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

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.

An expanded ultraluminous X-ray source catalogue

“…We used the Fourth XMM-Newton Serendipitous Source Catalog, Ninth Data Release, (4XMM-DR9, Webb et al 2020) as the basis for our ULX catalogue. It lists 810 795 individual detections of 550 124 unique sources discovered across 11 204 observations, and it represents an increase of 177 396 sources with respect to 3XMM-DR4, the resource used in the previous ULX study based in XMM-Newton data (Earnshaw et al 2019). The columns we used during the construction are:…”

“…More recent ones have been based on XMM-Newton and Chandra data. XMM-Newtonbased catalogues have been highly successful, yielding up to ∼ 300 ULX candidates (Walton et al 2011;Earnshaw et al 2019), but they are somewhat hampered by the low angular resolution of the observatory. On the other hand, works based on the Chandra telescope have provided better identification of individual sources thanks to its crisp angular resolution (e.g.…”

“…Of particular interest to us are the works of Earnshaw et al (2019) and Kovlakas et al (2020), since they both constructed ULXs samples from the largest and most recent XMM-Newton and Chandra samples available at the time. Earnshaw et al (2019) is, in fact, our main predecessor and inspiration regarding methodology.…”

“…Of particular interest to us are the works of Earnshaw et al (2019) and Kovlakas et al (2020), since they both constructed ULXs samples from the largest and most recent XMM-Newton and Chandra samples available at the time. Earnshaw et al (2019) is, in fact, our main predecessor and inspiration regarding methodology. They identify 384 ULX candidates within the fourth XMM-Newton data release 1 (3XMM-DR4, Rosen et al 2016), using the Third Reference Catalogue of Bright Galaxies (RC3, de Vaucouleurs et al 1991) and the Catalogue of Neighbouring Galaxies (Karachentsev et al 2004) as references for the host galaxies.…”

Searching for Extragalactic Exoplanetary Systems: the Curious Case of BD+20 2457

“…The universality of the aforementioned features are yet to be tested; it will be achieved only with the study of exoplanets in external galaxies. Up to now, only a few planetary mass objects have been found in extragalactic systems through microlensing events (see Dai & Guerras 2018;Bhatiani et al 2019;Di Stefano et al 2020). This is mainly due to the current impossibility to use the traditional detection methods, radial velocity, and transit, in resolved stellar sources at large distances.…”