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

Abstract

We present the Transiting Exoplanet Survey Satellite (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 ± 0.04 R_⊕ and 0.98 ± 0.06 R_⊕ in 0.86 day and 3.69 day orbits, respectively. Both planets are validated and characterized via ground-based follow-up observations. High Accuracy Radial Velocity Planet Searcher RV monitoring yields 97.7 percentile mass upper limits of 0.35 M_⊕ and 1.4 M_⊕ for planets b and c, respectively. The astrometric companion detected by the Cerro Tololo Inter-American Observatory/Small and Moderate Aperture Telescope System 0.9 m 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 James Webb Space Telescope and mass measurements via extreme-precision radial velocity. A third planet candidate of radius 0.9 ± 0.1 R_⊕ in a 4.97 day orbit is also identified in multicycle 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.

Additional Information

© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 June 24; revised 2021 October 20; accepted 2021 October 22; published 2022 March 7. We thank Daniel Foreman-Mackey for valuable suggestions on sampling parameterization. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This paper includes data collected with the TESS mission (Ricker et al. 2015), obtained from the MAST data archive at the Space Telescope Science Institute (STScI). Funding for the TESS mission is provided by NASAs Science Mission Directorate. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 526555. We acknowledge the use of TESS High Level Science Products (HLSP) produced by the Quick-Look Pipeline (QLP) at the TESS Science Office at MIT, which are publicly available from the Mikulski Archive for Space Telescopes (MAST). Funding for the TESS mission is provided by NASA's Science Mission directorate. We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. This research has made use of the Exoplanet Follow-up Observation Program website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This work makes use of observations from the LCOGT network. Part of the LCOGT telescope time was granted by NOIRLab through the Mid-Scale Innovations Program (MSIP). MSIP is funded by NSF. This research has made use of "Aladin sky atlas", developed at CDS, Strasbourg Observatory, France (Bonnarel et al. 2000), the SIMBAD database, operated at CDS, Strasbourg, France (Wenger et al. 2000), and the VizieR catalog access tool, CDS, Strasbourg, France (Ochsenbein et al. 2000). This research made use of the TOPCAT database visualization and management software, available at http://www.star.bris.ac.uk/~mbt/topcat/ (Taylor 2005). This research made use of Lightkurve, a Python package for Kepler and TESS data analysis (Lightkurve Collaboration et al. 2018). This work made use of tpfplotter by J. Lillo-Box (publicly available in www.github.com/jlillo/tpfplotter), which also made use of the Python packages astropy, Lightkurve, matplotlib and numpy. This research made use of exoplanet (Foreman-Mackey et al. 2021) and its dependencies (Agol et al. 2020; Kumar et al.2019; Astropy Collaboration et al. 2013, 2018; Kipping 2013; Luger et al. 2019; Salvatier et al. 2016; Theano Development Team 2016). The MEarth Team gratefully acknowledges funding from the David and Lucile Packard Fellowship for Science and Engineering (awarded to D.C.). This material is based upon work supported by the National Science Foundation under grant Nos. AST-0807690, AST-1109468, AST-1004488 (Alan T. Waterman Award), and AST-1616624, and upon work supported by the National Aeronautics and Space Administration under grant No. 80NSSC18K0476 issued through the XRP Program. This work is made possible by a grant from the John Templeton Foundation. The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. N. A.-D. acknowledges the support of FONDECYT project 3180063. T.D. acknowledges support from MIT's Kavli Institute as a Kavli postdoctoral fellow. K.H. acknowledges support from STFC grant No. ST/R000824/1. E.A.G. thanks the LSSTC Data Science Fellowship Program, which is funded by LSSTC, NSF Cybertraining Grant #1829740, the Brinson Foundation, and the Moore Foundation; her participation in the program has benefited this work. The material is based upon work supported by NASA under award number 80GSFC21M0002. This work was supported by the lead author's appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Universities Space Research Association under contract with NASA. Facilities: TESS - , CTIO:0.9 m - , CTIO:1.5 m - , WISE - , Small and Moderate Aperture Research Telescope System (SMARTS) - , LCOGT - , PEST - , MEarth - , VLT NaCo - , SOAR HRCam and SAM - , Gaia - , HARPS - , 2MASS - , MEarth-South - . Software: Python, IDL, NumPy (Harris et al. 2020), Matplotlib (Hunter 2007), Astropy (Astropy Collaboration et al. 2013, 2018), Lightkurve (Lightkurve Collaboration et al. 2018), DAVE (Kostov et al. 2019b), QATS, (Carter & Agol 2013; Kruse et al. 2019), TLS (Hippke & Heller 2019a, 2019b), AstroImageJ (Collins et al. 2017), vespa (Morton 2015), CDS aladin (Bonnarel et al. 2000), SIMBAD (Wenger et al. 2000) and VizieR (Ochsenbein et al. 2000), tpfplotter (Aller et al. 2020), exoplanet (Foreman-Mackey et al. 2021; Agol et al. 2020; Kumar et al. 2019; Astropy Collaboration et al. 2013, 2018; Kipping 2013; Luger et al. 2019; Salvatier et al. 2016; Theano Development Team 2016), forecaster (Chen & Kipping 2017), TESScut (Brasseur et al. 2019), Quick-Look Pipeline (Huang et al. 2020a, 2020b), TESS-plots (https://github.com/mkunimoto/TESS-plots).

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