Most Comets May be from other Solar Systems

The outer Solar System is full of comets from other planetary systems, say researchers, and this has implications for astronomers trying to explain the nature of these lonely interstellar wanderers.

The findings of a study led by planetary scientist Hal Levison, from the Southwest Research Institute in Boulder, Colorado, imply that some famous comets such as Halley, Hale-Bopp and McNaught might have originated around other stars in the early days of the Sun.

“People who study the chemical composition of comets have been trying to fit what they see into what they think was chemically happening in the proto-planetary disc around the Sun,” said Levison.

Not so lonely in space after all

“What we’re saying is: take a step back, because not all these guys may be from our Solar System.”

Most comets come from the Oort Cloud, a population of more than 400 billion comets lying nearly one light-year away - or roughly eight trillion kilometres from the Sun. This is equivalent to 50,000 times the distance between the Earth and the Sun and about a quarter of the way to Proxima Centauri, the nearest star.

It has long been thought comets in the Oort Cloud were originally part of the rocky debris surrounding Jupiter and Saturn, and were kicked into their present orbits by the gravity of the outer planets and nearby stars.

90% from another solar system

But traditional models have been unable to explain the abundance of comets in the Oort cloud, under-estimating their number by a factor of about 70.

“What we were interested in trying to explain was the large number of objects that we see in the Oort cloud today,” said Levinson.

“We calculate that number by looking at the number of what we call long period comets – these are some of the most famous and brightest comets in history.”

The study, published today in Sciencexpress, the online early edition of the journal Science, shows it is possible more than 90% of comets in the Oort Cloud were captured from other stars when the Sun spread apart from its ‘birth cluster’.

The Sun has sisters

Levinson said it is common for stars to be born in ‘clusters’, fed by a large cloud of gas held together by its own gravity. Around each of these stars will form comets, many of which will be stripped from the star by the gravity of the cluster.

When some of the stars grow old enough, they emit strong stellar winds that literally blow the gas from the cluster, destroying its gravitational hold on the stars. Once this occurs the cluster starts to disperse, and if a star leaves the cluster at the same velocity as a comet from another star, the comet can be captured.

Trillions of comets on average originally form around each star, and the study’s computer simulations show up to a quarter of this average can end up in an Oort Cloud.

Plenty of comets for everybody

The fact the Sun stole so many comets doesn’t mean other stars missed out on the chance to have an Oort Cloud of their own. “It’s not a zero sum game,” said Levison, and noted for all of the comets captured by stars when the cluster dispersed, there are many times more floating around in inter-stellar space.

Astronomer Simon O’Toole from the Anglo-Australian Observatory in Sydney said it is not a complete surprise the Oort Cloud contains comets from other solar systems, but the very large proportion of 'foreigners' estimated by the study was surprising.

“It’s a very intriguing result and an interesting idea, one that was actually suggested a while ago,” he said. “The great thing about the tremendous advances in computing power is that you can do calculations (like this) that you just couldn’t do, even 20 years ago.”