There are more than 4,000 known exoplanets, bewildering in their variety. Some are familiar to us, worlds analogous to Venus or Neptune. Others are like nothing in our own solar system, so we describe them as hot Jupiters, carbon planets, and super-Earths. Their names are an alphabet soup, derived from star catalogs, telescopic sky surveys, and space telescopes: CoRoT-7b, Kepler-10c, and TRAPPIST-1a. What are these worlds that orbit other stars?



Fifteen types of exoplanets are currently listed in the Unified Astronomy Thesaurus, and a few more names pop up in the literature. These types aren’t distinct; exoplanets can be described in more than one of these different ways. Five of the exoplanet types refer only to size (mass and diameter). In order, from smallest to biggest, they are rocky planets; super-Earths; mini-Neptunes; ice giants; and gas giants. Planets in the super-Earth and mini-Neptune size range may be ocean planets, having liquid-water oceans hundreds of kilometers deep. Some rocky planets in especially carbon-rich systems may be ultra-dry carbon planets, having rocks formed of compounds of silicon and carbon, different from our solar system’s silicon-and-oxygen rocks.

There are several exoplanet types that depend both on a planet’s size and its location within a stellar system. Hot Jupiters and hot Neptunes are big, gassy planets in tight, fast orbits around their stars. Traveling so close, they’d be tidally locked, with a hot hemisphere permanently facing the star and a cold hemisphere permanently facing out. (Objects close to large gravity sources get stretched a little bit, which slows their spin rate to match their orbital period. This is why we always see the same side of the Moon from Earth.)

There are cold Jupiters and cold Neptunes as well, orbiting farther out, just like the planets they’re named after. Lava planets are superdense larger-than-Earth worlds in close, hot orbits; they don’t seem to occur farther out. Some of them, known as Chthonian planets, are likely the remnant cores of evaporated hot Jupiters. (The name, pronounced “THO-nee-an,” refers to the underworld.) Habitable planets are Earth-sized ones that orbit at a distance from their star where liquid water could be stable, meaning they could be habitable to life as we know it, under the right circumstances. Every star has a habitable zone, a range of distances in which surface liquid water could be stable. If stars are small and dim, habitable planets can have close orbits and be tidally locked.

Finally, any size of planet can be found in a few especially strange locations. There could be Trojan planets, smaller co-orbital companions to larger planets, though none have been certainly identified yet. (Our solar system has Trojan asteroids co-orbiting with many of the planets, but they are all far too small to be planets.) Pulsars—ultradense, rapidly spinning neutron stars with unimaginably intense magnetic fields—are such weird stellar objects that pulsar planets get their own moniker, too. And finally there are free-floating planets, also known as rogue planets, which have been ripped free from their stars through an unlucky gravitational encounter with a more massive object and journey alone through the galaxy. They are hard to find and we’ve found only one so far, but there are probably lots of them out there.

Now that you’re acquainted with the different types of exoplanets, let’s look at some of the better-known examples and see what kinds they are.