All comets might share their place of birth, new research says. For the first time ever, astronomer Christian Eistrup applied chemical models to fourteen well-known comets, surprisingly finding a clear pattern. His publication has been accepted in the journal Astronomy & Astrophysics.
Comets: balls of ice or more?
Comets travel through our solar system and are composed of ice, dust, and small rock-like particles. Their nuclei can be as large as tens of kilometres across. ‘Comets are everywhere, and sometimes with very funky orbits around the Sun. In the past, comets even have hit the Earth,’ Christian Eistrup says. ‘We know what comets consist of and which molecules are present in them. They vary in composition, but are normally seen as just one group of icy balls. Therefore, I wanted to know whether comets are indeed one group, or whether different subsets can be made.’
A new take on comets
‘What if I apply our existing chemical models to comets?’, Eistrup thought during his PhD at Leiden University. In the research team, which included Kavli Prize winner Ewine van Dishoeck, he developed models to predict the chemical composition of protoplanetary discs – flat discs of gas and dust encompassing young stars. Understanding these discs can give insight into how stars and planets form. Conveniently, these Leiden models turned out to be of help in learning about comets and their origins.
‘I thought it would be interesting to compare our chemical models with published data on comets,’ says the astronomer. ‘Luckily, I had the help of Ewine. We did some statistics to pin down if there was a special time or place in our young solar system, where our chemical models meet the data on comets.’ This happened to be the case, and to a surprising extent. Where the researchers hoped for a number of comets sharing similarities, it turned out that all fourteen comets showed the same trend. ‘There was a single model that fitted each comet best, thereby indicating that they share their origin.’
And that origin is somewhere close to our young Sun, when it was still encircled by a protoplanetary disc and our planets were still forming. The model suggests a zone around the Sun, inside the range where carbon monoxide becomes ice – relatively far away from the nucleus of the young Sun. ‘At these locations, the temperature varies from 21 to 28 Kelvin, which is around minus 250 degrees Celsius. That’s very cold, so cold that almost all the molecules we know are ice.
‘From our models, we know that there are some reactions taking place in the ice phase – although very slowly, in a time-frame of 100,000 to 1 million years. But that could explain why there are different comets with different compositions.’
But if comets come from the same place, how do they end up in different places and orbits in our solar system? ‘Although we now think they formed in similar locations around the young Sun, the orbits of some of these comets could be disturbed – for instance by Jupiter – which explains the different orbits.’