Earth, Barely Habitable?

by Fraser Cain on January 9, 2008

Artist impression of a super Earth. Image credit: CfA
Our home planet has been often described in glowing, nurturing terms. A cradle for life, right in the goldilocks zone. But our planet is actually right on the edge of habitability. If it were any smaller, and a little less massive, plate tectonics might never have gotten started. It turns out, life needs plate tectonics.

Astronomers at the Harvard-Smithsonian Center for Astrophysics announced their research today at the Winter meeting of the American Astronomical Society. According to the team, plate tectonics only really get going when a planet gathers enough mass. And the Earth has just barely enough mass to enjoy plate tectonics.

“Plate tectonics are essential to life as we know it,” said Diana Valencia of Harvard University. “Our calculations show that bigger is better when it comes to the habitability of rocky planets.”

When a planet reaches a large enough size, huge chunks of the planet’s surface can float atop an ocean of boiling magma. These plates spread apart and crash into one another, lifting up gigantic mountain ranges like the Himalayas.

And without plate tectonics, we wouldn’t be here. The process enables complex chemistry and recycles carbon dioxide, which acts like a blanket to keep the Earth warm and hospitable for life. Carbon dioxide is locked into rocks, and then returned to the atmosphere when the rocks melt. Without this cycle, carbon dioxide would get locked away in rocks forever.

The researchers examined what would happen on different rocky planets. They looked at a range of planets, smaller than our planet, up to the so-called “super-Earths” – planets twice our size with 10 times the mass. Any bigger than that, and you start to get a gas planet.

According to their calculations, the Earth is barely habitable. If you get a planet with more mass, the plate tectonics really get rolling, and the carbon cycle becomes really active. A super-Earth could have globe-spanning rings of fire, bursting with hot springs and geysers. Life would have every opportunity to get started.

Of course, if we tried to visit a super-Earth, we’d find the gravity uncomfortable. We’d experience 3 times the gravity trying to walk around on the surface of the planet. Oh, my back.

But for native life forms, it would be paradise.

Original Source: CfA News Release


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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