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Using novel gas observations to probe exocomet composition

Space missions like the Rosetta space probe built by the European Space Agency, that recently reached and studied the comet 67P/Churyumov-Gerasimenko, address the profound question of how life came to Earth by quantifying the composition of comets in the Solar System. Comets are made of the pristine material from which planets were formed. By exploring the composition of comets, we can thus access the pristine composition of the building blocks of planets. Such experiments can also tell us whether a planet is likely to be habitable as, for instance, water is thought to have been delivered to Earth by asteroids/comets. Using comets as probes of the primordial Solar System leads to the exciting prospect of addressing the even broader question “can the physical characteristics, including possible habitability, of exoplanets be determined by measuring the composition of the comets in exoplanetary systems, i.e. exocomets?”

Recently, we gave predictions of the amount of gas (created from these exocomets) surrounding nearby planetary systems for which we can detect exoplanets. Using ALMA, a new facility able to detect tens of new systems with exocometary gas, we started comparing our predictions to actual observations to deduce the composition of exocomets in these extrasolar systems. From this, we eventually obtained significant constraints on the composition of exocomets, and thus on the material from which exoplanets were formed, in a large number of planetary systems.

The complexity of our method arises in actually detecting exocomets; imaging individual comets is difficult even in our own Solar System because these objects are cold and dark, but one solution is to observe signals from large systems of comets. In the Solar System, comets are constantly colliding within the Kuiper belt (a ring of comets just beyond Neptune), producing large amounts of dust and gas from the pulverised rock and ice. The same process occurs in other stars that are similar to the Sun, and the dusty remnants of these collisions have been observed for more than 30 years now. However, the gas component – which is critical for determining the amount of ice in comets – has remained too faint to be observed.

An extra-solar system composed of five planets and a belt composed of exocomets (similar to the Kuiper belt in our Solar System). Graphic by Amanda Smith, Institute of Astronomy. Used with permission.

This problem has been resolved recently with the ALMA interferometer, located in the Atacama Desert in Chile, which can now detect the gas in these extrasolar Kuiper belts. We recently showed the feasibility of extracting the composition of exocomets by observing extrasolar Kuiper belts (via emission lines of key species such as CO, oxygen, or carbon), and we now propose to extend this experiment to a plethora of systems to determine their cometary rock-to-ice ratios and compare with our own Solar System.

To do so, we predict the amount of gas around each planetary system that hosts its own version of a Kuiper belt. CO gas is released from the exocomets at the belt location. Due to strong impinging UV photons, CO is destroyed into carbon and oxygen atoms that then spread all the way to the star. The CO and atomic gas discs can be observed with ALMA, and with our model, we can quantify the gas production rate of these belts and then compare to what is observed. Doing this comparison allows us to fit the composition of comets in each system. Thanks to the ALMA facility, we hope that the number of gas detections will greatly increase in the next few years. Our work is the first step towards understanding the composition of exocomets for a large number of different planetary systems. And this will generate the first significant constraints on the building blocks of exoplanetary systems and will provide a census of the diversity of compositions beyond our own Solar System.

Featured image credit: Comet falling into white dwarf (artist’s impression) by NASA, ESA, and Z. Levy (STScI). CC BY 4.0 via Wikimedia Commons.

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