An exocomet, also known as an extrasolar comet, is a comet that originates from outside the Solar System. Exocomets are believed to be relatively rare objects, with only a handful of confirmed detections to date. Nevertheless, their study is important for understanding the formation and evolution of planetary systems beyond our own.
Exocometary activity has been detected around a number of different types of stars. Young, hot stars are thought to be the most likely hosts for exocometary activity, as they are more likely to have debris discs from which comets can form. However, exocomets have also been found around more evolved stars such as Beta Pictoris and Fomalhaut; in these cases it is thought that the comets may have been scattered inward by planets orbiting further out in the system.
The first confirmed exocomet was discovered around Beta Pictoris in 1992. Since then, a handful of other candidates have been identified through direct imaging or indirect evidence such as spectral features or variations in a star’s brightness (known as “transits”). The study of exocomets continues to be an active area of research, particularly with the advent of new observational facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA).
There are several proposed mechanisms for the formation of exocomets. One possibility is that they form in protoplanetary discs around young stars, similar to how comets form in our own Solar System’s Kuiper Belt. Another possibility is that they are formed through tidal interactions between planets and passing stars or stellar clusters (a process known as “dynamical capture”). It is also possible that some exocomets may be interstellar objects that have simply been scattered into planetary systems by chance encounters.
The composition of exocomets can provide valuable insights into the conditions under which they formed. For example, measurements of carbon monoxide (CO) and water vapour (H2O) emission from an exocomet around HD 172555 suggest that it contains material dating back to the time when its host star was still forming in its natal molecular cloud (“primordial” water). This provides strong evidence that at least some exocoments do indeed form in protoplanetary discs around young stars.
Exocoments offer a unique opportunity to study planetary systems beyond our own Solar System up close. By studying their composition and dynamics we can gain insights into planet formation processes and learn about the diversity of planetary systems in our Galaxy