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HD 20781

A dwarf star in the eastern regions of Fornax, close to Dalim or Alpha Fornacis in the sky, though in fact lying almost twice as far from the Solar System at a distance of some 117 light years. HD 20781 lies on the boundary between the G-type yellow and the K-type orange spectral classifications, making it somewhat cooler than the Sun, though it is nonetheless a relatively Sun-like star in other respects.

HD 20781 is part of a binary system, but one in which the two component stars are very widely separated from one another (the other component is individually designated HD 20782). At present, the two stars are separated by a distance of more than nine thousand Astronomical Units, or about 0.15 light years. Unusually for a binary system, each of the component stars is known to have its own planetary system. HD 20782 has a single known gas giant in orbit, but HD 20781 has a more extensive system of planets.

All the planets in the HD 20781 system occupy very close orbits around their star, and even the most distant, HD 20781 c, orbits more closely than Mercury around the Sun. This outermost planet is a gas giant comparable to Neptune, as is its slightly less massive inner neighbour HD 20781 b. Two further planets are suspected to exist, probably massive terrestial worlds of the 'super Earth' type. The orbits of these two innermost planets would be so close to HD 20781 that their 'years' would take a matter of days to complete, but the existence of these super Earths has yet to be definitively confirmed.


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