GJ 317: A Cool Dwarf and Its Two Known Worlds

GJ 317 is a star 49.6 light-years away. Its spectral type, M2.5 V, marks it as a small, cool red dwarf. It has a mass of 0.421 solar masses and a radius of 0.427 solar radii, both smaller than the Sun's, and a temperature of 3350 K, which is cooler than the Sun's. Two planets are known to orbit this host.

Both planets were found by the radial velocity method, which measures the wobble a planet's gravity induces in its star's motion; repeated wobbles reveal the planet and its orbital period. GJ 317 b was discovered in 2007 and has a minimum mass of 536 Earth masses, roughly 1.69 times Jupiter's mass, with an orbital period of 696 days. GJ 317 c was discovered in 2016 and has a minimum mass of 454 Earth masses, roughly 1.43 times Jupiter's mass, with an orbital period of 6500 days.

Among the listed planets, GJ 317 b has the shortest year, and GJ 317 c has the longest year, by reported estimates. GJ 317 b also has the largest minimum mass, while GJ 317 c has the smallest. As minimum masses, both values are lower bounds on the planets' true masses, and each is larger than Earth's.