A Second Planet Transiting LTT 1445A and a Determination of the Masses of Both Worlds

Winters, Jennifer G.; Cloutier, Ryan; Medina, Amber A.; Irwin, Jonathan M.; Charbonneau, David; Astudillo-Defru, Nicola; Bonfils, Xavier; Howard, Andrew W.; Isaacson, Howard; Bean, Jacob L.; Seifahrt, Andreas; Teske, Johanna K.; Eastman, Jason D.; Twicken, Joseph D.; Collins, Karen A.; Jensen, Eric L. N.; Quinn, Samuel N.; Payne, Matthew J.; Kristiansen, Martti H.; Spencer, Alton; Vanderburg, Andrew; Zechmeister, Mathias; Weiss, Lauren M.; Wang, Sharon Xuesong; Wang, Gavin; Udry, Stéphane; Terentev, Ivan A.; Stürmer, Julian; Stefánsson, Gudmundur; Shporer, Avi; Shectman, Stephen; Sefako, Ramotholo; Schwengeler, Hans Martin; Schwarz, Richard P.; Scarsdale, Nicholas; Rubenzahl, Ryan A.; Roy, Arpita; Rosenthal, Lee J.; Robertson, Paul; Petigura, Erik A.; Pepe, Francesco; Omohundro, Mark; Murphy, Joseph M. Akana; Murgas, Felipe; Močnik, Teo; Montet, Benjamin T.; Mennickent, Ronald; Mayo, Andrew W.; Massey, Bob; Lubin, Jack; Lovis, Christophe; Lewin, Pablo; Kasper, David; Kane, Stephen R.; Jenkins, Jon M.; Huber, Daniel; Horne, Keith; Hill, Michelle L.; Gorrini, Paula; Giacalone, Steven; Fulton, Benjamin; Forveille, Thierry; Figueira, Pedro; Fetherolf, Tara; Dressing, Courtney; Díaz, Rodrigo F.; Delfosse, Xavier; Dalba, Paul A.; Dai, Fei; Cortés, C. C.; Crossfield, Ian J. M.; Crane, Jeffrey D.; Conti, Dennis M.; Collins, Kevin I.; Chontos, Ashley; Butler, R. Paul; Brown, Peyton; Brady, Madison; Behmard, Aida; Beard, Corey; Batalha, Natalie M.; Almenara, Jose-Manuel
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The Astronomical Journal

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LTT 1445 is a hierarchical triple M-dwarf star system located at a distance of 6.86 pc. The primary star LTT 1445A (0.257 M ⊙) is known to host the transiting planet LTT 1445Ab with an orbital period of 5.36 days, making it the second-closest known transiting exoplanet system, and the closest one for which the host is an M dwarf. Using Transiting Exoplanet Survey Satellite data, we present the discovery of a second planet in the LTT 1445 system, with an orbital period of 3.12 days. We combine radial-velocity measurements obtained from the five spectrographs, Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, High Accuracy Radial Velocity Planet Searcher, High-Resolution Echelle Spectrometer, MAROON-X, and Planet Finder Spectrograph to establish that the new world also orbits LTT 1445A. We determine the mass and radius of LTT 1445Ab to be 2.87 ± 0.25 M ⊕ and ${1.304}_{-0.060}^{+0.067}$ R ⊕, consistent with an Earth-like composition. For the newly discovered LTT 1445Ac, we measure a mass of ${1.54}_{-0.19}^{+0.20}$ M ⊕ and a minimum radius of 1.15 R ⊕, but we cannot determine the radius directly as the signal-to-noise ratio of our light curve permits both grazing and nongrazing configurations. Using MEarth photometry and ground-based spectroscopy, we establish that star C (0.161 M ⊙) is likely the source of the 1.4 day rotation period, and star B (0.215 M ⊙) has a likely rotation period of 6.7 days. We estimate a probable rotation period of 85 days for LTT 1445A. Thus, this triple M-dwarf system appears to be in a special evolutionary stage where the most massive M dwarf has spun down, the intermediate mass M dwarf is in the process of spinning down, while the least massive stellar component has not yet begun to spin down.
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