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

TL;DR

This study characterizes a young, near-resonant planetary system, revealing early atmospheric loss and dynamical evolution, providing empirical evidence for models of planetary system development within the first few hundred million years.

Contribution

It presents detailed observations of the TOI-2076 system, demonstrating early atmospheric mass-loss and near-resonant dynamics, advancing understanding of planetary evolution shortly after formation.

Findings

Planets are near but not in mean-motion resonance.

Atmospheric envelopes decrease with increasing stellar insolation.

Helium outflows rule out water-world composition.

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.