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

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.