About this world
HD 191939 d is a sub-Neptune orbiting HD 191939, a yellow, Sun-like star (type G9V) 175 light-years from Earth. It is about 3 times the size of Earth and holds 2.8 Earth masses. It completes an orbit every 38.4 days at 0.21 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 540 K (267 °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 2020 by transit with Transiting Exoplanet Survey Satellite (TESS). A planet crosses its star and dims it by a tiny, periodic fraction. HD 191939 has 6 known planets; the others are HD 191939 b, HD 191939 c, HD 191939 e, HD 191939 g and HD 191939 f.