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Do external galaxies host planetary systems? Many lines of reasoning suggestthat the answer must be 'yes'. In the foreseeable future, however, the questioncannot be answered by the methods most successful in our own Galaxy. We reporton 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 afull, short-lived eclipse of a bright XRS. M51-ULS-1b has a most probableradius slightly smaller than Saturn. It orbits one of the brightest XRSs in theexternal 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 itorbits, M51-ULS-1, is young and massive. One of the binary components is astellar remnant, either a neutron star (NS) or black hole (BH), and the otheris a massive star. X-ray transits can now be used to discover more planets inexternal galaxies and also planets orbiting XRSs inside the Milky Way.

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