Composition and fate of short-period super-Earths: The case of CoRoT-7b

The discovery of CoRoT-7b, a planet of radius 1.68 +/- 0.09 R_E, mass 4.8 +/- 0.8 M_E and orbital period of 0.854 days demonstrates that small planets can orbit extremely close to their star. We use knowledge of hot Jupiters, mass loss estimates and models for the interior structure and evolution of planets to understand its composition, structure and evolution. The inferred mass and radius of CoRoT-7b are consistent with a rocky planet that would be depleted in iron relative to Earth. However, a one sigma increase in mass (5.6 M_E) and decrease in size (1.59 R_E) are compatible with an Earth-like composition (33% iron, 67% silicates). Alternatively, it is possible that CoRoT-7b contains a significant amount of volatiles. An equally good fit to the data is found for a vapor envelope equal to 3% (and up to 10%) by mass above an Earth-like nucleus. Because of its intense irradiation and small size, the planet cannot possess an envelope of H and He of more than 1/10,000 of its total mass. We show that the mass loss is significant (~ 10^11 g/s) and independent of planetary composition. This is because the hydrodynamical escape rate is independent of the atmosphere’s mean molecular mass, and owing to the intense irradiation, even a bare rocky planet would possess an equilibrium vapor atmosphere thick enough to capture stellar UV photons. This escape rate rules out the possibility of a H-He envelope as it would escape in only ~1 Ma. A water vapor atmosphere would escape in ~ 1 Ga, and thus it is a plausible scenario. The origin of CoRoT-7b cannot be inferred from present observations: It may have formed rocky; or be the remnant of a Uranus-like ice giant, or a gas giant with a small core that was stripped of its gaseous envelope.

💡 Research Summary

CoRoT‑7b, discovered by the CoRoT mission, is a remarkable example of an ultra‑short‑period super‑Earth with a radius of 1.68 ± 0.09 R⊕, a mass of 4.8 ± 0.8 M⊕, and an orbital period of only 0.854 days. This paper combines knowledge from hot‑Jupiter studies, atmospheric escape theory, and interior structure/evolution models to assess the planet’s composition, structure, and evolutionary history.

First, the authors compare the measured mass–radius pair with a suite of interior models. Within the 1σ error bars, a purely rocky planet with an Earth‑like Fe‑silicate ratio (≈33 % iron, 67 % silicates) fits the data if the mass is at the high end (≈5.6 M⊕) and the radius at the low end (≈1.59 R⊕). The nominal values (4.8 M⊕, 1.68 R⊕) are also compatible with a “iron‑poor” silicate world, where the iron fraction is significantly lower than Earth’s. Thus, the current uncertainties do not allow a decisive discrimination between an Earth‑like core‑mantle proportion and a more reduced iron content.

Second, the possibility of a volatile envelope is examined. A thin layer of high‑temperature water vapor (or super‑critical water) comprising 3 % to up to 10 % of the total mass can reproduce the observed radius while leaving an Earth‑like rocky nucleus underneath. Because the planet receives an extreme stellar flux (≈2 × 10⁴ W m⁻²) and its equilibrium temperature exceeds 2000 K, any such vapor envelope would be in a state of continuous evaporation and re‑condensation, forming a dense, optically thick “steam” atmosphere that absorbs the bulk of the stellar UV photons.

Third, the authors calculate the hydrodynamic escape rate using energy‑limited escape formalism, taking into account the intense extreme‑UV (EUV) irradiation from the host star. The resulting mass‑loss rate is ≈10¹¹ g s⁻¹, essentially independent of the mean molecular weight of the atmosphere. Consequently, a hydrogen‑helium (H‑He) envelope would be removed in less than a few million years, far shorter than the system’s age (≈1–2 Gyr). In contrast, a water‑vapor envelope would survive for roughly a gigayear, making it a plausible present‑day composition. The escape rate also implies that even a bare rocky planet would develop a transient equilibrium vapor atmosphere thick enough to capture stellar UV photons, ensuring that the loss process continues regardless of the underlying bulk composition.

Fourth, three formation/evolution pathways are discussed. (1) The planet formed as a rocky body and migrated inward, retaining only a thin steam atmosphere generated by surface vaporization. (2) It began as an ice‑giant analogue (Uranus/Neptune‑like) with a substantial water‑rich mantle; intense irradiation stripped away most of the H‑He envelope, leaving a water‑rich core capped by a modest vapor layer. (3) It originated as a gas‑giant with a relatively small solid core; photo‑evaporation removed the massive H‑He envelope, exposing the core and a residual water‑rich envelope. Each scenario requires different initial masses and compositions, but the present observations cannot uniquely favor one over the others.

The study highlights that for ultra‑short‑period super‑Earths, atmospheric escape is governed primarily by stellar EUV heating and is largely insensitive to atmospheric composition. This insight provides a robust framework for interpreting future transit spectroscopy of similar planets: detection of water‑related spectral features would support the vapor‑envelope hypothesis, whereas the absence of any detectable atmosphere would be consistent with a completely stripped rocky core. Moreover, CoRoT‑7b serves as a benchmark for bridging the gap between hot Jupiters and hot super‑Earths, illustrating how extreme irradiation can sculpt planetary interiors and atmospheres over gigayear timescales.