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A new study finds that many \u201cmini-Neptunes\u201d\u2014perhaps the most common planets in the galaxy\u2014are under so much pressure from their heavy atmospheres that the surface is likely compressed solid.<\/p>\n
As telescopes have become more powerful, it\u2019s turned out our solar system is not the only game in town: There are millions of other planets out there in the galaxy.<\/p>\n
But we\u2019re still teasing out clues about what they are actually like.<\/p>\n
One of the puzzles is a kind of planet that appears to be one of the most common types in the universe.<\/p>\n
Known as \u201cmini-Neptunes\u201d 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.<\/p>\n
Strangely, despite their abundance elsewhere, they have no analogue in our own solar system, making the population something of an enigma.<\/p>\n
But the new study, led by Professor Eliza Kempton with the University of Chicago, adds a new wrinkle to our best picture yet of these distant worlds.<\/p>\n
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.<\/p>\n
These planets still wouldn\u2019t be very fun for a human to stand on, though: The rocky surface is only solid because it\u2019s under tremendous pressure from the weight of a thick atmosphere.<\/p>\n
\u201cThis really upends a paradigm about these planets, which is interesting because there are so many of them in the universe,\u201d Kempton says. \u201cAt the bottom of it, quite literally, we\u2019re trying to understand what these objects are, because they don\u2019t exist in our solar system.\u201d<\/p>\n
Though we know planets outside our solar system\u2014known as exoplanets\u2014exist, they are so far away that even our most powerful telescopes can only pick up indirect signals, such as the dip in light when a planet crosses in front of its star.<\/p>\n
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\u2019 atmospheres by analyzing the light filtering through, and measure planets\u2019 gravitational effects on their host stars to find their masses.<\/p>\n
Finding so many mini-Neptunes surprised scientists who saw them around nearby stars, given their total absence from our own neighborhood.<\/p>\n
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 Associate Professor Edwin Kite previously predicted<\/a> these magma oceans may even begin to \u201ceat\u201d their own skies, limiting how large the planet can grow.<\/p>\n 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.<\/p>\n 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\u2019s atmosphere may contain larger molecules than simple hydrogen and helium, which implies the atmosphere would be heavier than previously thought\u2014much, much larger than Earth\u2019s thin shell.<\/p>\n 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\u2014much like carbon condenses into diamond deep beneath the Earth\u2019s surface.<\/p>\n 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.<\/p>\n \u201cIt\u2019s an either-or,\u201d says Kempton. \u201cYou can have this the-floor-is-lava scenario, or a solid surface, and you\u2019re going to have to take into account a number of other factors about a planet\u2019s atmosphere to try to figure out which regime it falls under.\u201d<\/p>\n These mini-Neptune planets are of special interest to scientists because of their sheer numbers, and what they imply about how planets form.<\/p>\n \u201cBefore 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,\u201d explains Nixon. \u201cBy following that logic, other solar systems should look like ours. But they don\u2019t.\u201d<\/p>\n 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.<\/p>\n \u201cIt gets back to why are we here\u2014how did Earth come to be?\u201d says Nixon. \u201cThis is a really fundamental piece for us to understand both other planets and our own.\u201d<\/p>\n The research appears in Astrophysical Journal Letters<\/a>.<\/p>\n