that the outer planet 55 Cancri d is inclined at 53° with respect to the plane of the sky. If these measurements are confirmed and the system is assumed to be coplanar, the true mass of 55 Cancri f would therefore be about 25% greater than this lower limit, at around 0.18 Jupiter masses.
Since the planet was detected indirectly through observations of its star, properties such as its radius, composition and temperature are unknown. With a mass half that of Saturn, 55 Cancri f is likely to be a gas giant with no solid surface. It orbits in the so-called "habitable zone," which means that liquid water could exist on the surface of a possible moon.