55 Cancri

Star

55 Cancri (Copernicus) - a Sun-like star 41 light-years away in Cancer, circled by five planets: a family ranging from a scorching super-Earth to a cold gas giant. Its innermost world, 55 Cnc e, transits the star, so we know its size and density - a dense, lava-covered world barely twice Earth's width, orbiting in just 18 hours.

Open in 3D viewSee 55 Cancri in the live, interactive 3D map

Key facts

Mass

1.80 × 10³⁰ kg

0.905 M☉

Radius

656,045 km

94% of the Sun's radius

Surface temperature

5,172 K (4,899 °C)

Luminosity

2.43 × 10²⁶ W

Spectral type

K0V (Sun-like, slightly cooler)

Science

Physical

Mass: 1.80 × 10³⁰ kg · 0.905 M☉
Radius: 656,045 km · 94% of the Sun's radius
Density: 1,522 kg/m³ · 1.5× water; Computed from mass and radius
Spectral type: K0V (Sun-like, slightly cooler)

Motion through space

Proper motion: 0.539 arcseconds per year · its drift across our sky; SIMBAD — 2020yCat.1350....0G, 2018A&A...616A...7S
Radial velocity: +27.4 km/s · moving away from the Sun; SIMBAD — 2020yCat.1350....0G, 2018A&A...616A...7S
Speed across our line of sight: 32.2 km/s; Computed from proper motion and parallax
Speed through space, relative to the Sun: 42.2 km/s; Computed from proper motion, parallax and radial velocity

Rotation

Note: Rotates about once every 40 days; a mature, fairly quiet star.

Composition

By mass: A hydrogen/helium star a little cooler and less luminous than the Sun and notably metal-RICH, which may have helped build its five planets. It has a distant red-dwarf companion (55 Cancri B) far out at ~1,000 AU.

Temperature

Surface: 5,172 K (4,899 °C)

Radiation & temperature

Luminosity: 2.43 × 10²⁶ W · 0.635 L☉
Surface temperature: 5,172 K (4,899 °C)
Peak wavelength: 560 nm · green-yellow visible light; Computed (Wien's law) from the surface temperature

Discovery & history

Method: Telescope
Name: '55 Cancri' is a Flamsteed number in Cancer; the star's IAU name is Copernicus and its planets carry names of famous scientists and instrument-makers.

The first planet here (55 Cnc b) was found in 1996, among the earliest exoplanets; the count grew to five by 2007, one of the richest systems then known. In 2011 the inner planet e was caught transiting, turning it into one of the most-studied rocky exoplanets. The star and planet e were named Copernicus and Janssen in the 2015 IAU public campaign.

Images

Where to find it — in Cancer, the Crab (IAU constellation chart). To the eye it is a single point of light; no telescope has imaged its surface. — IAU / Sky & Telescope, R. Sinnott & R. Fienberg (CC BY 3.0) · Wikimedia Commons

Images from open archives · credit shown per image