Sorry, Super-Earth Fans, There Are Only Three Classes Of Planet
“What’s really interesting is how the mass/radius relationship changes for these three different classes of world. Up to about double the Earth’s mass, or a size just ~25% larger than Earth’s radius, you have an opportunity to be Earth-like, with thriving life on the surface. Beyond that, you’ll have an enormous hydrogen/helium envelope, and be much more akin to Neptune, Uranus or Saturn. In other words, what we’ve been classifying as “super-Earths” aren’t anything like Earth at all, but are instead gas giant worlds, expected to be wholly inhospitable to life on their surfaces.”
Thanks to NASA’s Kepler mission, we’ve discovered literally thousands of worlds that lie beyond our own Solar System. Surprisingly, the majority of them aren’t like anything we have in our own backyard, but are somewhere in between Earth and Neptune in terms of size and mass. These worlds, usually broken into categories like “super-Earths” and/or “mini-Neptunes,” have often been viewed as new categories of planets, along with the “super-Jupiters” that we don’t see here, either. Yet these classifications are purely arbitrary, based on what we’ve seen and how we classified planets in our own neighborhood. What would we get if we classified them based on the properties that they actually possess, like mass and radius? We’d find, quite surprisingly, that there are only three classes of planet: Terran, Neptunian, and Jovian-like worlds.