Kavli Affiliate: Sara Seager
| First 5 Authors: Isobel S. Lockley, David J. Armstrong, Jorge Fernández Fernández, Sarah Millholland, Henrik Knierim
| Summary:
We present the discovery of three sub-Neptune planets around TOI-1117, a
Sun-like star with mass $0.97pm0.02M_{odot}$, radius $1.05pm0.03R_{odot}$,
age $4.42pm1.50$ Gyr and effective temperature $5635pm62$ K. Light curves
from TESS and LCOGT show a transiting sub-Neptune with a $2.23$ day period,
mass $M_b=8.90_{-0.96}^{+0.95}M_{oplus}$ and radius
$R_b=2.46_{-0.12}^{+0.13}R_{oplus}$. This is a rare ‘hot Neptune’ that falls
within the parameter spaces known as the ‘Neptunian Desert’ and the ‘Radius
Valley’. Two more planetary signals are detected in HARPS radial velocities,
revealing two non-transiting planets with minimum masses
$M_c=7.46_{-1.62}^{+1.43}M_{oplus}$ and $M_d=9.06_{-1.78}^{+2.07}M_{oplus}$,
and periods of $4.579pm0.004$ and $8.67pm0.01$ days. The eccentricities were
poorly constrained by the HARPS data, with upper limits $e_b=0.11$, $e_c=0.29$,
and $e_d=0.24$. However, dynamical simulations of the TOI-1117 system, suggest
that the orbits must be nearly circular to be stable. The simulations also show
that TOI-1117b and c are likely to be in a near 2:1 resonance. The multi-planet
nature of TOI-1117 makes it a more complex case for formation theories of the
Neptunian Desert and Radius Valley, as current theories such as
high-eccentricity migration are too turbulent to produce a stable,
non-eccentric, multi-planet system. Moreover, analysis of TOI-1117b’s
photoevaporation history found rocky core and H/He atmosphere models to be
inconsistent with observations, whilst water-rich scenarios were favoured.
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