About this world
HR 8799 d is a gas giant orbiting HR 8799, a white A-type star (type A5V) 135 light-years from Earth. It is about 1.2 times the size of Jupiter and holds 9.4 Jupiter masses. It completes an orbit every 101 years at 24 AU from its star, on a strongly elongated orbit (e = 0.6).
Its equilibrium temperature is about 1,300 K (1,027 °C): far too hot for liquid water on its surface. At its size it almost certainly keeps a deep envelope of hydrogen and helium. Its atmosphere has already been studied: 10 spectroscopic observations.
It was discovered in 2008 by direct imaging with W. M. Keck Observatory. The planet itself is photographed, its star blocked by a coronagraph. HR 8799 has 4 known planets; the others are HR 8799 e, HR 8799 c and HR 8799 b.