Vol. 572
In section 10. Planets and planetary systems

On the correlation between stellar chromospheric flux and the surface gravity of close-in planets

by A.F. Lanza A&A 572, L6


Revisiting the correlation between stellar activity and planetary surface gravity

by P. Figueira, M. Oshagh, V. Zh. Adibekyan, and N. C. Santos A&A 572, A51


It has been noticed a few years ago that the chromospheric activity of stars with exoplanets appears to be correlated with the gravity of the planet itself. Stars with strong-gravity planets seem to be more active. Two papers published today revisit this problem. First, P. Figueira and collaborators revisit the problem with a dataset that is about three times larger than the original one. They confirm the correlation, but with a pinch of salt: the correlation appears significant when only relatively massive, short-period planets are considered but remains tentative when including the whole dataset. Second, A. Lanza proposes that this correlation (and also that it is strongest for close-in planets) may be explained by planet evaporation: Some of the matter lost by the planet will remain trapped for some time in the star's coronal field and can efficiently absorb photons emitted in the Ca II H & K lines, which are the indicators of chromospheric activity. In the continuum, however, the absorption is negligible. Planets with lower gravity tend to lose more mass and to more strongly suppress the indicators of chromospheric activity, thus explaining why their stars appear less active. This study shows that one should be careful when using chromospheric activity of stars as an age indicator. It motivates the detection and accurate characterization of many more exoplanetary systems in order to refine possible statistical tests.