Polite planetary system saved Earth from fiery end

f conditions had been slightly different when our solar system was formed, the Earth might have been engulfed by the sun or flung into deep space.

A new, large-scale computer simulation that traces how planetary systems are born shows just how special we are, says University of Guelph astrophysicist Edward Thommes, the lead author of a paper published in today's edition of the journal Science.

Planetary systems are formed from disc-shaped clouds of gas and dust that surround stars. But the process and final product vary wildly.

The birth of our solar system was relatively peaceful compared with the violent encounters between fledgling planets that shaped other systems in our galaxy.

"We think in most of these systems it was a turbulent and chaotic process, with planets getting into each other's personal space and elbowing each other.

"A nice peaceful and polite and well-behaved planetary system like our own only happens more rarely," Dr. Thommes said.

He and two colleagues from Northwestern University in Illinois, Soko Matsumura and Frederic Rasio, are the first to model the formation of planetary systems from beginning to end, a process that in real life can take 10 million years.

The researchers used data that astronomers have collected from exoplanets, the 300 planets discovered outside our solar system.

They combined it with information about the gas discs that give rise to planetary systems.

Powerful computers were used to see what kind of systems would have arisen from various types of gas discs. The results matched the kinds of planetary systems astronomers have discovered in the night sky.

Many of these systems look very different from our own and are home to bizarre planets.

One of the largest exoplanets, in the constellation Hercules, is about as dense as balsa wood.

Another is hotter than some stars, and absorbs so much starlight its surface is probably blacker than charcoal.

The computer simulations show that the chemical composition and mass of the gas disc makes a huge difference in what kind of planets form.

"There are some where we see one gas giant after another forming and cruising into the star, almost like a convoy. It is probably good that didn't happen in our solar system," Dr. Thommes said.

As one planet approaches another, the gravitational pull they exert changes their orbits. Sometimes planets will get flung into deep space.

But in our solar system, said Dr. Thommes, the planets, including the gas giants Jupiter and Saturn, were relatively spread out, and far enough from the sun.

"Everyone has their personal space."

Until the early 1990s, the only known planets in the universe were the nine circling the sun.

(Two years ago, Pluto was demoted and that number dropped to eight.)

Most of the 300 exoplanets that have been detected are gas giants, big enough for astronomers to find using space- or land-based telescopes.

One technique involves looking for a star that is wobbling because of the gravitational pull exerted by an unseen planet. Astronomers can measure the size of the planet from the size of the wobble.

The technology is evolving, but mostly bigger planets have been spotted. They may have smaller, rockier, Earth-like neighbours that can't yet be detected, Dr. Thommes said.

"We think that any disc will produce some number of these guys."

But it is unclear what kind of neighbourhood is required for a planet to be capable of sustaining life. Jupiter and Saturn, for example, protect the Earth from incoming comets. Some systems don't have gas giants, he said, and produce puny planets like Neptune or Earth.

"Could that still support life?"

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