The Discovery of Four Eccentric Sub-Neptunes Close to M dwarfs from TESS

Sub-Neptunes with 2 − 3R⊕, which are commonly discovered around M dwarfs, are intermediate planets that bridge a gap between abundant rocky planets and rare Neptune- and Jupiter-sized ones. The orbital properties and bulk compositions of transiting sub-Neptunes give an inkling of how close-in planets formed and evolved. In this paper, we present the discovery of four sub-Neptunes orbiting M dwarfs (TOI-782, 1448, 2120, and 2406), three of which were newly validated by multicolor transit photometry. TOI-782 b, 1448 b, 2120 b, and 2406 b have a radius of Rp = 2.740+0.082 −0.079 R⊕, 2.769+0.073 −0.068 R⊕, 2.120 ± 0.067R⊕, and 2.830+0.068 −0.066 R⊕ and an orbital period of P = 8.02, 8.11, 5.80, and 3.08 days, respectively. We also obtain their (upper limit) masses at the 2σ confidence level from follow-up doppler observations using Subaru/IRD: Mp = 9.2+5.2 −4.8M⊕, < 24.2M⊕, < 6.7M⊕, and < 26.3M⊕ for TOI-782 b, 1448 b, 2120 b, and 2406 b. The mass-radius relationship of four sub-Neptunes bears testimony to the existence of volatile material in their interiors. Four sub-Neptunes with radii larger than the so-called “radius valley” are likely to retain a significant atmosphere and/or an icy mantle onto the core. Interestingly, four sub-Neptunes are in an eccentric orbit around fully-convective M-dwarfs older than 1 Gyr. The fact that tidal circularization of their orbits is not achieved over Gyrs suggests inefficient tidal dissipation in the interior.