Susceptibility of planetary atmospheres to mass loss and growth by planetesimal impacts
Approximately 20% of nearby stars host planetesimal belts orbiting 10s of au from the star, many of which have been shown to have similar composition to Solar System comets. Hot dust seen in several systems may be evidence that some of these planetesimals are perturbed into the inner regions, an interpretation which is reinforced by the detection of CO close to the CO2 sublimation radius in one system. Any planets in the inner regions of these systems, such as those in the habitable zone, will thus undergo bombardment. This can have a significant influence on their atmospheres, both through stripping of primordial atmospheres and delivery of volatiles to replenish a secondary atmosphere. Here we show how the parameter space of planet mass versus semimajor axis is divided into planets that are able to grow significant secondary atmospheres, and those for which any atmosphere would be depleted in impacts. Whether atmospheres grow or deplete significantly also depends on the total bombardment level, which is discussed in the context of observations of dust observations and how comet delivery is influenced by the outer planetary system architecture. Finally we consider application to the TRAPPIST-1 system which shows how, even for comet populations that are depleted below the level at which they can be detected in infrared observations, the outer planets in this system should have secondary atmospheres while those closer in should be absent.