What makes Earth so special?

By Clara Moskowitz

updated 1:13 p.m. ET July 8, 2008

Earth is one special planet.

It has liquid water, plate tectonics, and an atmosphere that shelters it from the worst of the sun's rays. But many scientists agree our planet's most special feature might just be us.

"It's the only planet we know of that has life," said Alan Boss, a planet formation theorist at the Carnegie Institution of Washington in Washington, D.C.

Though other bodies in our solar system, such as Saturn's moon Titan, seem like they could have once been hospitable to some form of life, and scientists still have hope of eventually digging up microbes beneath the surface of Mars, Earth is still the only world known to support life.

"So far, we haven't found it anywhere else," said Alex Wolszczan of Pennsylvania State University, who co-discovered the first planets beyond our solar system. He agreed that life was Earth's single most impressive characteristic.

None of this is a revelation, but understanding what's special about Earth is crucial for finding other planets out there and predicting what they might be like.

The fact that Earth hosts not just life, but intelligent life, makes it doubly unique. And the planet's intelligent life (humanity) has even developed rockets that enable travel beyond the planet, said Gregory Laughlin, astrophysicist and planet hunter at the University of California, Santa Cruz.

"During the last half century, the planet Earth has fashioned together tiny pieces of the metal in its crust, and has flung these delicately constructed objects to all of the other planets in the solar system," Laughlin said, adding that these achievements should be counted as an exemplary trait of our planet.

"From our anthropocentric viewpoint, we naturally separate ourselves from the planet that we live on, but if one adopts the point of view of an external observer, it is the 'planet' (taken as a whole) that has done these remarkable things," he told Space.com.

Water World
To enable life, this most special of attributes, planet Earth has a number of ideal features. It is unique among planets in our solar system for having water in its liquid form at the surface, in an amount conducive to life evolving.

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"The most impressive attribute of the Earth is the existence and amount of liquid water on its surface," said Geoffrey Marcy, an astronomer at the University of California, Berkeley who has helped discover dozens of extrasolar planets. No one knows why Earth has the exact amount of water it does, which is relatively small considering that water molecules outnumber silicate molecules in the galaxy, he said.

"The Earth is remarkable for its precisely-tuned amount of water, not too much to cover the mountains, and not so little that it's a dry desert, as are Mars and Venus, our 'sister' planets," he said.

Goldilocks planet
Earth's water is also special in that it has remained liquid for so long. How has Earth been able to hold on to its oceans while those on other planets freeze or fry?

"Many details as to why Earth is the only planet with liquid water in our solar system need to be worked out," said Diana Valencia, a graduate student in Earth and Planetary Sciences at Harvard University. "Certainly the distance to the sun has made it possible. A planet much farther in would receive too much energy from the sun, and a planet too far out would quickly freeze."

Our planet's Goldilocks-like "just right" location in the solar system has helped, as has its system of plate tectonics — the slip-sliding movements of Earth's crust that are thought to have created the planet's towering mountain ranges and plummeting ocean depths.

"The fact that Earth has plate tectonics allows for the carbon-silicate cycle to operate over geological timescales," Valencia said. "With the carbon-silicate cycle, the levels of carbon in the atmosphere get regulated to keep the surface temperature around that of liquid water."

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