  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.
