Astronomers have long puzzled over a strange and abundant type of planet found across the galaxy—known as “mini-Neptunes.”
These worlds are smaller than Neptune but larger than Earth and seem to be among the most common planets in the universe.
Yet, despite their abundance, there’s nothing quite like them in our own solar system.
For years, scientists believed most mini-Neptunes were covered by vast oceans of molten rock—essentially entire planets made of lava.
But a new study led by Professor Eliza Kempton from the University of Chicago, published in The Astrophysical Journal Letters, challenges that idea.
The research suggests that many of these planets might actually have solid surfaces, even though they’re cloaked under crushingly thick atmospheres.
“This really upends a paradigm about these planets,” said Kempton. “We’re trying to understand what these objects are, because they don’t exist in our solar system.”
Mini-Neptunes are thought to be made of a mixture of rock, metal, and gas, with atmospheres dominated by hydrogen, helium, and possibly water vapor.
They orbit close to their stars, making them hot—far too hot for life as we know it.
However, what lies beneath those thick clouds has remained a mystery because these planets are so distant that even powerful telescopes can only detect their presence indirectly.
Astronomers learn about exoplanets—planets orbiting stars beyond our Sun—by measuring tiny dips in starlight when a planet passes in front of its star or by studying how starlight filters through its atmosphere. They can also estimate a planet’s mass by observing the small wobbles it causes in its host star’s motion.
Originally, scientists assumed that the intense heat and pressure on these planets would melt rock into magma, forming global “lava oceans.”
But when Kempton and her colleagues—including first author Bodie Breza, then an undergraduate student, and Dr. Matthew Nixon, now at Arizona State University—took a closer look, they found a different picture emerging.
The turning point came from new observations of GJ 1214 b, a well-studied mini-Neptune orbiting a star in the constellation Ophiuchus. Using data from the James Webb Space Telescope (JWST), the researchers found that the planet’s atmosphere contained heavier molecules than previously thought. This suggested that the atmosphere was much denser and thicker than scientists had expected.
Under such extreme pressure, the rock beneath wouldn’t remain molten. Instead, it would re-solidify—much like how carbon becomes diamond deep inside Earth’s mantle. In other words, the immense weight of the atmosphere might crush the molten layer back into solid rock.
Intrigued by this discovery, the team ran simulations of many mini-Neptunes under different conditions.
Their models revealed that a significant number of these planets could have solid surfaces rather than seas of lava. Whether a mini-Neptune is a molten or solid world depends on its specific combination of temperature, atmospheric pressure, and composition.
“It’s basically an either-or situation,” Kempton explained. “You can have a ‘floor-is-lava’ world, or a solid-surface planet—and which one you get depends on the atmosphere’s details.”
Understanding these worlds is vital because mini-Neptunes are so common. Their existence—and their diversity—forces scientists to rethink how planets form and evolve.
“Before we found exoplanets, we thought every solar system would look like ours,” said Nixon. “But they don’t. Mini-Neptunes show us that planetary formation can take many different paths.”
By uncovering the hidden natures of these mysterious planets, astronomers are not only solving one of the biggest puzzles in exoplanet science—they’re also gaining new clues about how our own world, and perhaps others like it, came to be.
Source: University of Chicago.
