CoRoT-7b is the first characterised rocky super-Earth exoplanet and it orbits extremely close to its parent star, at a distance of only 2.56 million kilometres which translates to just 4.48 stellar radii of its parent star. CoRoT-7b is located so close to its parent star that the length of one year on this planet is a fleeting 20 hours and 29 minutes. The spin and orbit of CoRoT-7b are likely synchronized, resulting in a hemisphere of continuous daylight and a hemisphere of continuous night. CoRoT-7b is measured to have 1.58 times the diameter and 6.9 times the mass of the Earth.
CoRoT-7b is not expected to have any appreciable atmosphere as the scorching environment on the planet does not support the presence of significant amounts of volatiles that can make up an atmosphere. Any atmosphere on CoRoT-7b is expected to be extremely rarefied. Hence, the transport of heat by any planetary scale winds on CoRoT-7b will be unable to significantly change the temperature distribution on the dayside or provide heat to the nightside, leading to very low surface temperatures on the nightside of the planet. This enables a huge surface temperature difference between the dayside and nightside of the planet to be maintained.
At the sub-stellar point on the dayside hemisphere of CoRoT-7b, the estimated temperature is a roasting 2470 degrees Kelvin. The sub-stellar point on the surface of CoRoT-7b has a zenith angle of zero and on this spot the host star of CoRot-7b is always directly overhead, making the sub-stellar point the hottest spot on the surface of the planet. An ocean of molten rocks is believed to be present on the extremely hot star-facing hemisphere of CoRoT-7b. High temperatures of well over 2000 degrees Kelvin on most of the dayside hemisphere of CoRoT-7b mean that the viscosity of the molten rocks that make up the lava ocean is probably much closer to that of water that to that of Earth’s lavas.
In order to compute the extent of coverage of the lava ocean on CoRoT-7b, certain assumptions have to be made. If Coriolis forces are negligible, such a lava ocean will have radial symmetry around the sub-stellar point which enables its extent to be characterized solely by the zenith angle of the ocean’s shore from the sub-stellar point. The ocean’s shore is basically the location on the planet’s surface where the solidification of molten rocks begins to occur.
If the circulation within the lava ocean is extremely efficient in transporting heat, it could lead to an ocean with a uniform temperature. Assuming that the lowest possible temperature of such a lava ocean is 2150 degrees Kelvin, the zenith angle of the lava ocean’s shore will be about 75 degrees from the sub-stellar point. This corresponds to 37 percent of the planet’s surface area being covered by the lava ocean. This estimate of the ocean’s size is probably a maximum and it can be seen that lava ocean is limited to just the dayside of CoRoT-7b. This means that circulation within the lava ocean cannot carry any heat from the dayside to the nightside of the planet.
If heat transport within the lava ocean via circulation is not present, then the physical extent of the lava ocean on CoRoT-7b will be smaller. In this case, assuming that the solidification of molten rocks begins to occur at 2200 degrees Kelvin, the zenith angle of the lava ocean’s shore will be about 52 degrees from the sub-stellar point. This corresponds to 19 percent of the planet’s surface area being covered by the lava ocean.
Along the shores of the lava ocean, crystallization and condensation of molten rock can occur to create pieces of rocks that sink back to the ocean floor. Also, along the shores of the lava ocean, condensation of molten rock material onto the continental edges can cause the loaded continental edges to progressively sink as it base dissolves into the mantle of the planet. The transport of silicates from the melted base of the continental edges back to the ocean floor can close the circulation of materials. Compared to the Earth’s oceans, any form of wind driven waves on the lava ocean of CoRoT-7b will be very small due to the extremely rarefied atmosphere, the higher viscosity of lava as compared to water and the higher surface gravity of CoRoT-7b as compared to the Earth.
The nightside of CoRoT-7b will be extremely cold due to the lack of any form of mechanism that can efficiently transport heat from the dayside to the nightside of the planet. The only form of heating on the nightside of CoRoT-7b will be geothermal heating from the decay of radioisotopes within the planet. This leads to a surface temperature of between 50 to 75 degrees Kelvin on the frigid nightside of CoRoT-7b. The paper detailing this study is by Alain Leger et al (2011) and it is entitled “The extreme physical properties of the CoRoT-7b super-Earth”.
The existence of a lava ocean on CoRoT-7b should also be common to many small and very hot rocky planets that orbit extremely close to their host stars. A recently discovered planet called Kepler-10b has a lot of resemblance with CoRoT-7b, but its properties are expected to be even more extreme as it has a higher temperature at its sub-stellar point and possibly a larger lava ocean. To conclude, a new class of planets termed “lava-ocean planets” may be prevalent amongst small and very hot rocky worlds with ‘star-hugging’ orbits.