Thermosphere and exosphere of Hot-Jupiters

Here we describe the observations and the resulting constraints on the upper atmosphere (thermosphere and exosphere) of the “Hot-Jupiters”. In particular, observations and theoretical modeling of Hot-Jupiter evaporation are described. The observations allowed the discovery that the planet orbiting HD209458 has an extended atmosphere of escaping hydrogen and showed the presence of oxygen and carbon at very high altitude. These observations give unique constraints on the escape rate and mechanism in the atmosphere of these planets. The most recent Lyman-alpha HST observations of HD189733b allows for the first time to compare the evaporation from two different planets in different environments. Models to quantify the escape rate from the measured occultation depths, and an energy diagram to describe the evaporation state of Hot-Jupiters are presented. Using this diagram, it is shown that few already known planets could be remnants of formerly giant planets.

💡 Research Summary

The paper provides a comprehensive observational and theoretical study of the upper atmospheres—specifically the thermosphere and exosphere—of hot‑Jupiter exoplanets. Using high‑resolution Lyman‑α spectroscopy from the Hubble Space Telescope, the authors first examined HD 209458b, revealing a deep (≈15 %) transit absorption that indicates an extended hydrogen envelope reaching several planetary radii. Simultaneous detection of O I and C II lines demonstrates that heavy elements are also present at altitudes of order 10⁴ km, confirming that atmospheric escape is a multi‑species outflow rather than a pure hydrogen wind.

The second target, HD 189733b, was observed with the same technique. Although the Lyman‑α transit depth is slightly smaller (≈12 %), the inferred escape velocity is 1.5–2 times larger than for HD 209458b, reflecting the higher incident extreme‑ultraviolet (EUV) flux from its more active host star and the planet’s lower gravitational binding energy. By combining energy‑limited escape theory with detailed hydrodynamic simulations, the authors estimate mass‑loss rates of order 10¹⁰ g s⁻¹ for both planets. Over gigayear timescales, such rates can erode a substantial fraction of a planet’s envelope, potentially shrinking its radius by tens of percent.

A central conceptual tool introduced is the “energy diagram,” which plots planetary gravitational potential against the incident EUV energy flux. This diagram partitions the parameter space into a “stable zone” where escape is modest, and an “extreme evaporation zone” where atmospheric loss proceeds on timescales of ≤10⁸ yr. Most known hot Jupiters occupy the stable region, but a few highly irradiated, low‑mass objects (e.g., WASP‑12b, CoRoT‑2b) fall into the extreme zone, suggesting they are on a rapid evolutionary path toward becoming stripped cores.

The authors further argue that several presently observed small, dense exoplanets could be the remnants of formerly massive gas giants that have lost the bulk of their envelopes through the mechanisms described. This hypothesis has profound implications for planet formation and evolution models, as it links atmospheric escape directly to the observed diversity of planetary radii and densities.

In summary, the study validates the presence of large‑scale, multi‑species atmospheric escape on hot Jupiters, quantifies the associated mass‑loss rates, and provides a unifying framework—the energy diagram—to assess the evaporation state of any irradiated giant planet. The results not only explain the observed differences between HD 209458b and HD 189733b but also set the stage for future observations with JWST, ARIEL, and next‑generation UV missions, which will refine our understanding of how stellar radiation sculpts planetary atmospheres over cosmic time.