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Are we alone in the cosmos? Or might there be intelligent life elsewhere?

Last week, NASA scientists discussed in very concrete terms the steps to discovering life elsewhere in the universe over the next decade or two.

This year is the 20th anniversary of the discovery of a planet around a star like our own sun, 51 Pegasi. Since then, ground-based surveys and NASA's Kepler satellite have discovered nearly 2,000 confirmed "exoplanets," and thousands more candidates await confirmation.

Many of these planetary systems are quite unlike our own solar system. Some have large planets like Jupiter that orbit their stars far closer than Mercury, the innermost planet in our solar system. But smaller rocky planets like Earth, though harder to find, appear to be even more abundant.

Life on Earth developed in its oceans about a billion years after the planet formed. That suggests that rocky planets with liquid water on their surfaces might also have developed primitive forms of life.

Life as we know it is carbon-based and requires liquid water. Astronomers define the "habitable zone" around a star as the region within which liquid water can exist on a planet's surface. Any closer to the star, the water will boil into vapor; any farther and the water freezes into ice.

Extrapolating from discoveries to date, astronomers estimate there are perhaps 40 billion Earth-like, habitable-zone planets in our Milky Way galaxy alone.

Of course, there is a difference between single-celled organisms -- which developed 3.8 billion years ago and remained the most sophisticated form of life for another billion years or so -- and mammals, which appeared about 200 million years ago. And then the humans, who have existed for only 200,000 years.

Intelligent life that can communicate via radio waves with other intelligent life is less than 100 years old here on Earth.

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