Epsilon Eridani

From Growing Horizons
Epsilon Eridani
Location of ε Eridani (circled)
Spectral ClassK2V
Mass M0.82 M
Surface Temperature5084 K
Heim-Feynman Limit Distance4.8 AU
Distance to Sol10.52 ly

Epsilon Eridani (Latinized from ε 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 allows it to be visible from most of Earth's surface. At a distance of 10.5 light-years (3.2 parsecs) from the Sun, it has an apparent magnitude of 3.73. It is the third-closest individual star or star system visible to the unaided eye.

The star is estimated to be less than a billion years old. Because of its relative 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. It is 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 (8,500 °F), giving it an orange hue.

The Bayer designation ε Eridani (Latinised as Epsilon Eridani) 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 neighbour, 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 (0.29 pc).

The motion of Epsilon Eridani along the line of sight to Earth, known as the radial velocity, has been regularly observed for more than twenty years. Periodic changes in its value yielded evidence of a giant planet orbiting the star, making it one of the closest star systems with a candidate exoplanet. The discovery of the planet has been controversial because of the amount of background noise in the radial velocity data, particularly in the early observation, but many astronomers now regard the planet as confirmed. In 2016 it was given the alternative name AEgir.


Geophysical Data
Distance from Primary0.039 AU
Planet TypeGajan
Natural Satellites1
Mean Temperature13°C
Surface Water61%
Age.85 billion years
Demographics (2185)
  • 313.450 humans
  • 124.323 Quetzal
  • 28500 various