The more scientists learn about the conditions that make life possible on Earth, the more they realize how complex those factors are -- and how a relatively small change in one condition or another could have rendered the planet uninhabitable, Solomatov said.

"It's a very finely tuned system," he said. "Some of the factors are well known, but we still don't know what all the factors are."

Solomatov has a key part in a NASA-funded astrobiology research project aimed at better understanding the origin of life on Earth and the conditions in which life might be found elsewhere in the universe. The five-year, $4.9 million grant supports the work of a dozen researchers, headed by a team at the University of Washington.

The scientists come from a variety of fields, because life requires much more than water and the right mix of elements to survive and evolve into higher forms.

Solomatov's part of the project focuses on the role of plate tectonics -- the geologic process that results in the shifting of Earth's continental and oceanic plates. Only in recent years have scientists recognized the importance of plate tectonics in maintaining Earth's long-term temperature stability, through global recirculation of carbon dioxide from the planet's interior into the atmosphere, he said.

"Because carbon dioxide is a greenhouse gas, it helps to keep our planet warm," he said. "Of course, too much of it is not good, but without this cycle over the centuries the temperature would drop and you might have the 'Snowball Earth' scenario."

Plate tectonics also provides diverse geological environments, like mountains, which promote biodiversity, Solomatov said.

No other planets are known to have plate tectonics, although some may have had the feature earlier in their evolution, he said.

Whether plate tectonics might be essential to the development of higher forms of life is unknown, but Solomatov's theoretical modeling of the complex processes aims to shed light on a number of key questions, including: What planetary conditions allow for the formation of plate tectonics? Are oceans necessary for plate tectonics? When and how did plate tectonics begin on Earth?

The question of life on other planets, or even the habitability of other planets, has long captured our imagination, but we tend to be biased in our assumptions, Solomatov said.

"We think this is normal and there should be planets all around the universe like Earth," he said. "The more I work in this area, the more I realize the chances really are very slim."

It's not enough for a planet to be the right size, to have water, and to be located the right distance from a star of the right size. Without the giant planet Jupiter as a neighbor, and without our moon, Earth might not be the living planet that it is, Solomatov said.

Jupiter has protected Earth from too many cataclysmic asteroid collisions, he explained -- but on the other hand, a neighbor much larger that Jupiter would not allow formation of an Earth-like planet in the first place. Similarly, our moon is just the right size to help stabilize Earth's spin axis and, as a consequence, the Earth's climate. With a bigger moon or no moon at all, a planet similar to Earth in other respects might not sustain life.

The list of critical factors grows longer as scientists learn more.

"At the moment there are two camps of believers," Solomatov said. "One believes in the 'Rare Earth' hypothesis and the other thinks life is smart and can adapt to extreme conditions."

The "Rare Earth" hypothesis, which takes its name from a book by University of Washington scientists Peter Ward and Don Brownlee, holds that microbial life might be common in planetary systems, but advanced life is rare.

If pushed into one camp or the other, Solomatov would choose the "Rare Earth" believers.

"We don't have enough data yet but all the evidence we have now points out that the Earth is a very special place," he said. "Maybe we should take better care of our planet."

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