The LHS 1678 System: Two Earth-Sized Transiting Planets and an Astrometric Companion Orbiting an M Dwarf Near the Convective Boundary at 20 pc
Creators: name: Silverstein, Michele L.
name: Schlieder, Joshua E.
name: Barclay, Thomas
name: Hord, Benjamin J.
name: Jao, Wei-Chun
name: Vrijmoet, Eliot Halley
name: Henry, Todd J.
name: Cloutier, Ryan
name: Kostov, Veselin B.
name: Kruse, Ethan
name: Winters, Jennifer G.
name: Irwin, Jonathan M.
name: Kane, Stephen R.
name: Stassun, Keivan G.
name: Huang, Chelsea
name: Kunimoto, Michelle
name: Tey, Evan
name: Vanderburg, Andrew
name: Astudillo-Defru, Nicola
name: Bonfils, Xavier
name: Brasseur, C. E.
name: Charbonneau, David
name: Ciardi, David R.
name: Collins, Karen A.
name: Collins, Kevin I.
name: Conti, Dennis M.
name: Crossfield, Ian J. M.
name: Daylan, Tansu
name: Doty, John P.
name: Dressing, Courtney D.
name: Gilbert, Emily A.
name: Horne, Keith
name: Jenkins, Jon M.
name: Latham, David W.
name: Mann, Andrew W.
name: Matthews, Elisabeth
name: Paredes, Leonardo A.
name: Quinn, Samuel N.
name: Ricker, George R.
name: Schwarz, Richard P.
name: Seager, Sara
name: Sefako, Ramotholo
name: Shporer, Avi
name: Smith, Jeffrey C.
name: Stockdale, Christopher
name: Tan, Thiam-Guan
name: Torres, Guillermo
name: Twicken, Joseph D.
name: Vanderspek, Roland
name: Wang, Gavin
name: Winn, Joshua N.
Publisher: name: arXiv
Year: 2021
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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.
description: Published in The Astronomical Journal (31 pages, 21 figures, 11 tables, 3 appendices)