Abstract
Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently, they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra solar systems. In the solar system, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential compositional properties between solar system comets and exocomets to allow for the development of new observational methods and techniques. The paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. The compositional properties of solar system comets and exocomets are summarized before providing an observational comparison between them. Exocomets likely vary in their composition depending on their formation environment like solar system comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of solar system comets. Observations of gas around main sequence stars, spectroscopic observations of “polluted” white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with solar system comets. The recent interstellar visitor 2I/Borisov showed gas, dust and nuclear properties similar to that of solar system comets. This raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and solar system comets.
Highlights
Solar system comets are small bodies containing volatiles which sublimate on close approach to the Sun, creating a cloud of dust and gas
In this paper we provide an overview of the observational properties of solar system comets and exocomets and compare their similarities and differences
Compared to solar system comets, the information we have about exocomets is very limited
Summary
Solar system comets are small bodies containing volatiles which sublimate on close approach to the Sun, creating a cloud of dust and gas. Together with asteroids they are regarded as the unused building blocks of the solar system and much is to be gained by studying their composition and evolution as they provide important clues to the formation of the solar system (A’Hearn 2017; Eistrup et al 2019). The exocomet, known as 2I/Borisov, showed gas, dust and nuclear properties similar to that of solar system comets, but was enriched in CO (Bodewits et al 2020; Cordiner et al 2020)
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