A Second Planet Transiting LTT 1445A and a Determination of the Masses of Both Worlds
Butler, R. Paul; Kristiansen, Martti H.; Omohundro, Mark; Terentev, Ivan A.; Vanderburg, Andrew; Batalha, Natalie M.; Quinn, Samuel N.; Mayo, Andrew W.; Kane, Stephen R.; Huber, Daniel; Rosenthal, Lee J.; Howard, Andrew W.; Petigura, Erik A.; Behmard, Aida; Chontos, Ashley; Crossfield, Ian J. M.; Dalba, Paul A.; Rubenzahl, Ryan A.; Weiss, Lauren M.; Fetherolf, Tara; Horne, Keith; Collins, Karen A.; Jenkins, Jon M.; Crane, Jeffrey D.; Shectman, Stephen; Shporer, Avi; Collins, Kevin I.; Dai, Fei; Montet, Benjamin T.; Udry, Stéphane; Winters, Jennifer G.; Eastman, Jason D.; Jensen, Eric L. N.; Kasper, David; Twicken, Joseph D.; Bean, Jacob L.; Sefako, Ramotholo; Giacalone, Steven; Medina, Amber A.; Irwin, Jonathan M.; Charbonneau, David; Zechmeister, Mathias; Pepe, Francesco; Schwarz, Richard P.; Murgas, Felipe; Fulton, Benjamin; Wang, Sharon Xuesong; Cloutier, Ryan; Robertson, Paul; Roy, Arpita; Conti, Dennis M.; Astudillo-Defru, Nicola; Bonfils, Xavier; Teske, Johanna K.; Lovis, Christophe; Forveille, Thierry; Figueira, Pedro; Díaz, Rodrigo F.; Delfosse, Xavier; Almenara, Jose-Manuel; Isaacson, Howard; Močnik, Teo; Beard, Corey; Lubin, Jack; Dressing, Courtney; Hill, Michelle L.; Scarsdale, Nicholas; Stefánsson, Gudmundur; Brady, Madison; Seifahrt, Andreas; Stürmer, Julian; Lewin, Pablo; Murphy, Joseph M. Akana; Wang, Gavin; Massey, Bob; Cortés, C. C.; Payne, Matthew J.; Schwengeler, Hans Martin; Spencer, Alton; Mennickent, Ronald; Gorrini, Paula; Brown, Peyton
United States, Chile, France, Australia, Denmark, Germany, China, Switzerland, Russia, South Africa, United Kingdom, Spain, Portugal, Argentina
Abstract
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.