  The chromospheric emission of stars with close-in transiting planets has been
found to correlate with the surface gravity of their planets. Stars with
low-gravity planets have on average a lower chromospheric flux. We propose that
this correlation is due to the absorption by circumstellar matter that comes
from the evaporation of the planets. Planets with a lower gravity have a
greater mass-loss rate which leads to a higher column density of circumstellar
absorption and this in turn explains the lower level of chromospheric emission
observed in their host stars. We estimated the required column density and
found that planetary evaporation can account for it. We derived a theoretical
relationship between the chromospheric emission as measured in the core of the
Ca II H&K lines and the planet gravity. We applied this relationship to a
sample of transiting systems for which both the stellar Ca II H&K emission and
the planetary surface gravity are known and found a good agreement, given the
various sources of uncertainties and the intrinsic variability of the stellar
emissions and planetary evaporation rates. We consider implications for the
radial velocity jitter applied to fit the spectroscopic orbits and for the age
estimates of planetary systems based on the chromospheric activity level of
their host stars.
