A Giant Planet Around a Metal-Poor Star of Extragalactic Origin

Unlikely Planet Most known extrasolar planets orbit stars similar to the Sun. Very few planets have been detected around metal-poor stars whose abundances of elements other than hydrogen and helium are much lower than those of the Sun, or around stars that are at a late stage in their evolution. Setiawan et al. (p. 1642, published online 18 November) report the detection of a close-in giant planet around a metal-poor star belonging to a group of stars that formed in a satellite galaxy of the Milky Way. The star has gone past the red giant phase of stellar evolution, when stars like the Sun expand up to many times their original size, and so it is unclear why the planet was not engulfed by the star as it expanded. A planet is observed to orbit a star whose properties are different from those of all other known planet-hosting stars. Stars in their late stage of evolution, such as horizontal branch stars, are still largely unexplored for planets. We detected a planetary companion around HIP 13044, a very metal-poor star on the red horizontal branch, on the basis of radial velocity observations with a high-resolution spectrograph at the 2.2-meter Max-Planck Gesellschaft–European Southern Observatory telescope. The star’s periodic radial velocity variation of P = 16.2 days caused by the planet can be distinguished from the periods of the stellar activity indicators. The minimum mass of the planet is 1.25 times the mass of Jupiter and its orbital semimajor axis is 0.116 astronomical units. Because HIP 13044 belongs to a group of stars that have been accreted from a disrupted satellite galaxy of the Milky Way, the planet most likely has an extragalactic origin.