Exocomets: Now you see them, now you don't

Minor bodies such as Kuiper Belt objects, comets, and asteroids constitute the rocky and icy debris left over from the planet building phase of our solar system. The existence of reservoirs of small rocky bodies (i.e., asteroids/planetesimals) in orbits around young stellar systems is now well established, with their presence being required by current (exo)planetary formation theories. The initial proto-planetary disks that contain the reservoir of dust and gas required to form (exo)planets are short lived (<< 1 Myr) and thus the circumstellar debris disks observed around young stars of ages 10 – 50 Myr must be being continually replenished by collision and evaporation events amongst planetesimals. In such systems, the gravitation field associated with the newly formed exoplanets can potentially enable the disruption of large numbers of these kilometer-sized icy bodies into trajectories directed towards the young central star.

Present technology does not enable us to view images of these kilometer-sized infalling bodies, but the evaporation of gaseous products liberated from exocomets that occurs close to a star can potentially cause small disruptions in the ambient circumstellar disk plasma. For circumstellar disks that are viewed “edge-on” this evaporating material may be directly observed through transient (night-to-night and hour-to-hour) gas absorption features seen at rapidly changing velocities. Using high resolution spectrographs mounted to large aperture ground-based telescopes, we have discovered 15 young stars that harbor swarms of exocomets. In this lecture we briefly describe the physical attributes of comets in our own solar system and the instrumental observing techniques to detect the presence of evaporating exocomets present around stars with ages in the 10 – 100 Myr range. We note that this work has particular relevance to the dramatic fluctuations in the flux recorded towards “Tabby’s star” by the NASA Kepler Mission, that may be explained through the piling up of swarms of exocomets in front of the central star.