The Stumbling Dance of Two Planets Around TOI-4495 express
Two planets around the star TOI-4495 perform a stumbling dance: while the inner one completes two laps, the outer manages just one. But the rhythm is off — the inner planet speeds up and slows down, like a partner stumbling in her steps.
Using the transit method, watching as the planets briefly dim the star's light, astronomers weighed these worlds. The idea came from William Borucki. The inner planet weighs as much as 7.7 Earths, the outer as 23. Both are swathed in thick gas blankets, and their orbits lie nearly in the plane of the star's equator — a rare synchronicity.
The cause of the stumble probably lies in the past: collision with comet swarms may have knocked the system out of perfect resonance. Now the planets teeter on the edge, and any nudge could make them exchange energy, amplifying chaos.
Original abstract
as written by the authors arXiv:2601.02665v1
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 $$\lambda = -2.3^{+8.3}_{-7.8}\,\mathrm{deg}$$. Combined with the low mutual inclination constrained by the photodynamical analysis ($$\Delta I < 8.7\,\mathrm{deg}$$), 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.
A 2:1 orbital resonance also exists in our solar system: Jupiter's moons Io and Europa move in the same rhythm, but without stumbling.