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

For the hot exoplanets CoRoT-24b and CoRoT-24c, observations have provided transit radii R_T of 3.7 ± 0.4R_⊕ and 4.9 ± 0.5R_⊕, and masses of ≤ 5.7M_⊕ and 28 ± 11M_⊕, respectively. We study their upper atmosphere structure and escape applying an hydrodynamic model. Assuming R_T ≈ R_PL, where R_PL 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 <100 Myr, while R_PL shrinks till the escape rate becomes less than or equal to the maximum possible EUV-driven escape rate. For CoRoT-24b, R_PL must be therefore located at ≈1.9-2.2R_⊕ and high altitude hazes/clouds possibly extinct the light at RT. Our analysis constraints also the planet's mass to be 5-5.7M_⊕. For CoRoT-24c, R_PL and R_T lie too close together to be distinguished in the same way. Similar differences between R_PL and R_T may be present also for other hot, low-density sub-Neptunes.