HD 219134 b is a rocky super-Earth that, when it was found in 2015, was the closest known planet to cross in front of its star. HD 219134 is an orange dwarf star, a little smaller and cooler than the Sun, about 21 light-years away and bright enough to see without a telescope from a dark site. HD 219134 b orbits it every 3.09 days.
A team led by Fatemeh Motalebi found it with the HARPS-N spectrograph in the Canary Islands, using the radial velocity method, which measures the slight back-and-forth motion a planet's gravity gives its star. They then caught it crossing the star with NASA's Spitzer Space Telescope, which gave its size. With both mass and size known, they found it dense enough to be rocky. The same year a separate team led by Steven Vogt, using the Keck and Automated Planet Finder telescopes, reported six planets around the star. In 2017 a team led by Michaël Gillon found that HD 219134 c crosses the star too.
In 2019 a team led by Roxanne Ligi measured the star's size directly with the CHARA array of telescopes and revised the two transiting planets' radii downward, putting HD 219134 b at about 1.5 times Earth's. They found b more massive than c but possibly less dense, and suggested that heat from tidal flexing might have partly melted its interior. Earlier modelling by a team led by Caroline Dorn found that any atmosphere is unlikely to be dominated by hydrogen. At about 1,000 kelvin, its estimated equilibrium temperature, it is far too hot for liquid water. A 2026 study of the star suggests that its early X-ray output probably stripped the innermost planets of their original gas.