What makes Earth so special?

Plate tectonics and water are inextricably linked. Not only does plate tectonics enable liquid water to exist by way of regulating the temperature, but many scientists have argued water enables plate tectonics to happen.

"Without water the planet would be geologically dead," said Caltech's Mike Brown, discoverer of the newly reclassified "plutoid" object named Eris, which lies beyond Pluto in our solar system. "Water is what lubricates plate tectonics, which is what leads to the extreme difference between continents and seafloors, the large amount of earthquakes and volcanoes, fresh mountain-building. Venus has no water, no plate tectonics, no deep sea floor, no steep mountains, no continents, probably few earthquakes or volcanoes. A much less geologically interesting place!"

Another "just-right" aspect of Earth is its size: If it was much smaller, it wouldn't be able to hold on to our precious atmosphere, but much larger and it might be a gas giant too hot for life.

The presence of our big brother planet, Jupiter, farther out in the solar system blocking Earth from much of the incoming debris, has also helped Earth become a safe haven for life. Jupiter acts like a giant broom, sweeping the solar system of debris — rocks as small as cars and as huge as moons — that could snuff out life in one fatal blow. This protective effect was particularly helpful in the solar system's early years, when Earth still got pummeled but, scientists say, not nearly as bad as would have been the case without Jupiter.

A friendly moon
Life on Earth may also owe a debt to our nearest celestial neighbor, the moon.

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Earth's moon stabilizes our planet's rotation, preventing drastic movements of the poles that could cause massive changes in climate that some scientists think could have doomed any chance for budding life to form or evolve.

The moon also helpfully pulls the ocean's tides, which scientists suggest might have been the perfect place for early life to begin evolving to survive on land.

Though Earth has the necessary ingredients for life, it's unclear whether the development of life here might have been a one-time fluke, or if it's something that happens pretty much everywhere the conditions are right.

Rare Earth
All of these features make Earth special among known planets near and far.

"You hear all the time how Earth-like Mars is, but if you were taken to Mars you wouldn't feel happy there at all," said University of Washington astronomer Don Brownlee, author of the book "Rare Earth" (Springer, 2003). "It's not Earth-like. And Titan, when the [Huygens] probe landed, there was all this stuff in the media about how Earth-like it is. Earth-like? It is completely different. It has all this methane on the surface. Venus has about the same mass [as Earth], almost the same distance from the sun. But it's a totally different place — no oceans, no plate tectonics — and it's not a place you would want to be."

So far, we haven't seen any planet outside the solar system come very close to Earth either.

Of the nearly 300 new worlds glimpsed elsewhere in the galaxy, most are "hot Jupiters" — large planets that orbit close to their stars, on which life and liquid water are unlikely to exist.

"I doubt that in our galaxy typical stars have planets just like Earth around them," Brownlee said. "I'm sure there are lots of planets in the galaxy that are somewhat similar to Earth, but the idea that this is a typical planet is nonsensical."

Brownlee's view may be in the minority, however.

Not-so-special Earth
As our planet-hunting technology improves, many planet hunters expect to find Earth's twin. The search has led scientists to debate whether Earth is really as special as we think it is.

"In the past 10 years, everything has been pointing in the direction of, 'Hey, the solar system, which we thought was unique, is not unique at all,'" said Alan Boss.

Boss and many other scientists think it's likely that some form of life exists on some of those countless other planets out there.

"Certainly there will be other planets that support life," he said. "I think life is actually quite common. I think we're going to find there are literally billions of them in the galaxy."

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