Most of the planets discovered in other star systems aren’t considered good homes for life. They’re too hot, too cold, or too “gassy” to support life as we know it. But a few planets have better chances. They’re rocky worlds, like Earth. And most important, they’re in the habitable zone. That’s the distance from the parent star where temperatures are just right for liquid water — a key ingredient for life like that on Earth.

Defining the habitable zone is a bit tough. It depends on the size and temperature of the star, for example. A big, hot star has a wide zone, although it’s a long way from the star. A small, cool star, on the other hand, has a narrow zone that’s quite close to the star.

And conditions that might be habitable for one planet might not be for another. Rocky worlds that are more massive than Earth, for example, have stronger surface gravity. That would allow them to hold on to a thicker atmosphere, which could hold on to more heat — extending the size of its star’s habitable zone.

Uncertainties in these numbers mean there’s some wiggle room in the number of habitable-zone planets. But most lists include several dozen planets — up to perhaps 60 or more. And the list is likely to just keep growing, as planet hunters find more and more worlds that could be just right for life — planets that live in the habitable zone.

We’ll have more about life in the universe tomorrow.

Script by Damond Benningfield