Planetary system around the nearby M dwarf GJ 357 including a transiting, hot, Earth-sized planet optimal for atmospheric characterization

R. Luque, E. Pallé, D. Kossakowski, S. Dreizler, J. Kemmer, N. Espinoza, J. Burt, G. Anglada-Escudé, V. J. S. Béjar, J. A. Caballero, K. A. Collins, K. I. Collins, M. Cortés-Contreras, E. Díez-Alonso, F. Feng, A. Hatzes, C. Hellier, T. Henning, S. V. Jeffers, L. Kaltenegger, M. Kürster, J. Madden, K. Molaverdikhani, D. Montes, N. Narita, G. Nowak, A. Ofir, M. Oshagh, H. Parviainen, A. Quirrenbach, S. Reffert, A. Reiners, C. Rodríguez-López, M. Schlecker, S. Stock, T. Trifonov, J. N. Winn, M. R. Zapatero Osorio, M. Zechmeister, P. J. Amado, D. R. Anderson, N. E. Batalha, F. F. Bauer, P. Bluhm, C. J. Burke, R. P. Butler, D. A. Caldwell, G. Chen, J. D. Crane, D. Dragomir, C. D. Dressing, S. Dynes, J. M. Jenkins, A. Kaminski, H. Klahr, T. Kotani, M. Lafarga, D. W. Latham, P. Lewin, S. McDermott, P. Montañés-Rodríguez, J. C. Morales, F. Murgas, E. Nagel, S. Pedraz, I. Ribas, G. R. Ricker, P. Rowden, S. Seager, S. A. Shectman, M. Tamura, J. Teske, J. D. Twicken, R. Vanderspeck, S. X. Wang and B. Wohler. 2019. Planetary system around the nearby M dwarf GJ 357 including a transiting, hot, Earth-sized planet optimal for atmospheric characterization. Astronomy and Astrophysics 628 DOI: 10.1051/0004-6361/201935801

We report the detection of a transiting Earth-size planet around GJ 357, a nearby M2.5 V star, using data from the Transiting Exoplanet Survey Satellite (TESS). GJ 357 b (TOI-562.01) is a transiting, hot, Earth-sized planet (T-eq = 525 +/- 11 K) with a radius of R-b = 1.217 +/- 0.084 R-circle plus and an orbital period of P-b = 3.93 d. Precise stellar radial velocities from CARMENES and PFS, as well as archival data from HIRES, UVES, and HARPS also display a 3.93-day periodicity, confirming the planetary nature and leading to a planetary mass of M-b = 1.84 +/- 0.31 M-circle plus. In addition to the radial velocity signal for GJ 357 b, more periodicities are present in the data indicating the presence of two further planets in the system: GJ 357 c, with a minimum mass of M-c = 3.40 +/- 0.46 M-circle plus in a 9.12 d orbit, and GJ 357 d, with a minimum mass of M-d = 6.1 +/- 1.0 M-circle plus in a 55.7 d orbit inside the habitable zone. The host is relatively inactive and exhibits a photometric rotation period of P-rot = 78 +/- 2 d. GJ 357 b is to date the second closest transiting planet to the Sun, making it a prime target for further investigations such as transmission spectroscopy. Therefore, GJ 357 b represents one of the best terrestrial planets suitable for atmospheric characterization with the upcoming JWST and ground-based ELTs.

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