Published July 23, 2021 | Version v1
Conference paper Open

The LHS 1678 System: Two Small Planets and a Likely Brown Dwarf Orbiting a Nearby M Dwarf in Unconventional Circumstances

  • 1. NASA Goddard Space Flight Center
  • 2. NASA Goddard Space Flight Center, University of Maryland, Baltimore County
  • 3. University of Maryland, College Park
  • 4. Georgia State University, Research Consortium on Nearby Stars
  • 5. Research Consortium on Nearby Stars
  • 6. Center for Astrophysics | Harvard & Smithsonian
  • 7. NASA Goddard Space Flight Center, SETI Institute
  • 8. University of California, Riverside
  • 9. Vanderbilt University, Fisk University
  • 10. Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology
  • 11. University of Wisconsin-Madison
  • 12. Universidad Católica de la Santísima Concepción
  • 13. Univ. Grenoble Alpes


  • 1. Planetary Science Institute


We present the LHS 1678 (TOI-696) exoplanet system: two nearly Earth-sized transiting planets detected by TESS and a likely brown dwarf orbiting a bright M2 dwarf at 19.9 pc. The ultra-short-period LHS 1678 b (0.70 Earth radii, 0.9-day orbit) is a captivating target for emission spectroscopy observations with the JWST. LHS 1678 c (0.98 Earth radii, 3.7-day orbit) is in the Venus-zone and may be Venus density: a promising target for greenhouse effect studies. Both planets are favorable targets for EPRV mass measurements and for JWST transmission spectroscopy observations to study their atmospheres. The substellar companion, detected via CTIO/SMARTS 0.9m astrometry, is on a decades-long orbit and may someday eclipse the host star, revealing a rare system architecture in which more and less massive objects orbit in the same plane. There is also a candidate third planet detected in TESS multi-cycle data in near 4:3 resonance with LHS 1678 c. The host star is associated with an observed gap in the HR diagram tied to a change in M dwarf energy transport mechanisms. The effect of the associated stellar astrophysics on exoplanet evolution is currently unknown. In aggregate, LHS 1678 an exciting playground for comparative exoplanet science and understanding the formation and evolution of small, short-period exoplanets orbiting low-mass stars.



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