WASP-78 b

WASP-78 b is the only known planet of the bright yellow F-type star WASP-78 in Eridanus, more than two thousand light years from the Sun. The star lies southwestward of Orion in the sky, but at twelfth magnitude is far too faint to be detected with the naked eye. Its planet WASP-78 b follows a very close orbit around the star at a distance of 0.03 AU (that is, less than a tenth of the distance of Mercury from the Sun).

WASP-78 b is a gas giant with a mass comparable to that of Jupiter, but with nearly twice the diameter, meaning that it has a very low density. Its proximity to its star means that it belongs to the class of planets known as 'hot Jupiters' (and indeed it is one of the hottest extrasolar planets known, with an estimated temperature of some 2,350 K). The nature of its orbit is such that it is thought unlikely that the planet formed in its current extreme proximity to its star, but rather it originated with a more distant and eccentric orbital path that gradually decayed to its current close location around WASP-78.