HD 128311 and Its Two Planets

HD 128311 is a K0 star 53.3 light-years away with two planets, both found by the radial velocity method. That method measures the wobble a planet's gravity induces in its star's motion, and repeated wobbles reveal the planet and its orbital period. The star is 0.83 solar masses, 0.76 solar radii, and 4970 K, with an age of 6.4 billion years. Its mass and radius are smaller than the Sun's, and its temperature is cooler.

HD 128311 b was discovered in 2002, has a minimum mass of 562 Earth masses, and orbits every 453 days. Its minimum mass is larger than Earth's and roughly 1.77 times Jupiter's mass. HD 128311 c was discovered in 2005, has a mass of 1200 Earth masses, and orbits every 922 days. Its mass is larger than Earth's and roughly 3.79 times Jupiter's. Among the listed planets, b has the shortest year, at 453 days, and c the longest, at 922 days.