“How vast those Orbs must be, and how inconsiderable this Earth, the Theatre upon which all our mighty Designs, all our Navigations, and all our Wars are transacted, is when compared to them.” -Christiaan Huygens
It was quite a surprise when we started discovering all the exoplanets that were out there. While there’s a big gap between a world like Earth and a world like Uranus or Neptune in our Solar System – 17 times the mass and 4 times the radius – most of the worlds out there fall in between these two types. Super-Earths and Mini-Neptunes are the most common.
The numbers of planets discovered by Kepler sorted by their size distribution, as of May 2016, when the largest haul of new exoplanets was released. Super-Earth/mini-Neptune worlds are by far the most common. Image credit: NASA Ames / W. Stenzel.
But it turns out that what we’re calling a “Super-Earth” is a total misnomer! These worlds may be larger than Earth, but they’re much more like Uranus or Neptune than they are like our own. They have big gas envelopes surrounding them, and can even hold on to their young hydrogen and helium. The only way out is to boil the atmosphere away, and if you do, you’ve got a rocky core that’s close enough to its star to be roasted.
An artist’s illustration of a hot Jupiter world. If you’re hot enough to boil off the atmosphere, you can wind up with a rocky Super-Earth, but the temperatures will be so high that you’ll roast your planet. Image credit: ATG medialab, ESA.