What are the chances of finding life on another planet?

"We don't know what the chances are [that life exists on other planets], because we haven't found any," says Seth Shostak, senior astronomer with the Search for Extraterrestrial Intelligence (SETI) Institute.

To support the kind of life we know, a planet or moon would at least need liquid water. Then it could support the most basic life forms, like bacteria. Mars may have enough water to support life. But Venus, for example, is too hot for liquid water.

A planet might need air to support more-complex beings that breathe, although if these beings were structured differently than we are they might not necessarily require oxygen like we do.

The galaxy holds more than a trillion planets. "1 have hope [that life exists beyond Earth]. But science, in the end, rests on data, not hopes," says Shostak.

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