COROT-7b is an super-Earth extrasolar planet orbiting the yellow dwarf star COROT-7. It is located about 520 light years away from Earth in the constellation of Monoceros. The planet was first reported in February 2009. Until the announcement of Kepler-10b in January 2011, it was the smallest exoplanet to have its diameter measured, at 1.58 times that of Earth, which gives it a volume 3.95 times that of Earth's. It was the first potential extrasolar terrestrial planet ever found. The planet has an extremely short orbital period and revolves around its host star every 20 hours.
Discovery[]
COROT-7b was found by observing its parent star's periodic decrease in apparent magnitude caused by the planet's transit in front of the star as seen from Earth. Measuring this dip in brightness, together with a size estimate for the star, allows one to calculate the planet's size. (See Transit method.) The space mission COROT observed the star COROT-7, in the stellar field LRa01, from October 15, 2007 to March 3, 2008. During this period, 153 periodic transit signals of 1.3 h duration with a depth of 3.4 x 104 were registered. After 40 days of data acquisition, the Alarm mode pipeline algorithm detected the shallow signal of COROT-7b, starting the follow-up observations from the ground to get a confirmation of the planetary nature of the transiting object.
Characteristics[]
Although COROT-7b's mass remains uncertain, its radius and orbital period are well known. It orbits extremely closely to its host star (1/23rd the distance from the Sun to Mercury) with an orbital period of 20 hours, 29 minutes, and 9.7 seconds. It has a radius of 1.58 Earth radii. At the time of its discovery, it had the shortest orbit of any planet known, although shorter orbits have been since found.
COROT-7b has a maximum surface temperature between 1800 and 2600 °C (3300 to 4700 °F). Because of its extremely high temperature, it is speculated that the planet may be covered in lava. The composition and density of the planet, though weakly constrained, suggests COROT-7b is a rocky planet like Earth. It could belong to a class of planets that are thought to contain up to 40% water (in the form of ice and/or vapor) in addition to rock. However, the fact that it formed so close to its parent star may mean that it is depleted of volatiles. The planet is likely tidally locked to its star, meaning a portion of the planet permanently faces the star, while the other half is in a permanent darkness. Temperatures and geologic conditions on the other sides of the planet may be dramatically different. Theoretical work suggests that COROT-7b may be a chthonian planet (the remains of a Neptune-like planet from which much of the inital mass has been removed due to close proximity to its star). Other researchers dispute this, and conclude COROT-7b was always a rocky planet and not the eroded core of a gas or ice giant, due to the young age of the star system.
Any departure from circularity of its orbit (due to the influence of host star and neighboring planets) could generate intense volcanic activity similar to that of Io, via tidal heating.
A detailed study of the extreme properties of COROT-7b has been published, concluding that, despite the mass uncertainty, the planet is Earth-like in composition. The extreme proximity to the star should prevent the formation of a significant atmosphere, with the dayside hemisphere as hot as the tungsten filament of an incandescent bulb, resulting in the formation of a lava ocean. The researchers propose to name this new class of planets, COROT-7b being the first of them, "lava-ocean planets".
Model of the interior[]
The uncertainty in COROT-7b mass does not allow precise modelling of the planet structure. Nevertheless, educated guesses could still be put forth. Assuming a 5-Earth-masses planet, the planet was modeled to have convection in the mantle with a small core with no more than 15% the mass of the planet, or 0.7 Earth mass. The lower mantle above the core-mantle boundary has more sluggish convection than the upper mantle because the greater pressure causes fluids to become more viscous. The temperature of the upper convecting mantle is different from one side of the planet to the other with lateral temperature differences for downwellings up to several hundred kelvins. However, the temperature of the upwelling is unaffected by downwelling and surface temperature variations. On the permanent dayside of the tidally locked planet where the surface temperature is hot from continuously facing its sun, the surface takes part in convection, which is the evidence that all the surface of this hemisphere being covered in oceans of lava. On the permanent nightside, the surface is cool enough for the formation of the crust with pools of lava above the convective mantle with intense volcanism. The dayside of the planet has larger convection cells than the nightside. Researchers also investigated the physical state of the interior of COROT-7b, indicating as likely a solid iron core, thus a self-generated magnetic field should be absent on the planet.
Atmosphere[]
Because of the high temperatures on the illuminated side of the planet, and the likelihood that all surface voliates have been depleted, silicate rock vaporization may have produced a tenuous atmosphere.
A study suggests that it may "rain" pebbles onto the surface of the planet.