An extragalactic planet, also known as an extragalactic extrasolar planet is a rogue planet or a planet that is outside of the Milky Way galaxy. Due to the huge distance between such worlds, extragalactic planets are extremely hard and almost impossible to detect with current technology. The most distant known planets are SWEEPS-11 and SWEEPS-04, which are both located 27,710 light years away from the Sun in the constellation Sagittarius. The Milky Way is 100,000-180,000 light years in diameter, so this means that even galactic planets beyond 27,000 light years have not been detected yet, even though they certainly exist.

It is extremely improbable that extrasolar planets only exist in the Milky Way galaxy. In February 2018, scientists have found evidence that thousands of objects, ranging from Moon to Jupiter sized planets, exist in a quasar named RX J1131-1231, located 3.8 billion light years away from Earth.

HIP 13044 bEdit

HIP 13044 is a star around 2,000 light years away from Earth, but it is within the Milky Way galaxy which was found to have an exoplanet. However, this star 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, which means that the star is over 6 billion years old.

However, subsequent analysis of the data has revealed problems with the potential planetary detection: for example an erroneous barycentric correction had been applied (the same error which also led to claims of planets around HIP 11952 that were subsequently refuted). After applying the corrections, there is no evidence for a planet orbiting the star, although its existence has not been ruled out entirely.

Twin Quasar-related planetEdit

In 1996, a team at Harvard-Smithsonian Center for Astrophysics led by Rudy E. Schild discovered an anomalous fluctuation in one image's lightcurve, which led to a controversial and unconfirmable theory that there is a planet with approximately three Earth masses in the lensing galaxy, YGKOW G1. However, this cannot ever be confirmed, as the chance alignment that led to its discovery will never happen again. The planet, if it exists, lies about 4 billion light years away from Earth.

Andromeda Galaxy planetsEdit

A team of scientists have used gravitational microlensing to come up with a tentative detection of an extragalactic exoplanet in Andromeda, our nearest large galactic neighbour. The lensing pattern fits a star with a smaller companion, PA-99-N2, weighing just 6.34 times the mass of Jupiter. This suspected planet is the first announced in the Andromeda Galaxy.

Evidence of a population of rogue planetsEdit

A population of unbound planets between stars, with masses ranging from Moon to Jupiter masses, have been indirectly detected, for the first time, by astrophysicists from the University of Oklahoma, in the lensing galaxy that lenses quasar RX J1131-1231 by microlensing.

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