One of the key issues with discovering that Bennu is made of bits of an ocean world and that asteroids had to bring water into the inner solar system is that it implies things had to move around a lot. This problem is exacerbated when we look at alien solar systems and see gas giants snuggled next to stars and a lack of planets with masses between Earth and Uranus. Something has to be going on that shuffles worlds around and somehow kills off super-Earths.
Analysis of the systematic data acquired by the Kepler Space Telescope shows a distinctive lack of planets around two Earth-radii. These planets were detected as they passed in front of their home stars. By looking at how long it takes the planets to go from not in front of their stars to totally in front of their stars, scientists can measure their radii.
And while we see plenty of super-Earths and quite a few sub-Neptunes, the 2-Earth-radii systems just aren’t there in expected numbers.
By combining models that discuss how planets behave at different distances from their Sun with an analysis of possible planetary migration routes, researchers led by Remo Burn discuss how cold planets moving inwards can expand out past two Earth-radii as their ices melt and their atmosphere expands out… and potentially gets blasted away. In the other direction, a world moving outward will shrink as the ice in its atmosphere freezes and the overall world shrinks. No matter where you start, these 2-Earth-radii worlds are not long for this universe once they start to move.
This work appears in Nature Astronomy.
To understand better if this theory is the final answer, we’re going to need to better understand what worlds are made of. As JWST continues its work and is joined by large ground-based friends, we can get the needed data to confirm we’re on the right track.