CoRoT-7c

CoRoT-7c is an extrasolar planet which orbits the G-type main sequence star CoRoT-7, located approximately 489 light years away in the constellation Monoceros. It is either a super-Earth or a Neptune-like planet, orbiting at 0.046 AU from the star, taking 3.7 days or 89 hours to make one round trip around the star.

CoRoT-7c

Discovery
Discovered by	Queloz et al.
Discovery site	La Silla Observatory, Chile
Discovery date	August 24, 2009
Detection method	Radial Velocity (HARPS)
Orbital characteristics
Semi-major axis	0.046 AU (6,900,000 km)[1]
Eccentricity	0
Orbital period (sidereal)	3.698 ± 0.003[1] d
Semi-amplitude	unknown[1]
Star	CoRoT-7
Physical characteristics
Mass	13.289±0.689[2] ME

Discovery

The discovery of the planet was announced in February 2009, during the First Corot Symposium. It was discovered during the follow-up started in order to confirm the existence of CoRoT-7b, a super-Earth uncovered by the CoRoT mission. However, unlike CoRoT-7b, it was not detected by the transit method from the CoRoT satellite, but only by the radial velocity method using HARPS from La Silla Observatory, Chile. A posteriori search of transits of CoRoT-7c in the lightcurve of the star CoRoT-7 yielded a negative result, confirming the planet is not transiting. As a consequence no radius measurement is available, and no density and structure models of the planet can be established.

Characteristics

Like CoRoT-7b, the mass of CoRoT-7c is weakly constrained, since the radial velocity data is noisy due to the presence of stellar activity. Published mass measurements range from 8.4 Earth masses^[1] to 13.5 Earth masses,^[3] passing through 12.4 Earth masses^[4] and 13.1 Earth masses.^[5] This mass range encompasses the Super-Earths to Neptunes transition, so the nature of CoRoT-7c, either a rocky planet or an ice giant, remains unclear. However, if the larger mass estimates are correct, CoRoT-7c is probably a hot Uranus-like planet. Chances are high that planet's rotation is tidally locked to the orbital period, with one side of the planet always facing the star CoRoT-7 and the other one in permanent darkness.

A third planet, CoRoT-7d, is present in the system. Strong mutual gravitational forces could be exchanged between these planets, leading to powerful tidal forces.

Doubts about existence

A published study^[6] cast doubt on CoRoT-7c's existence, arguing that the combined presence of stellar activity and additional errors on HARPS radial velocity measurements preclude a meaningful search for additional companions besides CoRoT-7b. However, all the other studies seem to confirm the planet's existence: the signal in the HARPS data of CoRoT-7c is detected in analysis of different type, does not seem to be correlated with stellar activity and is stronger than the signal associated with the confirmed CoRoT-7b. The remarkable stability and precision showed by the HARPS spectrograph^[7] also speaks in favour of the detection. The status of CoRoT-7c seems very well confirmed.