One possibility for the event is that a star in the disk of M31 gravitationally lensed a red giant also in the disk. The lensing star would have a mass between 0.02 M☉ and 3.6 M☉ with the most likely value near 0.5 M☉. In this case the lens profile makes it likely[how?] that the star has a planet.[2][3]
Possible exoplanet
The possible exoplanet would have a mass of 6.34 Jupiter mass. If confirmed, it would be the first exoplanet found in another galaxy.[4] A similar event was seen in 1996 when a team of astronomers discovered an anomalous fluctuation in the Twin Quasar's lightcurve that seemed to be caused by a planet approximately three Earth masses in size in the quasar's lensing galaxy YGKOW G1. (However, the results remain speculative because the chance alignment that led to its discovery will never happen again; if that exoplanet could be confirmed, it would be the most distant known planet, 4 billion ly away.)[5]
^Ingrosso, G.; Paolis, F. DE; Novati, S. Calchi; Jetzer, PH.; Nucita, A. A.; Zakharov, A. F. (2012). "Detection of Exoplanets in M31 with Pixel-Lensing: The Event Pa-99-N2 Case". The Twelfth Marcel Grossmann Meeting. The Twelfth Marcel Grossmann Meeting: On Recent Developments in Theoretical and Experimental General Relativity. p. 2191. arXiv:1001.2105. Bibcode:2012mgm..conf.2191I. doi:10.1142/9789814374552_0433. ISBN978-981-4374-51-4.
^An, Jin H.; Evans, N. W.; Kerins, E.; Baillon, P.; Calchi Novati, S.; Carr, B. J.; Creze, M.; Giraud‐Heraud, Y.; Gould, A.; Hewett, P.; Jetzer, Ph.; Kaplan, J.; Paulin‐Henriksson, S.; Smartt, S. J.; Tsapras, Y.; Valls‐Gabaud, D. (February 2004). "The Anomaly in the Candidate Microlensing Event PA‐99‐N2". The Astrophysical Journal. 601 (2): 845–857. arXiv:astro-ph/0310457. Bibcode:2004ApJ...601..845A. doi:10.1086/380820. S2CID8312033.
