About this world
HD 21520 b is a sub-Neptune orbiting HD 21520, a yellow, Sun-like star 258 light-years from Earth. It is about 2.7 times the size of Earth and holds 18 Earth masses. It completes an orbit every 25.1 days at 0.17 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 637 K (364 °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 2024 by transit with Transiting Exoplanet Survey Satellite (TESS). A planet crosses its star and dims it by a tiny, periodic fraction.