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A Second Terrestrial Planet Orbiting the Nearby M Dwarf LHS 1140

K. Ment, J. A. Dittman, N. Astudillo-Defru, D. Charbonneau, J. M. Irwin, X. Bonfils, F. Murgas, J.-M. Almenara, T. Forveille, E. Agol, S. Ballard, Z. Berta-Thompson, F. Bouchy, R. Cloutier, X. Delfosse, R. Doyon, C. D. Dressing, G. A. Esquerdo, R. D. Haywood, D. Kipping, D. W. Latham, C. Lovis, E. R. Newton, F. Pepe, J. E. Rodriguez, N. C. Santos, T.-G. Tan, S. Udry, J. G. Winters, A. WŁnsche

Abstract
LHS 1140 is a nearby mid-M dwarf known to host a temperate rocky super-Earth (LHS 1140 b) on a 24.737-day orbit. Based on photometric observations by MEarth and Spitzer as well as Doppler spectroscopy from the High Accuracy Radial velocity Planet Searcher, we report the discovery of an additional transiting rocky companion (LHS 1140 c) with a mass of 1.81 ± 0.39 M and a radius of 1.282 ± 0.024 R on a tighter, 3.77795-day orbit. We also obtain more precise estimates for the mass and radius of LHS 1140 b, which are 6.98 ± 0.89 M and 1.727 ± 0.032 R. The mean densities of planets b and c are 7.5 ± 1.0 g cm−3 and 4.7 ± 1.1 g cm−3, respectively, both consistent with the Earth's ratio of iron to magnesium silicate. The orbital eccentricities of LHS 1140 b and c are consistent with circular orbits and constrained to be below 0.06 and 0.31, respectively, with 90% confidence. Because the orbits of the two planets are coplanar and because we know from previous analyses of Kepler data that compact systems of small planets orbiting M dwarfs are commonplace, a search for more transiting planets in the LHS 1140 system could be fruitful. LHS 1140 c is one of the few known nearby terrestrial planets whose atmosphere could be studied with the upcoming James Webb Space Telescope.

Keywords
planets and satellites: detection; planets and satellites: terrestrial planets; techniques: photometric; techniques: radial velocities; Astrophysics - Earth and Planetary Astrophysics E-Print

The Astronomical Journal
Volume 157, Number 1
2019 January

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Instituto de Astrof√≠sica e Ci√™ncias do Espa√ßo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia