About this world
K2-121 b is a Neptune-like planet orbiting K2-121, an orange dwarf 553 light-years from Earth. It is about 0.64 times the size of Jupiter and holds 0.16 Jupiter masses. It completes an orbit every 5.19 days at 0.052 AU from its star, closer than Mercury is to the Sun, on a strongly elongated orbit (e = 0.22).
Its equilibrium temperature is about 716 K (443 °C): far too hot for liquid water on its surface. At its size it almost certainly keeps a deep envelope of hydrogen and helium.
It was discovered in 2017 by transit with K2. A planet crosses its star and dims it by a tiny, periodic fraction.