A close-in giant planet escapes engulfment by its star

TL;DR

The discovery of a close-in giant planet orbiting a core-helium-burning red giant challenges previous assumptions, suggesting such planets can survive stellar expansion possibly due to stellar mergers or second-generation formation.

Contribution

This paper reports the first confirmed case of a close-in planet around a core-helium-burning red giant, indicating non-standard stellar evolution can allow planet survival.

Findings

8 Ursae Minoris b orbits at 0.5 au from its host star

The planet's orbit is nearly circular despite stellar expansion predictions

The system suggests possible stellar merger or second-generation planet formation

Abstract

When main-sequence stars expand into red giants, they are expected to engulf close-in planets. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars. Here we present the discovery that the giant planet 8 Ursae Minoris b orbits a core-helium-burning red giant. At a distance of only 0.5 au from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 au. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems.