The visitor from an ancient galaxy: A planetary companion around an old, metal-poor red horizontal branch star

TL;DR

This paper reports the discovery of a planet orbiting an ancient, metal-poor star from a disrupted galaxy, marking a unique case of planetary presence in such an environment and evolutionary stage.

Contribution

It presents the first detection of a planet around a very metal-poor, evolved star possibly originating from an extragalactic source, expanding understanding of planet formation in early universe conditions.

Findings

Planet has a minimum mass of 1.25 Jupiter masses.

Orbital semimajor axis is 0.116 AU.

Host star is a metal-poor, evolved star from a disrupted galaxy.

Abstract

We report the detection of a planetary companion around HIP 13044, a metal-poor red horizontal branch star belonging to a stellar halo stream that results from the disruption of an ancient Milky Way satellite galaxy. The detection is based on radial velocity observations with FEROS at the 2.2-m MPG/ESO telescope. The periodic radial velocity variation of P=16.2 days can be distinguished from the periods of the stellar activity indicators. We computed a minimum planetary mass of 1.25 Jupiter masses and an orbital semimajor axis of 0.116 AU for the planet. This discovery is unique in three aspects: First, it is the first planet detection around a star with a metallicity much lower than few percent of the solar value; second, the planet host star resides in a stellar evolutionary stage that is still unexplored in the exoplanet surveys; third, the planetary system HIP 13044 most likely has an extragalactic origin in a disrupted former satellite of the Milky Way.