Exocometary Activity Around Stars at Different Evolutionary Stages: Current Issues

Modern theories of planetary system formation predict a large population of planetesimals, which are remnants of the primordial matter of the protoplanetary cloud and, at the same time, embryos of small bodies that we observe in our solar system. The planetesimals play an important role in the dynamic and physical evolution of the planetary system. Gravitational scattering of planetesimals which are enriched with volatile elements might cause the volatile and organic compounds to enter the interior of the planetary system, triggering the formation of planetary atmospheres and further development of life. Small bodies inside planetary systems can evaporate due to the increasing insolation at close distances from the mother star, leading to the development of activity akin to the activity of comets in our solar system. The study of cometary activity in our solar system is aimed primarily at the investigation of the physical processes in the early stages of the development of the protoplanetary cloud. Until recently, it was not possible to study small bodies in other planetary systems because their small size makes it very difficult to detect by direct methods. Over the past 10 years, two space missions, Kepler and TESS (The Transiting Exoplanet Survey Satellite), have been equipped for continuous photometric monitoring to find exoplanets by transit, i. e. monitoring the changes in brightness of a star due to the passage of a smaller object across its disk. The presence of high-precision photometric measurements of the luminosity curves of about 200 000 stars, which are available in the public domain, potentially makes it possible to identify rather small changes in the luminosity curves of stars due to the passage of a body with gas-dust coma (exocomet) across the stellar disk. The article considers a number of issues related to the discovery and study of exocomets. The main detection methods based on the analysis of photometric and spectral series of observational data of space missions as well as ground-based observational complexes are considered. We provide a brief review of the main projects devoted to the results of theoretical modeling and experimental studies of the manifestations of exocometary activity. Known cases of manifestations of exocometary activity in planetary systems of stars of different spectral classes are described and the main characteristics of such stars and their planetary systems are given. We discuss the prospects for further research of these still very exotic objects. The importance of such research for understanding evolutionary processes in our own solar system is emphasized.