Solar systems like ours may be rare

By Jeff Hecht Our solar system is a Goldilocks among planetary systems. Conditions have to be just right for a disc of dust and gas to coalesce into such a set of neatly ordered planets, a new computer model suggests. Similar planetary systems are likely to be a minority in the galaxy, says model developer Edward Thommes of the University of Guelph in Ontario, Canada. Even so, if only 1% of the Milky Way’s hundreds of billions of stars have a terrestrial planet with a stable orbit in the habitable zone, the Earth could have plenty of company. Astronomers long thought planets orbited where they formed, with small terrestrial planets close to the star, gas giants near the middle, and smaller ice giants such as Neptune towards the edge of a ‘protoplanetary’ disc of gas and dust before it dissipated. But the discovery of “hot Jupiters” – gas giants orbiting close to their stars – since 1995 gave powerful evidence that planets could migrate. To understand why planetary systems differ, Thommes and colleagues decided to study how various types of protoplanetary discs evolved over time. Standard simulations need large amounts of computer time to model the complex interactions among multiple growing planets and their disc material. So Thommes reduced computing time by dividing the disc into a series of rings, rather than into smaller chunks, and by treating interactions between planets separately. This allowed runs to last 10 million simulated years – long enough for all the disc material that hadn’t collected into planets to fall into the star. The team used 100 different sets of disc properties – varying the disc’s mass from between 1% and 10% that of the Sun, as well as its viscosity. They found that discs with higher mass and viscosity tend to produce two or more gas giants which migrate inwards towards the star. Discs with lower mass and viscosity tend to dissipate earlier, producing larger numbers of much smaller planets, which jostle each other after the disc dissipates. Only in a few intermediate cases – six out of the 100 runs – did the model produce gas giants about as far from the Sun-sized model star as Jupiter is from our Sun. And in only one case did the gas giants resemble Jupiter and Saturn. This middle range “is more peaceful because there isn’t as much planetary migration and not quite as many planets forming”, Thommes told New Scientist. But he says the model can’t predict the exact fraction of solar-like planetary systems in the galaxy. That’s because planetary formation is a chaotic process, and initial disc conditions are poorly known. Other astronomers think the new study is an important advance. “The idea is probably qualitatively right,” says Hal Levison of the Southwest Research Institute in Boulder, Colorado, who calls Thommes’s technique “a breakthrough”. But he warns that the model needs a lot of refinement before it can produce solid estimates of the types of planetary systems. Journal reference: Science (vol 321,
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