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PLANET-MAKING This image shows three scenarios of how planets form in a disk. The left shows the violent version that produces “hot Jupiters” on eccentric orbits, which the authors suggest are typical. The right shows another typical, gas-strapped version where nothing bigger than Neptune grows. The middle version shows the conditions just right for producing planetary systems similar to the solar system.Image courtesy of Science, AAAS

Goldilocks isn’t the only one who demanded everything to be “just right.” The Earth and its fellow seven planets also needed perfect conditions to form as observed, and those right conditions occur rarely, a new computer simulation shows.

The new simulation, described in the Aug. 8 Science, is the first to trace from beginning to end how planetary systems form from an initial gas disk encircling a baby star.

“The really striking result of the new model is how chaotic and even violent the average story of a planet’s birth is,” says Edward Thommes, an astrophysicist now at the University of Guelph in Ontario, Canada.

The process is typically a big mess. “Planets get into each others' ‘personal space,’ gravitationally scattering each other. They compete with each other for gas from the disk that gives birth to them and lots of planets are lost along the way,” he says. “It's almost like reality TV.”

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BIRTHING EARTHThis artist's illustration shows a star with dusty rings, which could coalesce to form a rocky planet similar to Earth. New theoretical models suggest this type of planet formation is rare. Full StoryNASA, JPL-Caltech

All this violence and cutthroat competition seems to reduce the chances of forming a sedate solar system similar to the one in which the Earth lives, Thommes and his colleagues from Northwestern University in Evanston, Ill., conclude in the new study.

The simulation, based on data from the 307 exoplanets discovered to date, finds that initial gas disks last for a few million years — up to 10 million, at most. During that time, gas planets such as Jupiter and Saturn grow into giants, and rocky planets and icy planets coalesce too.

Modeling the growth of the planets and the gravitational interactions between them showed that if the disk that nurtures the planets has lots of gas, the system comes out containing a high number of “hot Jupiters,” which are similar to Jupiter but bigger and more gaseous. These extra-giant planets also tend to have eccentric, or extremely oblong, orbits, a result that matches current observations of exoplanets. Such orbits are not typical of planets in the solar system. Too little gas in the disk will produce nothing bigger than Earth-like planets, or possibly a few Neptunes, “ice giants” with little gas.

“An amount of gas in between those two is where there’s a relatively narrow range where systems like ours are the end result,” Thommes says. But, he cautions, although analogues to the solar system are less common, the team did still see a handful of them after 100 simulations.

“Although we may be weird, we’re by no means unique,” he says.

Shigeru Ida, a Tokyo Institute of Technology astrophysicist who models planetary formations, agrees. “This simulation has a good potential to be a powerful tool to explore the origin of extrasolar planetary systems and our solar system,” he says.

But planet formation consists of many different processes that leave many uncertainties in the theoretical modeling of these events, says Ida, who was not involved in the new research. Therefore, “it is too early to draw a statement such as ‘the solar system is special,’ ” he says.

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