The planets of the solar system come in three varieties: the rocky planets, like Earth; the giant planets, like Jupiter; and dwarf planets, like Pluto. The first two groups are well known. Except for Pluto, though, the third group is pretty much anonymous to everyone except scientists who study them.

Astronomers created the dwarf-planet category in 2006, after they began finding bodies similar to Pluto in the outer system. A dwarf planet must orbit the Sun and not some other object, and it must be mostly round. Today, the count varies. But most lists include at least eight members. And some say that scores of known objects could fit the category, with hundreds more awaiting discovery.

One definite dwarf planet is Ceres, in the asteroid belt, between the orbits of Mars and Jupiter. All the others are beyond Neptune, the most remote major planet. Most of them are in the Kuiper Belt – a doughnut that extends billions of miles beyond Neptune.

Pluto is the largest dwarf planet, and the closest to the Sun. The second largest is Eris. It’s actually a little heavier than Pluto, which means it’s also a little denser. We’ll have more about Eris tomorrow.

Most of these distant little worlds have icy surfaces. But a few could have active interiors, with possible oceans of liquid water. That means at least some of them are possible habitats for life – in the deep freeze of the outer solar system.

Script by Damond Benningfield