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Rare Earth: Did cosmic collisions reduce the chance for habitable planets?

File:AxialTiltObliquity.png

Earth's axial tilt as seen from sun/DNA-webmaster

At MSNBC (August 22, 2011), we are asked to consider, “Did cosmic collisions make habitable planets rare?”:

Long-ago collisions between clouds of gas and dust could explain why many alien solar systems have planets with strange, highly tilted orbits — and why habitable worlds may be rare in the universe, a new study suggests.

Most of the planets in our own solar system, including Earth, have relatively circular orbits and are lined up along a plane that isn’t tilted much from the sun’s equator. They also orbit in the same direction around the sun as our star spins.

But many other solar systems are not so neatly ordered, harboring planets that move in the opposite direction of their stars’ spin on highly tilted orbits.

And what if they didn’t?

Using computer models, the researchers showed that protoplanetary disks can become considerably tilted if they encounter another nearby cloud of material and suck in some of its mass. These collisions can also reverse the disks’ spin, leading to the odd orbits seen in many alien solar systems today.

Tilt. No life. For one thing, smaller, rocky planets that might harbour life then get chucked out into deep space, leaving gas giants close to the parent stars.

Our own solar system features a gentle tilt, a weighted planetary orbit average of 7 degrees, in relation to the sun’s equator. A precise collision …

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