Artist’s impression of a binary system consisting of a blue supergiant and a black hole (right). The black hole’s gravity pulls matter away from its binary companion, generating a stream that is deposited on the accretion disk orbiting the black hole. The X-ray emission observed in the binary system M51-ULS-1, located in the galaxy Messier 51, originates from the infall of matter towards a black hole or a neutron star [ESA/Hubble, M. Kornmesser, L. L. Christensen]

M51-ULS-1b, the First Extragalactic Planet

Creatively using archival data from Chandra and XMM-Newton X-ray space telescopes, a group of researchers detected the transit of a planet in a binary system consisting of a blue supergiant and a neutron star (or black hole). All this in M51, the Whirlpool Galaxy, located 28 million light-years away from us

Are there extragalactic planets?

Until the 1990s, the only known planets were those of the Solar System. Astronomers were pretty sure that other stars have planetary systems, too, but they had no proof. The existence of planets orbiting stars other than the Sun remained a simple possibility, a probable but not proven fact. Then, in the mid-90s, starting with the discovery of 51 Pegasi b, a real flood of exoplanets came to fill the gap of knowledge that has accompanied humanity since the dawn of time. In just a quarter of a century, as many as 4,284 planets were found orbiting Milky Way stars.

But each new knowledge brings with it new questions. We know today that our galaxy is teeming with planets. What about other galaxies? Just as it was doubtful that only the Sun, in the Milky Way, had a…