55 Cancri is a binary star system 41 light-years from the Sun in the Cancer zodiac constellation. It has the same mass as our Sun but is colder and less brilliant. The star is more concentrated in elements heavier than helium than our sun: it is classified as a rare “super metal-rich” main sequence star with an iron abundance of roughly 186 percent that of the sun. The Bayer designation is Rho1 Cancri (ρ1 Cancri), while the Flamsteed designation is 55 Cancri. A K-type star plus a smaller red dwarf star make up the system.
As of 2015, five extrasolar planets are thought to circle 55 Cancri A (designated 55 Cancri b, c, d, e, and f; named Galileo, Brahe, Lipperhey, Janssen, and Harriot, respectively). Based on chromospheric activity, one estimate puts the age at roughly 5,500 million years.
The 55 Cancri system is rather close to our solar system: the Hipparcos astrometry satellite calculated the parallax of 55 Cancri A as 79.80 milliarcseconds, which corresponds to a distance of 12.5 parsecs. The apparent magnitude of 55 Cancri A is 5.95, making it viewable using binoculars. Under really dark skies, it can only be seen with the naked eye. The red dwarf 55 Cancri B has a magnitude of 13 and is only seen through a telescope.
The Flamsteed designation for the system is 55 Cancri. The Bayer identification 1 Cancri (Latinised to Rho1 Cancri) and the Bright Star Catalogue designation HR 3522 are also attached to it. The two halves are labeled A and B, while A is frequently referred to as 55 Cancri. The first planet identified orbiting 55 Cancri A was designated by its discoverers as HR 3522b, albeit it is more popularly known as 55 Cancri b. It should be designated 55 Cancri Ab according to the rules for naming objects in binary star systems, and this more formal form is occasionally used to avoid confusion with the secondary star 55 Cancri B. In order of discovery, the other planets discovered were designated 55 Cancri c, d, e, and f.
NameExoWorlds, a process for providing proper names to specific exoplanets and their host stars, was initiated by the International Astronomical Union in July 2014. The new names were chosen through public nomination and vote. The IAU stated in December 2015 that the winning names for 55 Cancri A were Copernicus and Galileo, Brahe, Lipperhey, Janssen, and Harriot for its planets (b, c, d, e, and f, respectively).
The 55 Cancri system is relatively close to the Solar System: the parallax of 55 Cancri A was determined by the Gaia astrometry satellite to be 79.45 milliarcseconds, equating to a distance of 12.6 parsecs (41 light-years). 55 Cancri A has an apparent magnitude of 5.95, making it visible to the human eye only in an extremely dark sky. The red dwarf 55 Cancri B has a magnitude of 13 and is only seen through a telescope. The two components are separated by a distance estimated to be 1,065 AU (6.15 light-days). Despite their great distance, the two stars appear to be gravitationally connected since they have the same proper motion.