A Second Planet Transiting LTT 1445A and a Determination of the Masses of Both Worlds
Authors: 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 
Published: March 2022

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.