An extragalactic planet, also known as an extragalactic exoplanet, is a star-bound planet, or rogue planet, located outside of the Milky Way galaxy. Due to the huge distances to such worlds, they would be very hard to detect directly. However, indirect evidence suggests that such planets may exist. Nonetheless, the most distant known planets are SWEEPS-11 and SWEEPS-04, located in Sagittarius, approximately 27,710 light-years from the Sun, while the Milky Way is between 100,000–180,000 light years in diameter. This means that even galactic planets located further than that distance have not been detected.
HIP 13044 b
A planet with a mass of at least 1.25 times that of Jupiter had been potentially discovered by the European Southern Observatory (ESO) orbiting a star of extragalactic origin, even though the star now has been absorbed by our own galaxy. HIP 13044 is a star about 2,000 light years away in the southern constellation of Fornax, 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.
However, subsequent analysis of the data revealed problems with the potential planetary detection: for example an erroneous barycentric correction had been applied (the same error had 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. If it had been real, the Jupiter-like planet would have been particularly interesting, orbiting a star nearing the end of its life and seemingly about to be engulfed by it, potentially providing an observational model for the fate of our own planetary system in the distant future.
A microlensing event in the Twin Quasar gravitational lensing system was observed in 1996, by R. E. Schild, in the "A" lobe of the lensed quasar. It is predicted that a 3-Earth-mass planet in the lensing galaxy, YGKOW G1, caused the event. This was the first extragalactic planet candidate announced. This, however, is not a repeatable observation, as it was a one-time chance alignment. This predicted planet lies 4 billion light years away.
Andromeda galaxy planets
A team of scientists has used gravitational microlensing to come up with a tentative detection of an extragalactic exoplanet in Andromeda, our nearest large galactic neighbor. The lensing pattern fits a star with a smaller companion, PA-99-N2, weighing just around 6.34 times the mass of Jupiter. This suspected planet is the first announced in the Andromeda Galaxy.
Evidence of a population of rogue planets
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.
- Dai, Xinyu; Guerras, Eduardo (2 February 2018). "Probing Planets in Extragalactic Galaxies Using Quasar Microlensing". The Astrophysical Journal. 853 (2): L27. arXiv: . Bibcode:2018ApJ...853L..27D. doi:10.3847/2041-8213/aaa5fb.
- Zachos, Elaine (5 February 2018). "More Than a Trillion Planets Could Exist Beyond Our Galaxy - A new study gives the first evidence that exoplanets exist beyond the Milky Way". National Geographic Society. Retrieved 5 February 2018.
- Mandelbaum, Ryan F. (5 February 2018). "Scientists Find Evidence of Thousands of Planets in Distant Galaxy". Gizmodo. Retrieved 5 February 2018.
- "Planet from another galaxy discovered". ESO Press Release. 18 November 2010. Retrieved 17 November 2011.
- Klement, R.; Setiawan, J.; Thomas Henning; Hans-Walter Rix; Boyke Rochau; Jens Rodmann; Tim Schulze-Hartung; MPIA Heidelberg; ESTEC (2011). "The visitor from an ancient galaxy: A planetary companion around an old, metal-poor red horizontal branch star". The Astrophysics of Planetary Systems: Formation, Structure, and Dynamical Evolution. IAU Symposium. 276. Proceedings of the International Astronomical Union. pp. 121–125. arXiv: . Bibcode:2011IAUS..276..121K. doi:10.1017/S1743921311020059.
- Jones, M. I.; Jenkins, J. S. (2014). "No evidence of the planet orbiting the extremely metal-poor extragalactic star HIP 13044". Astronomy & Astrophysics. 562: id.A129. arXiv: . Bibcode:2014A&A...562A.129J. doi:10.1051/0004-6361/201322132.
- New Scientist (issue 2037), Do alien worlds throng faraway galaxy? Govert Schilling 06 July 1996
- Extrasolar Visions, "The Q0957+561 Planet" Archived 2012-03-30 at the Wayback Machine. (accessed 1 September 2009)
- Thaindian News, First extragalactic exoplanet may have been found by gravitational microlensing, 11 June 2009
- New Scientist, First extragalactic exoplanet may have been found, 10 June 2009