Scientists are rethinking Uranus and Neptune as new evidence suggests the so-called ice giants may contain far less ice than once believed.<\/p>\n
For decades, Uranus and Neptune have occupied their own category in the Solar System: the ice giants. <\/p>\n
The name suggests we know what lies beneath their blue atmospheres, but scientists are increasingly questioning whether these distant planets contain much ice at all.<\/p>\n
New models show that Uranus and Neptune could contain hot mixtures of rock<\/a>, iron, and hydrogen beneath their thick outer atmospheres while still matching what scientists observe about their size, mass, gravity, heat, and atmospheres.<\/p>\n Why the label may fail<\/p>\n Ravit Helled, a planetary scientist at the University of Zurich<\/a>, said the ice giant label is \u201ca little bit misleading.\u201d<\/p>\n \u201cWe really don\u2019t know what these planets are made of,\u201d Helled said.<\/p>\n Jonathan Fortney, a planetary astrophysicist at the University of California, Santa Cruz (UC Santa Cruz<\/a>), added that the evidence for deeply icy interiors is \u201call indirect, it\u2019s all circumstantial.\u201d<\/p>\n \u201cThis was always in the back of people\u2019s minds: that the planets<\/a> could be more complicated,\u201d Fortney added. <\/p>\n Small bodies in the outer Solar System appear to contain more rock and less ice than older models assumed.<\/p>\n Magma oceans offer another possibility <\/p>\n \u201cWe have these representatives in our own Solar System, and we realize that we don\u2019t know what they are made of,\u201d Helled said.<\/p>\n The question matters beyond our planetary neighborhood because worlds smaller than Neptune are common around other stars.<\/p>\n Edward Young is a planetary scientist at the University of California, Los Angeles (UCLA<\/a>).<\/p>\n \u201cWe thought, well, why should the formation of the ice giants and the sub-Neptunes be fundamentally different?\u201d Young said.<\/p>\n Rock and hydrogen may mix deep inside<\/p>\n In the new model, intense heat and pressure allow silicate rock, made from minerals containing silicon and oxygen, to mix with iron and hydrogen into one flowing material. <\/p>\n The authors call it a supercritical magma ocean, meaning the mixture is so hot and compressed that ordinary boundaries between liquid and gas no longer apply.<\/p>\n \u201cA rethinking of the astrophysics and the settings in which they were formed,\u201d Young said. <\/p>\n This magma would not resemble lava from an Earth volcano because it would contain large amounts of hydrogen and remain trapped under a deep atmosphere.<\/p>\n Models match many observations<\/p>\n The team built computer versions of both planets<\/a> as layers from the center outward.<\/p>\n They adjusted three features: the pressure where the magma ocean meets the atmosphere, the total hydrogen, and a layer that slows heat moving upward.<\/p>\n The researchers tested each version against six properties, including radius, temperature, internal heat, and small gravity variations that reveal how mass is spread inside a planet.<\/p>\n The best-fitting Neptune model contained about 13 percent hydrogen by mass, while the Uranus model contained about 14 percent. <\/p>\n In both cases, the magma ocean filled roughly the inner three quarters of the planet\u2019s radius, with a hydrogen-rich envelope above it.<\/p>\n Uranus and Neptune show important differences<\/p>\n The Uranus model matched the observations well, though one gravity value depended strongly on uncertain corrections for deep winds. <\/p>\n The Neptune model matched most observations, but its modeled temperature at a standard atmospheric pressure was about 22 degrees Fahrenheit (12 degrees Celsius) too warm.<\/p>\n The model gave Uranus a thicker heat-blocking layer than Neptune, which can account for Uranus<\/a> releasing much less internal heat. <\/p>\n However, the team adjusted that layer to match the heat data, so the result does not prove why Uranus is unusually faint.<\/p>\n Chemistry offers another clue<\/p>\n The model also produced an atmosphere rich in methane and hydrogen sulfide, a sulfur-bearing gas, but extremely poor in ammonia. <\/p>\n That pattern broadly resembles observations, and it could arise because nitrogen dissolves into the deep molten mixture while methane remains in the atmosphere.<\/p>\n Water can form where hydrogen reacts with hot rocky material, so a planet\u2019s present water does not provide a simple record of where it formed. <\/p>\n In the cold upper atmosphere, water would condense far below the visible clouds, making it difficult to detect from afar.<\/p>\n The paper links Uranus and Neptune with sub-Neptune planets around other stars. <\/p>\n A simple formation calculation found that larger growing worlds can collect more hydrogen, placing the outer planets near the high-mass end of the same family.<\/p>\n Other interior structures may still fit<\/p>\n Roberto Tejada Arevalo, an astrophysics PhD candidate at Princeton University<\/a>, has also modeled possible interiors for Uranus and Neptune.\u00a0<\/p>\n \u201cThere are so many mysteries with these planets,\u201d Tejada Arevalo said.<\/p>\n His separate models allow rock, ice, and gases to mix in ways that could also match the planets\u2019 heat and other properties.<\/p>\n Different models suggest different interiors<\/p>\n Uri Malamud, a planetary scientist at the Technion\u2013Israel Institute of Technology<\/a>, has proposed a different picture of what may lie inside Uranus and Neptune. <\/p>\n \u201cThey\u2019re just different ideas about what might be possible,\u201d Malamud said. His work points to methane-rich material and a more layered interior, with rock remaining more separate.<\/p>\n Vanesa Ram\u00edrez, an astronomy Ph.D. candidate at Leiden University<\/a>, has also examined what Uranus and Neptune may contain. \u201cWe are still discovering these planets,\u201d Ram\u00edrez said.<\/p>\n Her modeling suggests that both worlds could be more than 60 percent rock, while recent telescope measurements still allow several internal arrangements.<\/p>\n The magma ocean result therefore shows that interiors without large ice layers can fit current data, not that the traditional picture has been ruled out. <\/p>\n Material properties remain uncertain, and the paper estimates about a two-percentage-point uncertainty in its hydrogen amounts.<\/p>\n A future mission could settle the question<\/p>\n Voyager 2 supplied the only close measurements of Uranus and Neptune, and those brief visits happened about four decades ago. <\/p>\n A dedicated orbiter and atmospheric probe could measure gravity, magnetic fields, winds, temperatures, and gases in greater detail.<\/p>\n \u201cWe really need a flagship mission to put all these pieces of the puzzle together,\u201d Ravit Helled said. <\/p>\n Better measurements could separate models that now produce similar answers from limited data.<\/p>\n Even the name is up for debate<\/p>\n Edward Young prefers the name \u201cmiscible giants,\u201d referring to materials that can mix together. Uri Malamud has suggested \u201csubgiants,\u201d while Vanesa Ram\u00edrez prefers \u201cminor giants.\u201d <\/p>\n Ravit Helled has proposed \u201couter giants,\u201d and Roberto Tejada Arevalo favors \u201cmetal giants,\u201d using the astronomy term for elements heavier than hydrogen and helium.<\/p>\n \u201cThere were 10 of us in a room for a week. We didn\u2019t agree on any better names,\u201d Fortney said.<\/p>\n For now, the familiar label remains useful, but it should not be mistaken for a confirmed description of what lies below the clouds. <\/p>\n Only new observations can show whether Uranus and Neptune are magma ocean worlds.<\/p>\n The study is published as a preprint on arXiv<\/a> and is under review at The Astrophysical Journal.<\/p>\n \u2014\u2013<\/p>\n Like what you read? 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