Kavli Affiliate: Avi Shporer
| First 5 Authors: Michele L. Silverstein, Thomas Barclay, Joshua E. Schlieder, Karen A. Collins, Richard P. Schwarz
| Summary:
The nearby LHS 1678 (TOI-696) system contains two confirmed planets and a
wide-orbit, likely-brown-dwarf companion, which orbit an M2 dwarf with a unique
evolutionary history. The host star occupies a narrow "gap" in the HR diagram
lower main sequence, associated with the M dwarf fully convective boundary and
long-term luminosity fluctuations. This system is one of only about a dozen M
dwarf multi-planet systems to date that hosts an ultra-short period planet
(USP). Here we validate and characterize a third planet in the LHS 1678 system
using TESS Cycle 1 and 3 data and a new ensemble of ground-based light curves.
LHS 1678 d is a 0.98 +/-0.07 Earth radii planet in a 4.97-day orbit, with an
insolation flux of 9.1 +0.9/-0.8 Earth insolations. These properties place it
near 4:3 mean motion resonance with LHS 1678 c and in company with LHS 1678 c
in the Venus zone. LHS 1678 c and d are also twins in size and predicted mass,
making them a powerful duo for comparative exoplanet studies. LHS 1678 d joins
its siblings as another compelling candidate for atmospheric measurements with
the JWST and mass measurements using high-precision radial velocity techniques.
Additionally, USP LHS 1678 b breaks the "peas-in-a-pod" trend in this system,
although additional planets could fill in the "pod" beyond its orbit. LHS
1678’s unique combination of system properties and their relative rarity among
the ubiquity of compact multi-planet systems around M dwarfs makes the system a
valuable benchmark for testing theories of planet formation and evolution.
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