The Scorched Magma Ocean and Vaporized Rock Atmosphere of Super-Earth CoRoT-7b
Orbital Mechanics and Tidal Locking
Orbiting its host star at a distance of just 0.0172 astronomical units—roughly 2.5 million kilometers, or more than 20 times closer than Mercury orbits our Sun—CoRoT-7b completes a single orbital revolution in a mere 20 hours and 29 minutes. At this extreme proximity, gravitational tidal forces have brought the planet into a state of synchronous rotation, or 1:1 tidal locking. One hemisphere is condemned to perpetual stellar radiation, while the opposing hemisphere faces the permanent cold of deep space.
This tidal locking creates a profound environmental asymmetry. The dayside sub-stellar point experiences unrelenting solar flux, generating surface temperatures estimated between 2,300 and 2,600 Kelvin (over 2,000 degrees Celsius). Conversely, the nightside hemisphere, lacking atmospheric heat redistributive capabilities, plummets to thermal equilibrium temperatures around 50 to 70 Kelvin. The resulting thermodynamic engine drives extreme atmospheric and geological phenomena across the planetary surface.
Physical Dimensions, Mass, and Internal Architecture
Spectroscopic and transit observations yield precise physical parameters for CoRoT-7b. The world possesses a radius approximately 1.58 times that of Earth and a mass measured at roughly 4.8 Earth masses. These dimensions correspond to a bulk density of approximately 6.6 grams per cubic centimeter, slightly higher than Earth's average density of 5.51 g/cm³. This high density indicates an internal structure dominated by heavy inorganic material, primarily a large metallic iron-nickel core surrounded by a dense silicate mantle.
Geochemical models suggest that the severe thermal environment of CoRoT-7b has stripped away any primordial envelope of light volatile elements, such as hydrogen, helium, or water ice, that might have existed early in its formation. What remains is a stripped refractory core—a massive, ultra-dense rocky body undergoing continuous, intense thermal erosion on its illuminated face.
The Dayside Magma Ocean and Mineral Vaporization
On the permanently illuminated hemisphere of CoRoT-7b, temperatures far exceed the melting points of common rock-forming minerals, including olivine, pyroxene, and basalt. Consequently, the dayside surface does not consist of solid rock, but rather a continuous, churning ocean of liquid silicate magma hundreds of kilometers deep. The convective overturning of this magma sea constantly circulates superheated liquid material across the illuminated face.
At temperatures above 2,300 Kelvin, silicate liquid actively transitions into gaseous form through direct thermal vaporization. This process generates a tenuous, high-density mineral atmosphere. Unlike Earth's atmosphere composed of nitrogen and oxygen, or Venus's heavy carbon dioxide envelope, the atmosphere of CoRoT-7b is composed entirely of vaporized rock. Spectroscopic models indicate high concentrations of gaseous sodium, potassium, silicon monoxide, atomic oxygen, magnesium, and iron. This mineral vapor envelope operates under extreme pressure gradients driven by the extreme temperature delta between the illuminated and dark sides.
Atmospheric Circulation and Silicate Precipitation
The intense thermal heating at the sub-stellar point forces the superheated mineral vapor to expand rapidly and flow toward the cooler regions near the planetary terminator—the dividing line between night and day. As these mineral winds cross into the lower-temperature zones, the atmospheric dynamics change abruptly. Silicates and metallic elements reach their saturation vapor pressures and begin to condense out of the gas phase.
This condensation leads to the formation of high-altitude mineral cloud decks composed of microscopic liquid and solid droplets of enstatite, iron, and silica. As these clouds drift further toward the twilight boundary, the temperature drops rapidly, causing the condensed minerals to precipitate out of the sky. On CoRoT-7b, weather manifests not as liquid water or methane rain, but as a perpetual precipitation of solid rocky hail, corundum crystals, and molten silicate droplets falling back into the margins of the magma ocean or accumulating along the cold crust of the terminator.
The Rigid Nightside Terranes
Beyond the terminator lies a drastically different realm. The permanent nightside of CoRoT-7b is dominated by a frozen, highly fractured crust of solid basaltic and ultramafic rock. Lacking direct stellar irradiation, heat transport from the dayside magma ocean via internal thermal conduction is insufficient to maintain liquid surface conditions across the entire nightside.
However, the extreme temperature gradient at the terminator line generates severe thermal stress within the lithosphere. Sub-surface tectonic fractures, cooling cracks, and potential magmatic intrusions penetrate the nightside crust. Gravitational tidal flexure—caused by slight orbital eccentricities induced by companion planets in the system, such as CoRoT-7c—may additionally generate internal tidal heating, sustaining sub-crustal magmatic activity and localized volcanism across the dark hemisphere.
Scientific Significance of CoRoT-7b
CoRoT-7b serves as a benchmark laboratory for understanding planetary structure and atmosphere-crust interactions under extreme conditions. It demonstrated that terrestrial planets could exist in close-in orbital regimes previously thought to be occupied exclusively by gas giant worlds. The survival of CoRoT-7b's heavy iron-silicate core provides critical empirical constraints on theories of atmospheric photo-evaporation, mass loss, and the evolutionary pathways of Super-Earth planets orbiting close to high-energy host stars.
