A recent study led by Professor Eliza Kempton at the University of Chicago has challenged previous assumptions about a prevalent type of planet in the galaxy known as “mini-Neptunes.” These planets, which are slightly smaller than Neptune and composed of rock, metal, and thick atmospheres rich in hydrogen, helium, and possibly water, are common elsewhere in the universe but have no equivalent in our solar system.
Previously, scientists believed that mini-Neptunes were covered by global oceans of molten magma due to their high temperatures and dense atmospheres. However, the new research suggests that many of these planets may actually have solid surfaces. According to Kempton, "This really upends a paradigm about these planets, which is interesting because there are so many of them in the universe. At the bottom of it, quite literally, we’re trying to understand what these objects are, because they don’t exist in our solar system."
The study analyzed data from distant exoplanets—planets outside our solar system—using indirect observations such as changes in starlight when a planet passes in front of its star and atmospheric analysis through light filtering. The team focused on GJ 1214 b, a mini-Neptune orbiting a star in the constellation Ophiucus. Data from the James Webb Space Telescope indicated that this planet’s atmosphere contains larger molecules than previously thought. This would result in an atmosphere much heavier than Earth’s.
The pressure from such an atmosphere would be so great that it could turn molten rock into solid rock beneath the surface. The researchers created simulations with different planetary conditions and found that many mini-Neptunes previously thought to be lava worlds might instead have solid surfaces.
Kempton explained: "It’s an either-or. You can have this the-floor-is-lava scenario, or a solid surface, and you’re going to have to take into account a number of other factors about a planet’s atmosphere to try to figure out which regime it falls under."
Mini-Neptunes attract scientific interest due to their abundance and what they reveal about planetary formation processes. Postdoctoral researcher Matthew Nixon stated: "Before we found any exoplanets, we had a nice neat story about how solar systems form based on how our solar system formed. We thought that would apply to other solar systems. By following that logic, other solar systems should look like ours. But they don’t." He added: "It gets back to why are we here—how did Earth come to be? This is a really fundamental piece for us to understand both other planets and our own."
The study was published on November 5th in Astrophysical Journal Letters by Bodie Breza (first author), Matthew Nixon (now at Arizona State University), and Eliza Kempton.
Funding for this research came from the Alfred P. Sloan Foundation, NASA, and the Heising-Simons Foundation.