M51-ULS-1b: The First Candidate for a Planet in an External Galaxy

TL;DR

This paper reports the first candidate for a planet in an external galaxy, discovered through X-ray transit observations of a bright X-ray source in M51, suggesting a new method for extragalactic planet detection.

Contribution

It introduces a novel approach using X-ray eclipses to identify planets in external galaxies, demonstrating the first such candidate in M51.

Findings

First extragalactic planet candidate identified via X-ray eclipse

Planet candidate has a radius slightly smaller than Saturn

Method can be used to find more planets in external galaxies

Abstract

Do external galaxies host planetary systems? Many lines of reasoning suggest that the answer must be 'yes'. In the foreseeable future, however, the question cannot be answered by the methods most successful in our own Galaxy. We report on a different approach which focuses on bright X-ray sources (XRSs). M51-ULS-1b is the first planet candidate to be found because it produces a full, short-lived eclipse of a bright XRS. M51-ULS-1b has a most probable radius slightly smaller than Saturn. It orbits one of the brightest XRSs in the external galaxy M51, the Whirlpool Galaxy, located 8.6 Megaparsecs from Earth. It is the first candidate for a planet in an external galaxy. The binary it orbits, M51-ULS-1, is young and massive. One of the binary components is a stellar remnant, either a neutron star (NS) or black hole (BH), and the other is a massive star. X-ray transits can now be used to discover more planets in external galaxies and also planets orbiting XRSs inside the Milky Way.