About this world
HD 219666 b is a Neptune-like planet orbiting HD 219666, a yellow, Sun-like star (type G5V) 307 light-years from Earth. It is about 4.9 times the size of Earth and holds 18 Earth masses. It completes an orbit every 6.03 days at 0.064 AU from its star, closer than Mercury is to the Sun.
Its equilibrium temperature is about 1,076 K (803 °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: 4 spectroscopic observations.
It was discovered in 2018 by transit with Transiting Exoplanet Survey Satellite (TESS). A planet crosses its star and dims it by a tiny, periodic fraction.