The LHS 1678 System: Two Earth-Sized Transiting Planets and an Astrometric Companion Orbiting an M Dwarf Near the Convective Boundary at 20 pc

Michele L. Silverstein , Joshua E. Schlieder (1) , Thomas Barclay (2) , Benjamin J. Hord , Wei-Chun Jao , Eliot Halley Vrijmoet , Todd J. Henry , Ryan Cloutier , Veselin B. Kostov , Ethan Kruse (3) , Jennifer G. Winters , Jonathan M. Irwin , Stephen R. Kane , Keivan G. Stassun , Chelsea Huang , Michelle Kunimoto , Evan Tey , Andrew Vanderburg , Nicola Astudillo-Defru , Xavier Bonfils (4) , C. E. Brasseur , David Charbonneau , David R. Ciardi , Karen A. Collins , Kevin I. Collins , Dennis M. Conti , Ian J. M. Crossfield , Tansu Daylan , John P. Doty , Courtney D. Dressing , Emily A. Gilbert , Keith Horne , Jon M. Jenkins , David W. Latham , Andrew W. Mann , Elisabeth Matthews , Leonardo A. Paredes , Samuel N. Quinn , George R. Ricker , Richard P. Schwarz , Sara Seager , Ramotholo Sefako , Avi Shporer , Jeffrey C. Smith , Christopher Stockdale , Thiam-Guan Tan , Guillermo Torres , Joseph D. Twicken , Roland Vanderspek , Gavin Wang , Joshua N. Winn

1 ObsGE - Observatoire Astronomique de l'Université de Genève
2 ARC-CREST - NASA Ames Research Center Cooperative for Research in Earth Science in Technology
3 University of Washington [Seattle]
4 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

We present the TESS discovery of the LHS 1678 (TOI-696) exoplanet system, comprised of two approximately Earth-sized transiting planets and a likely astrometric brown dwarf orbiting a bright ($V_J$=12.5, $K_s$=8.3) M2 dwarf at 19.9 pc. The two TESS-detected planets are of radius 0.70$\pm$0.04 $R_\oplus$ and 0.98$\pm$0.06 $R_\oplus$ in 0.86-day and 3.69-day orbits, respectively. Both planets are validated and characterized via ground-based follow-up observations. HARPS RV monitoring yields 97.7 percentile mass upper limits of 0.35 $M_\oplus$ and 1.4 $M_\oplus$ for planets b and c, respectively. The astrometric companion detected by the CTIO/SMARTS 0.9m has an orbital period on the order of decades and is undetected by other means. Additional ground-based observations constrain the companion to being a high-mass brown dwarf or smaller. Each planet is of unique interest; the inner planet has an ultra-short period, and the outer planet is in the Venus zone. Both are promising targets for atmospheric characterization with the JWST and mass measurements via extreme-precision radial velocity. A third planet candidate of radius 0.9$\pm$0.1 $R_\oplus$ in a 4.97-day orbit is also identified in multi-Cycle TESS data for validation in future work. The host star is associated with an observed gap in the lower main sequence of the Hertzsprung-Russell diagram. This gap is tied to the transition from partially- to fully-convective interiors in M dwarfs, and the effect of the associated stellar astrophysics on exoplanet evolution is currently unknown. The culmination of these system properties makes LHS 1678 a unique, compelling playground for comparative exoplanet science and understanding the formation and evolution of small, short-period exoplanets orbiting low-mass stars.

Domains: Astrophysics [astro-ph]

Michele L. Silverstein, Joshua E. Schlieder, Thomas Barclay, Benjamin J. Hord, Wei-Chun Jao, et al.. The LHS 1678 System: Two Earth-Sized Transiting Planets and an Astrometric Companion Orbiting an M Dwarf Near the Convective Boundary at 20 pc. 2021. ⟨insu-03458317⟩