Anextragalactic planet, also known as anextragalactic exoplanet or anextroplanet,[1][2][3] is astar-boundplanet orrogue planet located outside of theMilky Way Galaxy. Due to the immense distances to such worlds, they have been very hard to detect directly. However, indirect evidences suggest that such planets exist.[4][5][6][7][8] Nonetheless, the most distant individually confirmed planets areSWEEPS-11 andSWEEPS-04, located inSagittarius, approximately 27,710light-years from the Sun, while the Milky Way is about 87,400 light-years in diameter. This means that even galactic planets located further than that distance have not been individually confirmed.
A microlensing event in theTwin Quasargravitational lensing system was observed in 1996, byR. E. Schild, in the "A" lobe of the lensed quasar. It is predicted that a3-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.[9][10]
A team of scientists has used gravitational microlensing to come up with a tentative detection of an extragalacticexoplanet inAndromeda, theMilky Way's 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.[11][12]
Candidates around extragalactic black-holes and X-ray binaries
In September 2020, the detection of a candidate planet orbiting thehigh-mass X-ray binary M51-ULS-1 in theWhirlpool Galaxy was announced. The planet was detected byeclipses of the X-ray source,[1] which consists of a stellar remnant (either aneutron star or ablack hole[2]) and a massive star, likely aB-typesupergiant. The planet is0.7RJ or around 50,000 kilometers in radius.[14] and orbit at a distance of some tens ofAU.[15][16] The study ofM51-ULS-1b as the first known extragalactic planet candidate was published inNature in October 2021.[17]
Thesubdwarf starHD 134440, which is currently located ingalactic halo and has extragalactic origin, was found to have a significantly higher metallicity than the similar star HD 134439. In 2018, research by Henrique Reggiani and Jorge Melendez concluded that this may have resulted from an engulfment of orbiting planets by HD 134440.[18]
The bright giant starBD+20 2457 was proposed to host twosuper-Jupiter planets or brown dwarfs, although the claimed planetary system is not dynamically stable.[19] As BD+20 2457 is a halo star possibly having formed in theGaia Enceladus, which are galactic remains of a former galaxy, the star and its planets might be extragalactic in origin.[20]
A planet with a mass of at least 1.25 times that ofJupiter had been potentially discovered by theEuropean Southern Observatory (ESO) orbiting a star of extragalactic origin, even though the star currently has been absorbed by our own galaxy.HIP 13044 is a star about 2,000 light years away in the southern constellation ofFornax,[21] part of theHelmi streamof stars, a leftover remnant of a small galaxy that collided with and was absorbed by the Milky Way over 6 billion years ago.[22]
However, subsequent analysis of the data revealed problems with the potential planetary detection: for example an erroneousbarycentric correction had been applied (the same error had also led to claims of planets aroundHIP 11952 that were subsequently refuted). After applying the corrections, there is no evidence for a planet orbiting the star.[23] 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 (cf.Future of Earth).
^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. Vol. 276. Proceedings of the International Astronomical Union. pp. 121–125.arXiv:1011.4938.Bibcode:2011IAUS..276..121K.doi:10.1017/S1743921311020059.
