What *IS* the Radius of hot Neptune Corot-24b?

Identifying the "true" radius of the hot sub-Neptune CoRoT-24b by mass loss modelling

Authors:

Lammer et al

Abstract:

For the hot exoplanets CoRoT-24b and CoRoT-24c, observations have provided transit radii RT of 3.7±0.4 R⊕ and 4.9±0.5 R⊕, and masses of ≤5.7 M⊕ and 28±11 M⊕, respectively. We study their upper atmosphere structure and escape applying an hydrodynamic model. Assuming RT≈ RPL, where RPL is the planetary radius at the pressure of 100 mbar, we obtained for CoRoT-24b unrealistically high thermally-driven hydrodynamic escape rates. This is due to the planet's high temperature and low gravity, independent of the stellar EUV flux. Such high escape rates could last only for less than 100 Myr, while RPL shrinks till the escape rate becomes less than or equal to the maximum possible EUV-driven escape rate. For CoRoT-24b, RPL must be therefore located at ≈1.9−2.2 R⊕ and high altitude hazes/clouds possibly extinct the light at RT. Our analysis constraints also the planet's mass to be 5−5.7 M⊕. For CoRoT-24c, RPL and RT lie too close together to be distinguished in the same way. Similar differences between RPL and RT may be present also for other hot, low-density sub-Neptunes.

CoRoT-24: a new Transiting Multi-planet System

Transiting exoplanets from the CoRoT space mission: XXIV. CoRoT-24: A transiting multi-planet system

Authors:

Alonso et al

Abstract:

We present the discovery of a candidate multiply-transiting system, the first one found in the CoRoT mission. Two transit-like features with periods of 5.11 and 11.76d are detected in the CoRoT light curve, around a main sequence K1V star of r=15.1. If the features are due to transiting planets around the same star, these would correspond to objects of 3.7±0.4 and 5.0±0.5 R_earth respectively. Several radial velocities serve to provide an upper limit of 5.7 M_earth for the 5.11~d signal, and to tentatively measure a mass of 28+11−11 M_earth for the object transiting with a 11.76~d period. These measurements imply low density objects, with a significant gaseous envelope. The detailed analysis of the photometric and spectroscopic data serve to estimate the probability that the observations are caused by transiting Neptune-sized planets as >26× higher than a blend scenario involving only one transiting planet, and >900× higher than a scenario involving two blends and no planets. The radial velocities show a long term modulation that might be attributed to a 1.5 M_jup planet orbiting at 1.8~A.U. from the host, but more data are required to determine the precise orbital parameters of this companion.