Epsilon Eridani

**Epsilon Eridani**, formally named Ran, is a star in the southern constellation of Eridanus, at a declination of 9.46° south of the celestial equator. This star is located only about 10.5 light-years (ly) away in the northeastern part (03:32:55.84-09:27:29.74, ICRS 2000.0) of Constellation Eridanus, the River -- west of Rana (Delta Eridani) and northwest of Zaurak. Somewhat smaller and cooler than our own Sun, Sol, Epsilon Eridani is also less luminous. In Earth's night sky, however, it is clearly visible to the naked eye as the third closest star viewable without a telescope.

Epsilon Eridani is the closest star known to have a planet: and maybe two of them. It is the only system that has both precise Doppler data and long-baseline astrometric positional observations, which combined yield the orbital tilt and the true planetary mass. Given its closeness and the knowledge of where it is relative to its star, the planet around Epsilon Eridani may become the first ever actually imaged.

## Habitable Zone

The distance from Epsilon Eridani where an Earth-type rocky planet may have liquid water on its surface has been estimated to be between 0.47 and 0.91 AU (Jones and Sleep, 2003) -- between the orbital distances of Mercury and Earth in the Solar system. In that distance range from the star, such a planet would have an orbital period shorter an Earth year. According to alternative calculations performed for the NASA Star and Exoplanet Database, the inner edge of Epsilon Eridani's habitable zone could be slightly farther out from the star at around 0.507 AUs from the star, while the outer edge lies even farther out at around 1.005 AUs. Given the apparent youth of this star system, however, it is likely that only primitive, single-celled organisms like bacteria that can survive heavy meteorite or cometary bombardment would be likely to survive on any Earth-type planet that has cooled sufficiently to allow carbon-based lifeforms to develop unless silicon-based lifeform or another exotic flavor is commonplace. (For an illustrated discussion, see Christoph Kulmann's web page on the potential habitable zone around **Epsilon Eridani**)

## Planetary system

Previous studies indicate that Epsilon Eridani hosts a disk of planetary debris (can take the form of gas and dust as well as small rocky and icy bodies) and two planets, the Jupiter-mass planet Epsilon Eridani b (AEgir) and a low-mass planet candidate called Epsilon Eridani c.One of the two possible planets previously detected around Epsilon Eridani (planet "b" or "A" for the Spitzer team) was discovered in 2000 and is thought to be a Jupiter-class object in orbit around this nearby star at an average distance of 3.4 AUs (or almost three and half times Earth's orbital distance from the Sun), which is just outside the innermost asteroid belt. Although some researchers previously suggested that "Epsilon Eridani b" moves in an exaggerated ellipse ranging between 1 and 5 AUs around its host star, such an orbit woul cross and quickly disrupt and "clear out" the newly discovered innermost asteroid belt; hence, the Spitzer team of astronomers argues that planet b must have relatively circular orbit that keeps it just outside the inner asteroid belt, such as the eccentricity of e= 0.25 +/- 0.23 reported by Butler et al in 2006 (Backman et al, 2008).