M. Wang, F. Dai, H. Liu, H. Chen, Z. Hu, E. Petigura, S. Giacalone, E. Lee, M. Goldberg, A. Leleu, A. W. Mann, M. G. Barber, J. N. Winn, K. A. Collins, C. N. Watkins, R. P. Schwarz, H. Relles, F. P. Wilkin, E. Palle, F. Murgas, A. Shporer, R. R. Sefako, K. Horne, H. Osborn, Y. Alibert, L. Fossati, A. Fortier, S. G. Sousa, A. Brandeker, P. F. L. Maxted, A. Goldenberg
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, probably established through convergent disk migration. Over time, however, these resonant chains are often disrupted, mirroring the Nice model proposed for the Solar System. Here 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 to 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 (from stripped to 1%, 5% and 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 their primordial envelopes. Additionally, previous detections of metastable helium outflows rule out a pure water-world scenario for the TOI-2076 planets. Our finding provides direct observational evidence that the dynamical and atmospheric reshaping of compact planetary systems begins early and offers an empirical anchor for models of their long-term evolution.
Nature Astronomy
Volume 2026
2026 February
