Epsilon Eridani

Epsilon Eridani (ε Eri, ε Eridani) is a star in the southern constellation Eridanus, along a declination 9.46° south of the celestial equator. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years (ly), it has an apparent magnitude of 3.73. It is the third closest of the individual stars or star systems visible to the unaided eye and was the closest star known to host a planet until the discovery of Alpha Centauri Bb. Its age is estimated at less than a billion years. Because of its youth, Epsilon Eridani has a higher level of magnetic activity than the present-day Sun, with a stellar wind 30 times as strong. Its rotation period is 11.2 days at the equator. Epsilon Eridani is smaller and less massive than the Sun, and has a comparatively lower level of elements heavier than helium. Astronomers categorize it as a main-sequence star of spectral class K2, which means that energy generated at the core through nuclear fusion of hydrogen is emitted from the surface at a temperature of about 5,000 K, giving the star an orange hue.

The motion of this star along the line of sight to the Earth, known as the radial velocity, has been regularly observed for more than twenty years. Periodic changes in this data yielded evidence of a giant planet orbiting Epsilon Eridani, making it one of the nearest extrasolar system with a candidate exoplanet. This object, Epsilon Eridani b, was formally announced in 2000 by a team of astronomers led by Artie Hatzes. Current data indicate that this planet orbits with a period of about 7 years at a mean separation of 3.4 astronomical units (AU), where 1 AU is the mean distance between the Earth and the Sun. Although this discovery has been controversial because of the amount of background noise in the radial velocity data, many astronomers now regard the planet as confirmed.

The system includes two belts of rocky asteroids: one at about 3 AU and a second at about 20 AU, whose structure may be maintained by a hypothetical second planet, Epsilon Eridani c. Epsilon Eridani harbors an extensive outer debris disk of remnant planetesimals left over from the system's formation.

The designation for this star was established in 1603 by Johann Bayer. It may be a member of the Ursa Major Moving Group of stars that share a similar motion through the Milky Way, implying these stars shared a common origin in an open cluster. Its nearest neighbor, the binary star system Luyten 726-8, will have a close encounter with Epsilon Eridani in approximately 31,500 years when they will be separated by about 0.93 ly. As one of the nearest Sun-like stars with the potential for a planet that may harbor life, Epsilon Eridani has been the target of SETI searches. The star appears in science fiction stories and has been suggested as a destination for interstellar travel.