Planetary migration and evolving atmospheres might hold the key to the puzzling "radius valley" in exoplanet data.
For years, astronomers have been baffled by a curious gap in the size distribution of planets beyond our solar system. This "radius valley" sees a scarcity of exoplanets about twice the size of Earth, sandwiched between smaller "super-Earths" and larger "sub-Neptunes."
New research, published in Nature Astronomy, suggests a novel explanation: planetary migration combined with the behavior of water in these distant worlds.
While atmospheric loss due to stellar radiation was previously considered the culprit, this study highlights the crucial role of planetary migration. Simulations show that icy sub-Neptunes, born farther from their stars, migrate inwards as they heat up. This causes their ice to melt, forming thick water vapor atmospheres, inflating their apparent size and contributing to the radius valley.
Meanwhile, rocky super-Earths closer to the star lose their atmospheres, effectively "shrinking" in size. Both processes, happening simultaneously, create the observed dearth of planets around two Earth radii.
This research marks a significant step forward, but further work is needed. Telescopes like James Webb and Extremely Large Telescope could help confirm the simulations by directly measuring planetary compositions. Understanding the behavior of water under extreme conditions remains crucial for accurate modeling.
If these findings hold true, the existence of "water worlds" with vast oceans in cooler regions becomes a possibility. Such planets could be prime targets for future searches for potential life beyond Earth.