The TOI-1117 multiplanetary system: 3 sub-Neptunes, 1 in both the Neptunian Desert and Radius Valley

We present the discovery of three sub-Neptune planets around TOI-1117, a Sun-like star with mass $0.97\pm 0.02\,{\mathrm M}_{\odot }$, radius $1.05\pm 0.03\,{\mathrm {R}}_{\odot }$, age $4.42\pm 1.50$ Gyr, and effective temperature $5635\pm 62$ K. Light curves from Transiting Exoplanet Survey Satellite and Las Cumbres Observatory Global Telescope show a transiting sub-Neptune with a 2.23-d period, mass $M_b=8.90_{-0.96}^{+0.95}\,\mathrm{ M}_{\rm{\oplus }}$ and radius $R_b=2.46_{-0.12}^{+0.13}\,\mathrm{ R}_{\rm{\oplus }}$. 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 (High Accuracy Radial velocity Planetary Searcher) radial velocities, revealing two non-transiting planets with minimum masses $M_c=7.46_{-1.62}^{+1.43}\,\mathrm{ M}_{\rm{\oplus }}$ and $M_d=9.06_{-1.78}^{+2.07}\,\mathrm{ M}_{\rm{\oplus }}$, and periods of $4.579\pm 0.004$ and $8.67\pm 0.01$ d. 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-1117 b and c are likely to be in a near 2:1 resonance. The multiplanet 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, and multiplanet system. Moreover, analysis of TOI-1117 b’s photoevaporation history found rocky core and H/He atmosphere models to be inconsistent with observations, whilst water-rich scenarios were favoured.