A new study finds that many "mini-Neptunes" - perhaps the most common planets in the galaxy - are under so much pressure from their heavy atmospheres that the surface is likely compressed solid. Illustration courtesy NASA/JPL-Caltech/R. Hurt (IPAC)
As telescopes have become more powerful, it's turned out our solar system is not the only game in town: There are millions of other planets out there in the galaxy.
But we're still teasing out clues about what they are actually like.
One of the puzzles is a kind of planet that appears to be one of the most common types in the universe. Known as "mini-Neptunes" because they run a little smaller than Neptune in our solar system, these planets are made of some mix of rock and metal, with thick atmospheres mostly made of hydrogen, helium, and perhaps water. Strangely, despite their abundance elsewhere, they have no analogue in our own solar system, making the population something of an enigma.
But a new study published Nov. 5, led by Prof. Eliza Kempton with the University of Chicago, adds a new wrinkle to our best picture yet of these distant worlds.
Though it was previously thought these planets are generally covered in planet-wide oceans of molten magma, Kempton found the surfaces of many of them may actually be solid.
These planets still wouldn't be very fun for a human to stand on, though: The rocky surface is only solid because it's under tremendous pressure from the weight of a thick atmosphere.
"This really upends a paradigm about these planets, which is interesting because there are so many of them in the universe," Kempton said. "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."
However, scientists have come up with creative ways to interpret the data we do have. For example, they can get a sense of the molecules in planets' atmospheres by analyzing the light filtering through, and measure planets' gravitational effects on their host stars to find their masses.
Finding so many mini-Neptunes surprised scientists who saw them around nearby stars, given their total absence from our own neighborhood.
Due to the high temperatures and heavy atmospheres, it was thought these planets likely have global seas of molten magma on their surfaces, like the Earth briefly did. UChicago Assoc. Prof. Edwin Kite previously predicted these magma oceans may even begin to "eat" their own skies, limiting how large the planet can grow.
But digging deeper into the data, a team of researchers including Kempton; then-undergraduate student Bodie Breza, the first author on the paper; and postdoctoral researcher Matthew Nixon (now a 51 Pegasi b postdoctoral fellow at Arizona State University) realized the story might be more complicated.
The group first realized the potential twist when analyzing a planet called GJ 1214 b, which circles a faraway star in the constellation Ophiucus. Recent data from the James Webb Space Telescope suggests this planet's atmosphere may contain larger molecules than simple hydrogen and helium, which implies the atmosphere would be heavier than previously thought - much, much larger than Earth's thin shell.
That blanket of heavy atmosphere would create extremely high-temperature, high-pressure conditions. In fact, the pressure would be so high that the data suggests the rock would transition from molten magma to solid rock again - much like carbon condenses into diamond deep beneath the Earth's surface.
Surprised, the team wondered what this meant for other planets. By creating a series of simulated planets with different conditions, they found that a substantial portion of these mini-Neptunes that were previously assumed to be lava worlds may, in fact, have solid surfaces.
"It's an either-or," said Kempton. "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."
"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," explained Nixon. "By following that logic, other solar systems should look like ours. But they don't."
Scientists, therefore, want to understand how mini-Neptunes form and what they look like now to have a more complete picture of how planets form in general. This can guide, among other things, the search for habitable planets.
"It gets back to why are we here - how did Earth come to be?" said Nixon. "This is a really fundamental piece for us to understand both other planets and our own."
Research Report:Not All Sub-Neptune Exoplanets Have Magma Oceans
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