Artist’s rendition of Gliese 876d, the first super-Earth to be discovered, if it is a solid planet. Credit: Trent Schindler, National Science Foundation.

The easiest planets to find are the biggest ones. The larger a planet is, the larger the effect on its parent star, which is what we can see. It’s also easiest to spot planets close to their stars, since they move faster there. This is why most of the earliest exoplanets discovered were hot jupiters.

But more sensitive instruments have allowed us to find smaller and smaller planets by spotting ever-tinier changes in the light from their stars. We now know that hot jupiters are not very common, although many of the smaller planets are still pretty hot.

By small, I don’t mean really small planets, like Mars and Mercury. Almost all of the planets we’ve found are still bigger than Earth, though often not by much. The next larger planet than Earth in our Solar System is Uranus, which is 14.5 times Earth’s mass, but Neptune is slightly smaller in diameter and has a more serious-sounding name, so astronomers like to call the warmest exoplanets that are Neptune-sized “hot neptunes“.

The first hot neptune to be discovered was Mu Arae c in 2004. It’s a little smaller than Neptune at about 11 times Earth’s mass, and it’s hotter than Mercury, but like Neptune, it’s expected to be gaseous. (We don’t know for sure because we don’t know how big across it is, but what we know about planet formation tells us that Mu Arae c is probably a gas giant.)

But the hot neptunes were just an opening act for a weird new class of planets that has been getting a lot of attention lately: the super-Earths. The “Earths” part doesn’t mean that they’re Earth-like, but just that they are solid, like Earth. The “super” part means that they’re bigger than Earth, but in-between Earth and Neptune, like in the example below.

Size comparison of Earth, Neptune, and the super-Earth CoRoT-7b. Credit: Aldaron (Wikipedia).

Nothing like that exists in our Solar System, and we still don’t know much about the super-Earths. We only know the diameters of a few of them, from things like the Kepler spacecraft, and we don’t really know what they’re made of. Some of them are definitely rocky, like Earth, and some of them are definitely gaseous “mini-neptunes”, but some of them are in the middle, where it’s not so clear. They could be rocky or gaseous, or even made of water–more on that later.