Kepler K2 Campaign 9: II. First space-based discovery of an exoplanet using microlensing

Specht, D.; Poleski, R.; Penny, M. T.; Kerins, E.; McDonald, I.; Lee, Chung-Uk; Udalski, A.; Bond, I. A.; Shvartzvald, Y.; Zang, Weicheng; Street, R. A.; Hogg, D. W.; Gaudi, B. S.; Barclay, T.; Barentsen, G.; Howell, S. B.; Mullally, F.; Henderson, C. B.; Bryson, S. T.; Caldwell, D. A.; Haas, M. R.; Van Cleve, J. E.; Larson, K.; McCalmont, K.; Peterson, C.; Putnam, D.; Ross, S.; Packard, M.; Reedy, L.; Albrow, Michael D.; Chung, Sun-Ju; Jung, Youn Kil; Gould, Andrew; Han, Cheongho; Hwang, Kyu-Ha; Ryu, Yoon-Hyun; Shin, In-Gu; Yang, Hongjing; Yee, Jennifer C.; Cha, Sang-Mok; Kim, Dong-Jin; Kim, Seung-Lee; Lee, Dong-Joo; Lee, Yongseok; Park, Byeong-Gon; Pogge, Richard W.; Szymański, M. K.; Soszyński, I.; Ulaczyk, K.; Pietrukowicz, P.; Kozłowski, Sz; Skowron, J.; Mróz, P.; Mao, Shude; Fouqué, Pascal; Zhu, Wei; Abe, F.; Barry, R.; Bennett, D. P.; Bhattacharya, A.; Fukui, A.; Fujii, H.; Hirao, Y.; Itow, Y.; Kirikawa, R.; Kondo, I.; Koshimoto, N.; Matsubara, Y.; Matsumoto, S.; Miyazaki, S.; Muraki, Y.; Olmschenk, G.; Ranc, C.; Okamura, A.; Rattenbury, N. J.; Satoh, Y.; Sumi, T.; Suzuki, D.; Silva, S. I.; Toda, T.; Tristram, P. J.; Vandorou, A.; Yama, H.; Beichman, C.; Bryden, G. and Novati, S. Calchi (2023). Kepler K2 Campaign 9: II. First space-based discovery of an exoplanet using microlensing. Monthly Notices of the Royal Astronomical Society, 520(4) pp. 6350–6366.



We present K2-2016-BLG-0005Lb, a densely sampled, planetary binary caustic-crossing microlensing event found from a blind search of data gathered from Campaign 9 of the Kepler K2 mission (K2C9). K2-2016-BLG-0005Lb is the first bound microlensing exoplanet discovered from space-based data. The event has caustic entry and exit points that are resolved in the K2C9 data, enabling the lens–source relative proper motion to be measured. We have fitted a binary microlens model to the Kepler data, and to simultaneous observations from multiple ground-based surveys. Whilst the ground-based data only sparsely sample the binary caustic, they provide a clear detection of parallax that allows us to break completely the microlensing mass–position–velocity degeneracy and measure the planet’s mass directly. We find a host mass of 0.58 ± 0.04M⊙ and a planetary mass of 1.1 ± 0.1MJ. The system lies at a distance of 5.2 ± 0.2 kpc from Earth towards the Galactic bulge, more than twice the distance of the previous most distant planet found by Kepler. The sky-projected separation of the planet from its host is found to be 4.2 ± 0.3 au which, for circular orbits, deprojects to a host separation $a = 4.4^{+1.9}_{-0.4}$ au and orbital period $P = 13^{+9}_{-2}$ yr. This makes K2-2016-BLG-0005MLb a close Jupiter analogue orbiting a low-mass host star. According to current planet formation models, this system is very close to the host mass threshold below which Jupiters are not expected to form. Upcoming space-based exoplanet microlensing surveys by NASA’s Nancy Grace Roman Space Telescope and, possibly, ESA’s Euclid mission, will provide demanding tests of current planet formation models.

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