MEANWHILE, IN ANOTHER GALAXY
M51-ULS-1B DISCOVERED: 2020 DETECTION METHOD:
TRANSIT
Most of the exoplanets found so far are relatively nearby in cosmic terms – generally within about 3,000 light years. That’s still well within our Milky Way, but there must be countless exoplanets in other galaxies, too. The first potential extragalactic planet was discovered in 2020, lurking in the Whirlpool Galaxy (Messier 51) an incredible 23 million light years away. Called M51-ULS-1B, it was detected by the conventional transit method, but using X-ray observations rather than visible light. The planet orbits an X-ray binary system consisting of an ordinary Sun-like star and a much more compact object such as a neutron star or black hole. These systems are powerful emitters of X-rays, which is why they can be detected at enormous distances.