An extragalactic planet, also known as an extragalactic extrasolar planet, is a planet that exists outside of the Milky Way Galaxy. Because of the huge distance between these worlds, these planets are almost impossible to detect with current technology. Nevertheless, a few candidates have been found, and indirect evidence suggests they do in fact exist. The most distant confirmed extrasolar planets found as of February 2019 are SWEEPS-04 and SWEEPS-11, both located in the constellation of Sagittarius around 27,710 light years away from Earth. The Milky Way Galaxy is around 100,000-180,000 light years in diameter, so even planets beyond 27,710 light years have not been detected yet.
Candidates[]
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In 1996, a team at Harvard-Smithsonian Center for Astrophyics led by Rudy E. Schild discovered an anomalous fluctuation in one image's lightcurve. One controversial and uncomfirmable theory was that there may be a planet of approximately 3 Earth masses in the lensing galaxy, YGKOW G1. However, this theory is unprovable and can never be confirmed, because the chance alignment that led to its discovery will never happen again. The planet, if it exists, is approximately 4 billion light years away from Earth.
Andromeda Galaxy planet[]
In 1999, a team of scientists have used gravitational microlensing to come up with a tentative detection of an extragalactic planet in the Andromeda Galaxy, the closest spiral galaxy to the Milky Way. The lensing pattern fits a star with a smaller companion, PA-99-N2 b, which has a mass 6.34 times that of Jupiter. If the planet exists, it lies 2,185,247 light years away from Earth.
HIP 13044 b[]
A planet orbiting a star around 2,000 light years from Earth, HIP 13044, was discovered by the European Southern Observatory in 2010. While the star is currently in the Milky Way, it is part of the Helmi stream of stars, a leftover remnant of a small galaxy that collided with and was absorbed by the Milky Way over 6 billion years ago. The star itself is over 6 billion years old. This would mean that the planet had originally originated in another galaxy.
However, further analysis of the data has revealed problems with the potential planetary detection. For example, an erroneous barycentric correction had been applied, and after applying the corrections, there is no evidence for any planet orbiting the star.
RX J1131-1231 rogue planets[]
In 2018, a population of rogue planets were indirectly detected for the first time by astrophysicists from the University of Oklahoma in the lensing galaxy that lenses the quasar RX J1131-1231 by microlensing. The planets have masses ranging from the Moon to several Jupiter masses.
M51-ULS-1[]
In September 2020, it was announced that a planet orbiting the high-mass X-ray binary M51-ULS-1 in the Whirlpool Galaxy was discovered. This planet was detected by eclipses of the X-ray source, which consists of a stellar remnant (either a neutron star or a black hole, and a massive star, likely a blue supergiant.