“To raise new questions, new possibilities, to regard old problems from a new angle, requires creative imagination and marks real advance in science.” -Albert Einstein
When we first began looking for exoplanets, we were expecting that other solar systems would be like ours: with inner, small, rocky worlds and outer, large gas giants. What we found surprised us in a couple of ways. Not only could planets of any size appear anywhere, as they seem to, but the most common mass for a planet was somewhere between two and ten Earth masses, something we don’t have a single example of in our Solar System.
While a visual inspection shows a large gap between Earth-size and Neptune-size worlds, the reality is you can only be about 25% larger than Earth and still be rocky. Anything larger, and you’re more of a gas giant. Image credit: Lunar and Planetary Institute.
Does this mean something unusual happened in our Solar System? Did we have some oddity of our formation that prevented us from having the most common class of world? Up until this year, that was the consensus opinion. But thanks to a new paper by two Columbia University astrophysicists, we’re realizing that the big problem is not our Solar System, but our way of classifying planets.
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.