Are we not the only Earth out there?

If another Earth exists in the universe, wouldn't it need to look like, well, Earth? Sure, but the odds of finding a blue world exactly 7,926 miles (12,756 kilometers) across and tilted on its axis nearly 24 degrees seem about as remote as finding an Elvis Presley impersonator who looks good in sequined leather and can snarl out a tune better than the King himself.

It doesn't hurt to look, of course, and astronomers are doing just that. The idea isn't necessarily to find an exact match, but a close one. For example, astronomers have discovered several so-called "super-Earths" -- planets that are slightly larger than our home. Gliese 581g stands as a perfect example. It's about three times the mass of Earth, which makes it a far better match than planets as large as Jupiter or Saturn.

In fact, behemoths like Jupiter and Saturn are known as gas giants because they're nothing more than giant balls of hydrogen, helium and other gases with little or no solid surface. Gas giants, with their stormy, multicolored atmospheres, may offer spectacular sights, but they'll never make good digs. Smaller planets, including Earth and super-Earth lookalikes, are much more likely to become incubators of life. Astronomers refer to these pipsqueaks as terrestrial planets because they possess heavy-metal cores surrounded by a rocky mantle. Terrestrial planets tend to stick close to their host stars, which means they have smaller orbits and much shorter years.

Terrestrial planets are also more likely to lie in the Goldilocks zone. Also called the habitable zone or life zone, the Goldilocks region is an area of space in which a planet is just the right distance from its home star so that its surface is neither too hot nor too cold. Earth, of course, fills that bill, while Venus roasts in a runaway greenhouse effect and Mars exists as a frozen, arid world. In between, the conditions are just right so that liquid water remains on the surface of the planet without freezing or evaporating out into space. Now the search is on to find another planet in the Goldilocks zone of another solar system. And astronomers have a couple of tricks they're not afraid to use.