55 Cancri

55 Cancri is a binary star system 41 light-years from the Sun in the constellation Cancer. Its two stars are a K-type primary (55 Cancri A, also called Copernicus) and a smaller red dwarf companion (55 Cancri B). As of 2025, five confirmed exoplanets and one candidate orbit the primary, while two exoplanets orbit the secondary, making the system unusually rich in planets. The system goes by several designations: Flamsteed number 55 Cancri, Bayer designation Rho1 Cancri, and Bright Star Catalogue number HR 3522. The two components are labeled A and B, though the primary is often simply called 55 Cancri. The first planet found around the primary was initially called HR 3522b but is more commonly known as 55 Cancri Ab; the other planets are designated Ac, Ad, Ae, and Af in order of discovery. In 2014, the IAU launched a public naming process, and in December 2015 the winning names were announced: Copernicus for the primary star, and Galileo, Brahe, Lipperhey, Janssen, and Harriot for its planets Ab, Ac, Ad, Ae, and Af respectively. These names, submitted by the Royal Netherlands Association for Meteorology and Astronomy, honor Copernicus, Galileo, Tycho Brahe, Thomas Harriot, and telescope pioneers Hans Lipperhey and Zacharias Janssen. (The IAU initially announced "Lippershey" for 55 Cancri Ad, but corrected it to Lipperhey in January 2016 after recognizing the 19th-century error.) In July 2016, the IAU’s Working Group on Star Names formally recognized these names, and the star is now listed in the IAU Catalog of Star Names. The system is relatively close to the Solar System: Gaia measured the primary’s parallax at 79.45 milliarcseconds, giving a distance of 12.6 parsecs (41 light-years). 55 Cancri A has an apparent magnitude of 5.95, just visible to the naked eye under very dark skies. The red dwarf 55 Cancri B is magnitude 13 and requires a telescope. The two stars are separated by 85 arcseconds, corresponding to about 1,065 AU (6.15 light-days), and share a common proper motion, confirming they are gravitationally bound.

55 Cancri A is a K0IV-V star—either a main-sequence or subgiant star. It is slightly smaller, less massive, cooler, and less luminous than the Sun. Its chromosphere emits little in visible light but varies in X-rays. It is unusually metal-rich, with 186% of the Sun’s iron abundance, classifying it as a rare "super metal-rich" (SMR) star.