Observations of Exocomets

Abstract

Active small bodies in extrasolar systems, the extrasolar analogues of Solar System comets, provide insights into the orbital evolution and physical processes shaping planetary systems. Since the discovery of exocomets around $β$ Pictoris, these small, icy bodies have shown the potential to become key probes for understanding planetary formation and migration. This review presents an overview of current observational techniques used to detect exocomets, focusing on individual systems and large-scale searches. We discuss photometric methods that identify exocomet transits through asymmetric light curves and spectroscopic techniques revealing cometary gases via time-variable absorption lines. Despite progress, significant open questions remain regarding the physical properties, occurrence rates, and similarities between exocomets and their Solar System counterparts. This review explores future opportunities in observational exocomet research, highlighting advancements required to further our understanding of these active small bodies and their role in the context of planetary system evolution.

Figures (4)

• Figure 1: Illustration of an exocomet transit.
• Figure 2: Examples of exocomet transit detections using photometry (top) and spectroscopy (bottom). The lower plots also show deep narrow absorption from stable circumstellar gas, which indicates the systemic velocity (20 km s$^{-1}$). The plots on the left are adapted from Lecavelier1999 and Beust1990, showing the model predictions for both observation techniques. The plots on the right show exocomet detections using real data, agreeing with the model predictions. Top-right: grey lines are superimposed Transiting Exoplanet Survey Satellite Ricker2015 data showing two exocomet detections in photometry with different depths from LecavalierDesEtangs2022. Bottom-right: the plot is adapted from 2014Natur.514..462K showing superimposed High Accuracy Radial velocity Planet Searcher 2003Msngr.114...20M spectra focused on the Ca ii K line in grey and a reference spectrum i.e. free of any exocomet features in red. Any variable absorption features are attributed to exocomet transits.
• Figure 3: Example of gas absorptions in HD 110058 observed under different resolutions. The spectrum shows the wide rotationally broadened stellar absorption signature, a narrow and deep circumstellar disc absorption, and a slightly wider interstellar absorption just blueward of the circumstellar disc line. Left: HARPS observation, $R\approx115000$, absorptions are resolved. Middle: FEROS observation, $R\approx48000$, absorptions are fairly resolved and detected at lower intensity. Right: X-Shooter observation, $R\approx5500$, absorptions are no longer resolved and detection is difficult.
• Figure 4: Synthetic phase-angle dependence of the linear polarisation degree in a form suggested by 1993LPICo.810..194L.