Mini-Neptunes. Super-Earths. There’s a huge diversity of exoplanets out there, many of them unlike anything we have in our Solar System. So how does a single physical process—the aggregation of bodies within a disk of gas and dust—produce so many different outcomes?

That’s a question tackled by a paper in this week’s Nature Astronomy. An international team of researchers has modeled the formation of planets early in the history of exosolar systems. And they find it’s possible to radically change the water content of planets based on the amount of a radioactive element present in the material forming the exosolar system. The difference, they suggest, can determine whether a system is filled with ocean worlds or whether it winds up looking more like our own Solar System.

Wet or dry?

We already have some idea of what sets the level of water on a planet. The material in a planet-forming disk is heated both by collisions among its material and from the inside-out by the star once it ignites. Different materials will freeze out at specific distances from the star, creating multiple snow lines for water, carbon dioxide, methane, and more. Depending on which side of the snow lines an exoplanet forms, it will have more or less of these materials.