Most common planets in galaxy may have solid surfaces, not oceans

For years, astronomers assumed that mini-Neptunes—planets smaller than Neptune, scattered throughout the galaxy—were covered in global oceans of molten magma. A new study suggests they got that wrong.

Mini-Neptunes are strange objects. They're everywhere in the universe, yet our solar system has nothing like them. They're made of rock and metal wrapped in thick atmospheres of hydrogen, helium, and possibly water. And until recently, scientists thought their surfaces were basically lava worlds, baked by the heat pressing down from above.

But when Professor Eliza Kempton and her team at the University of Chicago took a closer look at the data—particularly observations from the James Webb Space Telescope—the picture shifted. They found that many of these planets likely have solid surfaces after all. The twist: that surface is only solid because the atmospheric pressure is so extreme it forces the rock to compress, much like carbon becomes diamond deep inside Earth.

The breakthrough came while studying GJ 1214 b, a mini-Neptune orbiting a distant star in Ophiucus. Recent telescope data suggested its atmosphere contains heavier molecules than previously thought—a denser blanket than anyone had calculated. That changes everything. Instead of a thin, hot layer above molten rock, you get crushing pressure that actually pushes the magma back into solid form.

"It's an either-or," Kempton explains. "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."

The team ran simulations with different atmospheric conditions and found that a significant portion of mini-Neptunes previously written off as lava worlds likely have solid ground beneath them—though standing on that ground would still be a terrible idea. The pressure would be crushing, the temperatures extreme.

Why does this matter? Mini-Neptunes are the most common planets we've found in the galaxy. Understanding what they're actually like is fundamental to understanding how planets form. Our solar system's neat story—rocky planets close to the sun, gas giants farther out—doesn't match what we're seeing elsewhere. These mini-Neptunes exist in a category that doesn't exist here, which means our whole model of planetary formation is incomplete.

"Before we found any exoplanets, we had a nice neat story," says Matthew Nixon, a co-author now at Arizona State University. "We thought that would apply to other solar systems. But they don't."

That gap in understanding matters because how planets form shapes what kinds of planets exist—and which ones might be habitable. The more accurately astronomers can map the landscape of distant worlds, the better they can search for places where life might actually exist.