TOI-4495: A Pair of Aligned, Near-Resonant Sub-Neptunes that Likely Experienced Overstable Migration

TL;DR

TOI-4495 hosts a sub-Neptune and a Neptune-like planet in a near-$2:1$ MMR configuration, offering a testbed for migration and resonance-breaking scenarios. The authors perform a photodynamical analysis of TESS transits to constrain masses and eccentricities, and obtain a well-aligned orbit around the host star from the Rossiter–McLaughlin effect and Doppler shadow measurements, with a small projected obliquity of $\lambda = -2.3^{+8.3}_{-7.8}$ degrees. They find a circulating resonant angle, a measurable free eccentricity for the inner planet $e_b = 0.078^{+0.020}_{-0.013}$, and argue that resonant overstability plus potential perturbations (e.g., planetesimal scattering) may explain the architecture. To preserve the observed eccentricity over $\sim 1.9$ Gyr, the inner planet would require a reduced tidal quality factor $Q' \gtrsim 10^5$, consistent with a thick gaseous envelope and formation history involving disk migration followed by dynamical excitation.

Abstract

We report the discovery of a sub-Neptune and a Neptune-like planet ($R_b = 2.48^{+0.14}_{-0.10}\,R_\oplus$, $R_c = 4.03^{+0.23}_{-0.15}\,R_\oplus$) orbiting the F-type star TOI-4495. The planets have orbital periods of 2.567 days and 5.185 days, lying close to a 2:1 mean-motion resonance (MMR). Our photodynamical analysis of the TESS light curves constrains the planetary masses to $M_b = 7.7 \pm 1.4\,M_\oplus$ and $M_c = 23.2 \pm 4.7\,M_\oplus$. The measured masses and radii indicate the presence of volatile-rich gaseous envelopes on both planets. The Rossiter-McLaughlin effect and the Doppler shadow of TOI-4495 c reveal a well-aligned orbit with a projected stellar obliquity of $λ= -2.3^{+8.3}_{-7.8}\,\mathrm{deg}$. Combined with the low mutual inclination constrained by the photodynamical analysis ($ΔI < 8.7\,\mathrm{deg}$), the planetary orbits are likely coplanar and aligned with the host star's spin axis. We show that the planets are near, but not in, the 2:1 MMR, with a circulating resonant angle. We also find substantial free eccentricity for the inner planet, TOI-4495 b ($e_b = 0.078^{+0.020}_{-0.013}$). Given the observed proximity to the 2:1 resonance and the more massive outer planet, TOI-4495 b and c are particularly susceptible to resonant overstability, which can convert resonantly excited eccentricity into free eccentricity. However, additional mechanisms (e.g., planetesimal scattering) may be required to further excite the eccentricity by $\sim 4\%$. To prevent tidal damping from reducing the eccentricity below the observed level over the star's lifetime (1.9 Gyr), the reduced tidal quality factor of TOI-4495 b must be $Q' \gtrsim 10^5$, consistent with the presence of a thick envelope on the planet.