An Adolescent and Near-Resonant Planetary System Near the End of Photoevaporation

Abstract

Young exoplanets provide vital insights into the early dynamical and atmospheric evolution of planetary systems. Many multi-planet systems younger than 100 Myr exhibit mean-motion resonances, likely established through convergent disk migration. Over time, however, these resonant chains are often disrupted, mirroring the Nice model proposed for the Solar System. We present a detailed characterization of the ~200-Myr-old TOI-2076 system, which contains four sub-Neptune planets between 1.4 and 3.5 Earth radii. We demonstrate that its planets are near but not locked in mean-motion resonances, making the system dynamically fragile. The four planets have comparable core masses but display a monotonic increase in hydrogen and helium (H/He) envelope mass fractions (stripped-1%-5%-5%) with decreasing stellar insolation. This trend is consistent with atmospheric mass-loss due to photoevaporation, which predicts that the envelopes of irradiated planets either erode completely or stabilize at a residual level of ~1% by mass within the first few hundred million years, with more distant, less-irradiated planets retaining most of primordial envelopes. Additionally, previous detections of metastable helium outflows rule out a pure water-world scenario for TOI-2076 planets. Our finding provides direct observational evidence that the dynamical and atmospheric reshaping of compact planetary systems begin early, offering an empirical anchor for models of their long-term evolution.

Figures (13)

• Figure 1: Transit timing variations and radial velocity observation, and measured masses and radii of TOI-2076 planets. (A-D) Transit timing variations of TOI-2076 planets, observations from TESS, LCO, and CHEOPS are denoted as open circles, squares, and triangles. The median and 1$\sigma$ interval of photodynamical posterior are shown as blue solid lines and shaded area. (E-H) phase-folded radial velocity data (triangle: HARPS-N, diamond: HIRES, square: Automated Planet Finder) of TOI-2076 planets, with stellar activity signals removed. The solid line and shaded area are the median and 1$\sigma$ interval of RV posterior. Black errorbars are binned RV data in 8 evenly-spaced bins over a planetary period. (I) The masses and radii of TOI-2076 planets from joint photodynamics and RV model are shown in circles, colored by their equilibrium temperatures. Grey circles are exoplanets from the Exoplanet Archive (as of 2025-05-23) with mass/radius constraints better than 4$\sigma$. Mass-radius relations with different compositions are overplotted Chen2016Zeng2019Aguichine2024.
• Figure 1: Planetary mass and eccentricity constraints from different observational sources. Top: marginalized planet mass constraints from TTV (blue), RV (gray), and joint photodynamical and RV model (red). First three panels in bottom show the planet mass and eccentricity 1 and 2$\sigma$ contours plot, color indexed as in top panels. Bottom rightmost panel shows the marginalized distribution of eccentricity from the joint photodynamical and RV model.
• Figure 1: LCO light curves of TOI-2076 planets. The LCO $z_s$ band light curves of TOI-2076 bcd planets. For each panel, the top row shows the raw fluxes (gray dots) and fluxes binned to 10 minutes (black dots), with the relevant planet index shown on top left. Blue lines are transit models with systematic-induced trends and light blue lines show the trends separately. Middle rows shows detrended light curves with best-fit transit model (orange), and bottom row shows light curve residuals.
• Figure 2: Mean-motion resonance diagram of TOI-2076 planets. (A) The Hamiltonian map of first-order mean-motion resonance Nesvorny2016. Resonant regime is shown as the gray area. The 2:1 TOI-2076 b-c pair (blue points) lies beyond the resonant regime. (B) The integrated orbit of TOI-2076 b-c on the action-angle phase space at $\delta=-6.3$. The trajectory encloses the origin, suggesting TOI-2076 b-c has a circulating resonant angle. (C) The second-order mean-motion resonance Hamiltonian map Hadden2019. Resonance regime is shown in gray. The 5:3 TOI-2076 c-d (blue points) is near-resonant while the 7:5 TOI-1136 e-f (orange points) lies in the true second-order resonant regime. (D) The integrated orbits of TOI-2076 c-d on the action-angle phase space at $J^*=-2\times10^{-4}$, which also shows the TOI-2076 c-d have a circulating resonant angle.
• Figure 2: Radial velocity data modeled with Keplerian planet signal and stellar activity with Gaussian Process. (A) An overview of RV analysis of TOI-2076 with a Gaussian process (GP) activity model over the combined HIRES (orange triangle), APF (blue circle), and HARPS-N (green diamond) RV series of TOI-2076, overlaid with mean prediction from GP activity model (black line). The dark and light gray region show the 1$\sigma$ and 2$\sigma$ uncertainty band of GP model. (B)RV residuals after subtracting the activity and planetary signals. Legends are the same as those in panel A. (C-D) zoom-in view of GP model at two observing seasons. Legends are the same as those in panel A.