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M51-ULS-1b is a giant extragalactic planet candidate orbiting the high-mass X-ray binary M51-ULS-1, located around 23,000,000 (23 million) light years (7,051,832 parsecs) from Earth in the Whirlpool Galaxy (also known as Messier 51).

If the planet is confirmed, it would be the first extragalactic planet and the furthest extrasolar planet ever found, far exceeding both SWEEPS-04 and SWEEPS-11, both located around 27,710 light years from Earth.


The planet, if it exists, would be a circumbinary planet, meaning it orbits both binary system members of M51-ULS-1, which consists of a stellar remnant (either a neutron star or a black hole), and a massive star, probably a blue supergiant. The planet is speculated to be slightly smaller in radius and mass than the planet Saturn, which would make it a gas giant. [2] The planet orbits at "tens of AUs"[1] from its stars. However, despite the great orbital distance, the planet would be heavily irradiated by ionizing radiation, and would receive irradiation comparable to that of a Hot Jupiter orbiting a Sun-like star at 0.05 astronomical units.

It is located in the Whirlpool Galaxy, also known as Messier 51. While the object very likely exists, it is not clear if it is a gas giant, a brown dwarf, or a low-mass star.

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