HD 219134 c is one of two rocky planets known to cross in front of HD 219134, an orange dwarf star about 21 light-years away. It orbits every 6.76 days, just outside its sibling HD 219134 b. Being so close to the Sun and transiting a star bright enough to see without a telescope makes both planets valuable for comparison with rocky planets in our own solar system.
It was first found in 2015 from the star's wobble, which reveals a planet's minimum mass but not its size. A team led by Fatemeh Motalebi detected it with the HARPS-N spectrograph, and a team led by Steven Vogt independently found it with telescopes in Hawaii and California. In 2017 a team led by Michaël Gillon used NASA's Spitzer Space Telescope to show that it also transits. With both its mass and size known, they found it to be rocky, and a 2019 study led by Roxanne Ligi, which measured the star directly, refined its radius to about 1.4 times Earth's.
Modelling by a team led by Caroline Dorn found that any atmosphere on HD 219134 c is unlikely to be rich in hydrogen, and that the planet can be entirely rocky. Ligi's team found it less massive than HD 219134 b but possibly denser. It receives over 60 times the light Earth gets from the Sun, according to the NASA Exoplanet Archive, with an equilibrium temperature of about 780 kelvin, so it is far too hot for liquid water. A 2026 study suggests the star's strong X-ray output early in its life likely stripped the inner planets of any original hydrogen envelope.