A low-eccentricity migration pathway for a 13-h-period Earth analogue in a four-planet system

Serrano, Luisa Maria; Gandolfi, Davide; Mustill, Alexander J.; Barragán, Oscar; Korth, Judith; Dai, Fei; Redfield, Seth; Fridlund, Malcolm; Lam, Kristine W. F.; Díaz, Matías R.; Grziwa, Sascha; Collins, Karen A.; Livingston, John H.; Cochran, William D.; Hellier, Coel; Bellomo, Salvatore E.; Trifonov, Trifon; Rodler, Florian; Alarcon, Javier; Jenkins, Jon M.; Latham, David W.; Ricker, George; Seager, Sara; Vanderspeck, Roland; Winn, Joshua N.; Albrecht, Simon; Collins, Kevin I.; Csizmadia, Szilárd; Daylan, Tansu; Deeg, Hans J.; Esposito, Massimiliano; Fausnaugh, Michael; Georgieva, Iskra; Goffo, Elisa; Guenther, Eike; Hatzes, Artie P.; Howell, Steve B.; Jensen, Eric L. N.; Luque, Rafael; Mann, Andrew W.; Murgas, Felipe; Osborne, Hannah L. M.; Palle, Enric; Persson, Carina M.; Rowden, Pam; Rudat, Alexander; Smith, Alexis M. S.; Twicken, Joseph D.; Van Eylen, Vincent; Ziegler, Carl
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Nature Astronomy

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It is commonly accepted that exoplanets with orbital periods shorter than one day, also known as ultra-short-period (USP) planets, formed further out within their natal protoplanetary disks before migrating to their current-day orbits via dynamical interactions. One of the most accepted theories suggests a violent scenario involving high-eccentricity migration followed by tidal circularization. Here we present the discovery of a four-planet system orbiting the bright (V = 10.5) K6 dwarf star TOI-500. The innermost planet is a transiting, Earth-sized USP planet with an orbital period of ~13 hours, a mass of 1.42 ± 0.18 M⊕, a radius of 1.16 6−0.058+0.061R⊕ ? and a mean density of 4.8 9−0.88+1.03g cm−3 ?. Via Doppler spectroscopy, we discovered that the system hosts 3 outer planets on nearly circular orbits with periods of 6.6, 26.2 and 61.3 days and minimum masses of 5.03 ± 0.41 M⊕, 33.12 ± 0.88 M⊕ and 15.0 5−1.11+1.12M⊕ ?, respectively. The presence of both a USP planet and a low-mass object on a 6.6-day orbit indicates that the architecture of this system can be explained via a scenario in which the planets started on low-eccentricity orbits then moved inwards through a quasi-static secular migration. Our numerical simulations show that this migration channel can bring TOI-500 b to its current location in 2 Gyr, starting from an initial orbit of 0.02 au. TOI-500 is the first four-planet system known to host a USP Earth analogue whose current architecture can be explained via a non-violent migration scenario.
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