page_title: 55 Cancri
section_title: Planetary system
hierarchical_section_title: 55 Cancri / Planetary system
context_page_description: 55 Cancri is a binary star system located 41 light-years away from the Sun in the zodiac constellation of Cancer. It has the Bayer designation Rho¹ Cancri; 55 Cancri is the Flamsteed designation. The system consists of a K-type star and a smaller red dwarf.
  As of 2015, five extrasolar planets are believed to orbit 55 Cancri A.
context_section_description: The 55 Cancri system was the first known to have four, and later five planets, and may possibly have more.  The innermost planet, e, transits 55 Cancri A as viewed from Earth. The next planet, b, is non-transiting but there is tentative evidence that it is surrounded by an extended atmosphere that does transit the star.
  In 1997, the discovery of a 51 Pegasi-like planet orbiting 55 Cancri A was announced, together with the planet of Tau Boötis and the inner planet of Upsilon Andromedae. The planet was discovered by measuring the star's radial velocity, which showed a periodicity of around 14.7 days corresponding to a planet at least 78% of the mass of Jupiter. These radial velocity measurements still showed a drift unaccounted-for by this planet, which could be explained by the gravitational influence of a more distant object.
  In 1998 the discovery of a possible dust disk around 55 Cancri A was announced. Calculations gave the disk radius at least 40 AU, similar to the Kuiper belt in the Solar System, with an inclination of 25° with respect to the plane of the sky. However, the discovery could not be verified and was later deemed to be spurious, caused instead by background galaxies.
  After making further radial velocity measurements, a planet orbiting at a distance of around 5 AU was announced in 2002. This planet received the designation 55 Cancri d. At the time of discovery, the planet was thought to be in an orbit of mild eccentricity (close to 0.1), but this value was increased by later measurements. Even after accounting for these two planets, a periodicity at 43 days remained, possibly due to a third planet. Measurements of the star suggested that this was close to the star's rotation period, which raised the possibility that the 43-day signal was caused by stellar activity. This possible planet received the designation 55 Cancri c.
  55 Cancri e was announced in 2004. With 8.3 Earth masses, it is a large super-Earth which was originally thought to have an orbital period of 2.8 days, though it was later found that this was an alias of its true period of 0.74 days by observations of e transiting in 2011. This planet was the first known instance of a fourth extrasolar planet in one system, and was the shortest-period planet until the discovery of PSR J1719-1438 b. The measurements that led to the discovery of this planet also confirmed the existence of 55 Cancri c.
  In 2005, Jack Wisdom combined three data sets and drew two distinct conclusions: that the 2.8 day planet was an alias and that there was a Neptune-scale planet with a period near 261 days. Fischer et al. (2008) reported new observations that they said confirmed the existence of the 2.8 day planet, as first reported by McArthur et al. (2004), and a 260-day Neptune-sized planet, as first reported by Wisdom (2005). However, Dawson and Fabrycky (2010) concluded that the 2.8 day planet was indeed an alias, as suggested by Wisdom (2005), and that the correct period was 0.7365 days.
  In 2007, Fisher et al. confirmed the existence of the 260 day planet proposed in 2005 by Wisdom. This planet, 55 Cancri f, was the first occurrence of a fifth extrasolar planet in one system. With a similar mass to c, it has a 260-day orbit, towards the inner edge of 55 Cancri A's habitable zone. The planet itself is not thought to be conducive to life, but hypothetical moons in principle could maintain at least microbial life.
  The planet e's eccentricity is poorly defined; varying values between 0 and 0.4 does not significantly improve the fit, so an eccentricity of 0.2 was assumed. Taking interactions between the planets into account results in a near-zero orbital eccentricity.
  Astrometric observations with the Hubble Space Telescope measured an inclination of 53° of the outer planet d, though this result relies on the precise orbital parameters which have been substantially revised since this was published. The observed transits of e suggest an orbit normal inclined within 9° to the line-of-sight, and a possible detection of the transit of an extended atmosphere around 55 Cancri b wou
caption: Comparison of the orbits of the inner planets of 55 Cancri A (black) with the planets of the Solar System.