Hiisi

A gas giant of the 'hot Jupiter' class, Hiisi is the only known planet of the star Horna or HAT-P-38, a relatively Sun-like yellow dwarf star. Horna and its planet Hiisi lie more than eight hundred light years from the Solar System within the constellation of Triangulum. Hiisi follows an orbit very close to its parent star, with a distance averaging just 0.05 AU (a little under eight million kilometres), placing it much closer to Horna than Mercury is to the Sun. This close and rapid orbit means that a 'year' on Hiisi lasts just four days and fifteen hours.

In terms of size and mass, Hiisi is somewhat comparable to Saturn, in that it is rather smaller than Jupiter and much less massive. This means that, like Saturn, the planet has a comparatively low density. Unlike Saturn, Hiisi's upper atmosphere appears to be relatively clear of dense cloud cover, and water vapour has been detected within that atmosphere.

Hiisi's name was assigned as part of the international NameExoWorlds project. Names for the star HAT-P-38 and its planet were chosen by the people of Finland, who gave the name Horna ('hell') to the star, and Hiisi to its planet. Hiisi is a name with a complex etymology, originally describing nature spirits and sacred places, though in combination with Horna it more specifically suggests evil spirits or devils.