Astronomers have determined a way to tell the chemical composition of exocomets in a large number of nearby planetary systems for the first time, using the comparison of a new gas model to recent ALMA data.

Space missions – such as the Rosetta probe that recently studied the comet 67P/Churyumov-Gerasimenko at close quarters – can help address the profound question of how life came to Earth, by quantifying the chemical composition of comets in the Solar System. Comets are of prime importance as they are made of the same material from which the planets were formed; but because they are too small for geological processes to have taken place, and having lived most of their lives out in the deep freeze of space, they remain pristine examples of the pre-planetary conditions. Thus their composition directly traces the building blocks of planets – and as icy asteroids and comets are thought to have delivered water to the Earth, their properties also relate to whether or not  a planet is likely to be habitable. 

There are, of course, other planetary systems beyond our own, and so we can widen the idea of using comets as tracers of the properties of the primordial Solar System to other worlds. This leads to the exciting prospect of determining the physical characteristics of these systems, including the possible habitability of exoplanets, by measuring the composition of exocomets (comets contained in exoplanetary systems).

Recently, Quentin Kral, Luca Matra, Mark Wyatt and Grant Kennedy, a team of astronomers from the Institute of Astronomy (Cambridge) predicted the amount of gas released from such exocomets that would surround nearby known planetary systems.  With the advent of the Atacama Large Millimeter Array (ALMA), it is possible to collect observations of the exocometary gas in such systems, providing important comparisons to the predictions. It allows the astronomers to put 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 the science team’s method arises in actually detecting exocomets; imaging individual distant comets is very difficult in our own Solar System, as they are cold and dark but is totally impossible in exoplanetary systems. Instead one solution is to observe signals from an ensemble of comets. In the Solar System, comets are constantly colliding within the Kuiper belt (a ring of icy bodies just beyond Neptune), to produce large amounts of dust and gas that escape from the pulverised rock and ice. The same process occurs around other stars that are similar to the Sun, and the dusty remnants of these collisions have been observed for more than 30 years now as debris discs. Until recently, however, the gas component – which is critical for determining the amount of ice in comets – has remained too faint to be observed. This problem has now been resolved with the ability of the ALMA interferometer to detect the gas in the extrasolar Kuiper belts. 

The model they developed allows the astronomers to predict the amount of gas around each planetary system that is known to host its own version of a Kuiper belt. Comparison to the observed properties of each dusty belt allows them to quantify the rate of the production of gas from these belts. Comparing observations to predictions also provides a way to deduce the composition of comets in each system. Thanks to the ALMA facility, it is expected that the number of gas detections from exocomet belts will greatly increase over the next few years. Therefore, they now intend to extend this experiment to a plethora of other planetary systems, to determine their cometary rock-to-ice ratios for comparison with our own Solar System.

Then at last astronomers can make the first steps towards understanding how the composition of exocomets varies with the diversity of planetary systems in the Galaxy. This has the potential to generate the first significant constraints on the building blocks of solar systems beyond our own.

The research paper Predictions for the secondary CO, C and O gas content of debris discs from the destruction of volatile-rich planetesimals by Kral et al will be published in the MNRAS.

Local contact: Dr Quentin Kral, Institute of Astronomy, Cambridge University

Figure: (click to obtain high-resolution version)

An illustration of an extra-solar system composed of 5 planets and a belt composed of exocomets (similar to the Kuiper belt in our Solar System). 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 their model, the scientists can then assess the composition of exocomets releasing this gas (and compare the results to the composition of comets in our Solar System). Credit: Amanda Smith, Institute of Astronomy