A SwRI-led study published research in The Planetary Science Journal that used argon-40 measured in the atmospheres of Uranus and Neptune to estimate the rock below their icy layers and to investigate how and where the ice giants came to be.
Dr. Olivier Mousis, a planetary scientist in SwRI’s Solar System Science and Exploration Division in Boulder, Colorado, led the work. The team concluded that argon-40 readings from the two atmospheres likely say more about how the planets formed than about whether the material underneath is rocky or icy.
The argon-40 in the two atmospheres might have come from the ancient cloud that built our solar system roughly 4.6 billion years ago, or produced by radioactive breakdown of potassium-40 inside deep rock across billions of years. The scientists blended argon-40 production math with a model of the early nebula that followed primordial argon as it condensed, trapped and moved inside ice particles while the nebula changed, and those particles helped build Uranus and Neptune. Computer runs showed argon-rich solids could have formed close to an argon frost line at the outer edge of the cloud, where cold turns argon gas into solid ice, and that grains collecting there could shape the makeup of planet-forming material.
Mousis said decades of study have left little understanding of what lies beneath the atmospheres of Uranus and Neptune and of their physical composition, and he suggested the planets may contain far more rock than their ice giant labels imply. He said primordial argon brought to Uranus and Neptune would drown out radioactive-decay contributions from within the planets, and that argon-40 from the nebula is more abundant than scientists thought.
The paper’s other authors include Dr. Robin Canup of SwRI and Dr. Christopher Glein, also of SwRI and based in the Space Science Division in San Antonio, Texas, along with Dr. Mark Hofstadter of the Space Science Institute and Dr. Jonathan Lunine, tied to the California Institute of Technology and to NASA’s Jet Propulsion Laboratory. Researchers cite the work as O. Mousis et al., Primordial and Radiogenic Argon 40 in Uranus and Neptune, published in The Planetary Science Journal in 2026 with DOI 10.3847/psj/aeb2ee.
Canup said knowing what the formative material consisted of matters for rebuilding the planets’ formation histories, and that conditions in the early solar system can leave chemical marks in atmospheres billions of years after the planets formed.
The scientists said future missions such as NASA’s planned Uranus Orbiter and Probe could measure noble gases and isotope ratios directly, going beyond what remote sensing can reliably measure, and that measuring nitrogen, carbon and other components could test ideas about how ice giants formed. Mousis said argon-40 readings could help answer what the planets consist of and where their building blocks came together, and that probes carrying sensitive enough mass spectrometers could bring science closer to both answers.
An international group with SwRI staff reported in Astronomy & Astrophysics that the two ice giants have different chemical origins even though they are similar in size, mass and atmospheric conditions. Neptune’s deep atmosphere holds more oxygen than Uranus’s deep atmosphere, creating different carbon-to-oxygen ratios between the two worlds.
The scientists compared atmospheric chemistry using carbon-monoxide observations, including recent Atacama Large Millimeter/Submillimeter Array readings, and models tracking how dust and gas chemistry evolved in the planet-forming disk. Mousis said scientists cannot directly reach the deep atmospheres, but chemical tracers can probe conditions below visible clouds, and carbon monoxide is especially useful because it is common in the upper troposphere. He said not being able to study the atmospheres in situ fundamentally limits understanding of what the planets consist of, their interiors and how they formed.
Models show Neptune’s building blocks started closer to the young Sun and later moved outward, challenging the older view that both planets formed from nearby material where they now orbit. Mousis said Neptune’s chemistry does not match formation where it sits now from the same reservoir as Uranus, and that its building blocks traveled a long way before they accreted, meaning two similar worlds took distinct evolutionary paths.
The researchers said planets of similar size are common around other stars, making Uranus and Neptune key benchmarks for how planets form across the galaxy, and that this supports NASA’s planned Uranus Orbiter and Probe, which is scheduled to launch in the next decade. Mousis co-authored the paper and advised first author Tom Briand, and he said the two worlds are more like cousins than twins, with different histories.
Scientists had long believed Uranus and Neptune came from one shared pool of cosmic material close to where they now orbit. The paper Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways? appeared in the September 2026 issue of Astronomy & Astrophysics. The two worlds have no solid surfaces; thick layers of gas made of water, ammonia and methane extending thousands of miles down toward their interiors wrap them, and a thin lighter layer on top gives a greenish-blue appearance.
Scientists believe the two planets formed at roughly the same time, about 4.5 billion years ago. Modeling by SwRI showed that different volatile compounds condensed into solids at different distances from the center of the disk, creating separate chemical reservoirs as the material cooled and came together into planets.
NASA and other U.S. agencies produced the 2023-2032 Planetary Science Decadal Survey for the National Research Council, which recommends the Uranus Orbiter and Probe as a high-priority Flagship-class concept; Robin Canup co-chaired that survey.
EarthSky reported that researchers said Neptune’s building blocks formed much closer to the sun than Uranus’s, while Uranus formed in place. EarthSky added that an Oxford study from 2024 found Neptune is a lighter greenish-blue compared with Uranus, and that researchers from the University of Zurich and NCCR PlanetS suggested earlier in 2026 that the planets could be rocky deep inside instead of being ice giants. The outlet also noted that the Atacama Large Millimeter/Submillimeter Array, which supplied recent readings, is in Chile’s Atacama Desert. “Uranus and Neptune may resemble twins today, but their chemistry is telling us that they are more like cousins with very different histories,” Olivier Mousis said.
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