Computer simulations put Solar System in its place
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: August 8, 2008

Traditional theories of Solar System formation assume our neighbourhood to be pretty run of the mill, but in a new study using data from 300 exoplanets discovered orbiting other stars, our planetary haven turns out to be one of a kind.

Before the discovery of the first exoplanets, our own Solar System's planets were the only ones known to us, so astronomers had no reason to think it unusual. But thanks to powerful computer simulations based on the data acquired from 300 exoplanets discovered over the last 15 years, theoretical astronomers from Northwestern University have been able to understand planetary system formation in a much broader sense, and have made the alarming discovery that planets being thrown mercilessly into the Sun or jettisoned into deep space could have been the destiny of our Solar System had its initial conditions been just a slightly bit different.

"Now we know that other planetary systems don't look like the Solar System at all," says Frederic Rasio. "The shapes of the exoplanets' orbits are elongated, not nice and circular. Planets are not where we expect them to be. Many giant planets similar to Jupiter, known as 'hot Jupiters,' are so close to the star they have orbits of mere days. Clearly we needed to start fresh in explaining planetary formation and this greater variety of planets we now see."

The 'Goldilocks' simulations. Left: the protoplanetary disc that is too massive; right: the disc which is not massive enough, and centre: the 'just right' conditions that lead to the formation of rocky and gaseous planets in a configuration similar to our own Solar System. Image: Thommes et al. Science, 8 August, 2008.

The simulations are the first to consider the formation of planetary systems from start to finish, and focus on the growth of planets from the generic disc of gas and dust left behind after the formation of a central star, and the subsequent gravitational interactions of the planets as they evolve. From around one hundred simulations, the average planetary system was found to be full of chaos, violence and drama that could not have produced the comparatively calm nature of our Solar System. In fact, just six percent of the simulations produce a result that “broadly resembles the Solar System,” says Edward Thommes, lead author on the paper published in the journal Science this week.

The simulations show the nascent gas disc that gives birth to the planets forcing them callously toward the central star, where they crowd together or are engulfed. And if the planet escapes that ordeal then there is always the cut-throat competition for gas among growing planets, a chaotic process that produces a rich variety of planet masses. Or, if a planet encroaches upon another planet’s territory, they frequently become locked into dynamical resonances that drive the orbits of all participants into increasingly elongated loops around the parent star. Such gravitational elbowing can result in a slingshot encounter that flings the planets across the system like a giant game of planetary pinball, with unlucky planets being ejected off the game board completely and out into deep space.

"Such a turbulent history would seem to leave little room for the sedate Solar System, and our simulations show exactly that," says Rasio. "Conditions must be just right for the Solar System to emerge."

The familiar collection of planetary bodies that make up our Solar System, but is this the norm for solar system formation? New simulations conducted by theoretical astronomers in America and Canada suggest not. Image: NASA/CXC/SAO.

This Goldilocks porridge of ‘just right’ ingredients falls largely to the mass of the original gas disc. Too massive a gas disc and planet formation is an anarchic mess, producing hot jupiters and noncircular orbits galore. Too low mass a disc, and nothing bigger than an ice giant like Neptune, with only a small amount of gas, will grow. These extremes explain the properties of some of the strange exoplanet systems already uncovered, but in reality, it is our own Solar System’s serene behaviour that sticks out like a sore thumb.

“What we'd really like to know is, what is the real range of initial properties in those discs?” says Thommes  “We picked a broad range for our simulations that we believe covers all possibilities, which means that the real distribution sits somewhere inside that range. To actually predict a number for the percentage of Solar System analogues, we need to know what that distribution is; until then, all we can say is that Solar Systems are ‘uncommon’.”

The researchers propose that systems like ours result right near the boundary between discs that form the gas giants like Jupiter and Saturn, and discs that only produce small planets, nothing bigger than Neptune. “When this happens, the planets which do manage to form never have their orbits changed very much by interaction with the gas disc, simply because by the time they're born, there isn't all that much gas left,” Thommes tells Astronomy Now. “This also means the biggest planets aren't pushed toward each other, and so never invade each others' ‘personal space’, avoiding strong gravitational interactions. It's exactly that sort of gravitational elbowing which is the cause for eccentric gas giant orbits seen in most of our simulations, and in most of the exoplanetary systems which have been discovered to date.”

And when it comes to the chances of finding other habitable exoplanetary systems, Thommes thinks that the questions we need to ask next are: how important is the larger planetary system environment, i.e in our Solar System, how important are Jupiter and Saturn?  “For example, it's often pointed out that Jupiter acts as ‘bouncer’ to protect us against comets from the outer reaches of the Solar System. But many of those comets were put out there in the first place by Jupiter,” he says. “So the question is a bit tricky, but the good news is, a lot of the clues are right in our own back yard!”

The results of the 100 simulations are presented in this week's issue of the journal Science.