Epsilon Indi C: a moon as massive as 70 Jupiters?

The star Epsilon Indi is just barely visible with the naked eye in the southern sky. It is at a distance of 11.8 light-years from the Earth and has approximately the same size as the Sun but is somewhat older. It has also turned the usual classification of star, planet, and moon upside down.

This is because what we see is only Epsilon Indi A, but the star is orbited by a brown dwarf with a mass of 75 Jupiters and this brown dwarf has, in turn, a companion with a mass of 70 Jupiters.

Thus, Epsilon Indi B should actually be called a “planet” and Epsilon Indi C would then be a “moon.” Brown dwarfs are stars that do not contain enough hydrogen to sustain fusion reaction. But the exact limit for sustaining fusion reaction is not known with certainty and Epsilon Indi could be the specimen that tips the scales.

Previously it was assumed that the transition had to occur at a mass of 70 to 73 Jupiter masses. With surface temperatures of 1200 and 850 Kelvin, respectively, Epsilon Indi B and C are definitely not stars; in any case, they don’t obtain any energy from nuclear fusion.

The star system Epsilon Indi consists of two brown dwarfs orbiting their common center of mass, and together, the pair orbit a star similar to the Sun (picture: Roberto Molar Candanosa and Sergio Dieterich, courtesy of the Carnegie Institution for Science)

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