Earth-sized planets are elusive. Of the thousands of worlds discovered in other star systems, only a couple of hundred are within about 20 percent of Earth’s diameter. And fewer than a dozen are within 20 percent of Earth’s mass, which is how astronomers categorize planets.

That could be because Earth-sized planets are fairly small and lightweight. That makes them tougher to find than big, heavy planets similar to Jupiter and Saturn. Or perhaps it’s because Earth-sized worlds just aren’t that common. It’ll take a lot more planet discoveries to let us know for sure.

Planets that are similar to Earth’s size and mass probably are built a lot like Earth, too. They’re probably dense balls of rock and metal. Whether they have conditions like Earth depends on several factors, including the type of star they orbit and how far from the star they are.

As an example, consider the closest Earth-sized planet, which appears to orbit a star that’s 12 light-years away. Known as GJ 273c, the planet is about 20 percent heavier than Earth. It’s so close to its parent star, though, that surface conditions are much too hot for life like that on Earth.

Finding an Earth-sized planet with Earth-like conditions is the prize goal for planet hunters. Such worlds are considered the best places to look for signs of life — planets that could be “twins” to our own life-filled world.

We’ll have more about exoplanets tomorrow.

Script by Damond Benningfield