[SwRI] Uranus and Neptune are often considered planetary twins due to their many similar characteristics, including size, mass and atmospheric conditions, and have long been thought to have pulled from the same cosmic reservoir of materials when forming.
New research by an international team of scientists, including Southwest Research Institute (SwRI) staff, suggests the ice giants have significantly different chemical origins despite their similarities.
The research, published in the September 2026 issue of Astronomy & Astrophysics, details how Neptune’s deep atmosphere appears substantially richer in oxygen than Uranus’s, hinting at different origins for the materials that make up the two planets. These ice giants do not have solid surfaces like the Earth.
Instead, Uranus and Neptune are enveloped by thick blankets of gas made up of heavy elements like water, ammonia and methane, which extend thousands of miles deep toward their interiors. A thin outer layer of lighter elements provides the planets with their greenish-blue appearance.
“Uranus and Neptune may resemble twins today, but their chemistry is telling us that they are more like cousins with very different histories,” said Dr. Olivier Mousis, a program director in SwRI’s Solar System Science and Exploration Division located in Boulder, Colorado, and a co-author of the paper.
Mousis also served as a doctoral co-advisor for the paper’s first author, Tom Briand. “Their atmospheric compositions may be connected to different regions of the protoplanetary disk where the planets formed, which would make their similarities a noteworthy coincidence.”
The researchers contrasted the two planets’ chemical compositions using atmospheric chemistry models and observations of carbon monoxide levels, including recent measurements from the Atacama Large Millimeter/Submillimeter Array (ALMA). The researchers found that Neptune’s deep atmosphere has more oxygen than Uranus’s, resulting in significantly different carbon-to-oxygen ratios between the two planets. The researchers compared the chemical differences in oxygen using models that illustrate the chemical evolution of dust and gases in the protoplanetary disk.
Eddy diffusion coefficient computed on Uranus (left) and Neptune (right) as a function of pressure and latitude following the prescription of Wang et al. (2015). The dashed line corresponds to the transition latitude between the fast- and slow-rotating regimes. For each degree of latitude, a different thermal profile was computed with the corresponding value of y top CH4 using the fits presented in Sect. 2.4. This would result in a different deep oxygen abundance required to fit the CO upper tropospheric value depending on latitude. For simplicity, the Kzz profiles shown here were computed with the thermal profiles obtained using the deep oxygen abundance derived by Venot et al. (2020) for each planet.– [astro-ph.EP]
“We can’t directly access the deep atmospheres of Uranus and Neptune, but chemical tracers allow us to probe the conditions hidden beneath their visible clouds,” Mousis said. “Studying carbon monoxide, or CO, is particularly useful because of its abundance in the upper troposphere. Using this data, we can model the chemical evolution of the protoplanetary disk and the origins of its materials.”
Uranus and Neptune are thought to have formed around the same general timeframe in our solar system’s history, around 4.5 billion years ago. SwRI’s models provided insight into how the protoplanetary disk’s different volatile compounds condensed into solids at different distances to create the chemically distinct reservoirs that formed planets as materials cooled and coalesced.
Mousis compared the inferred carbon-to-oxygen ratios of the planets using protoplanetary disk models, simulating the conditions that lead to planetary formations. Scientists have traditionally thought that Uranus and Neptune formed from materials originating in their nearby cosmic neighborhoods, but these new models point to Neptune’s building blocks originating closer to the young Sun before experiencing an outward migration.
“Neptune’s chemistry is difficult to reconcile with the idea that it formed where we see it today, pulling from the same reservoir of cosmic materials as Uranus,” Mousis said. “Instead, our models lead us to believe that its building blocks traveled a long way to where Neptune could accrete them. In short, two superficially similar worlds may have reached their present locations and formations through two very distinct evolutionary paths.”
Mousis added that due to a lack of in situ access to the atmospheres of Uranus and Neptune, the full scope of understanding regarding their composition, interiors and formations are fundamentally limited.
“Our findings have implications beyond Neptune and Uranus and even far beyond our Solar System,” Mousis said. “Planets comparable in size to Uranus and Neptune are common around other stars, which is why Uranus and Neptune are essential benchmarks for understanding how planets form and evolve throughout the galaxy, and it is precisely why missions like NASA’s planned Uranus Orbiter and Probe, expected to launch sometime in the next decade, are so important.”
The Uranus Orbiter and Probe is a mission concept recommended as a high-priority Flagship-class mission by the 2023-2032 Planetary Science Decadal Survey, a publication produced by NASA and other U.S. agencies for the U.S. National Research Council. Dr. Robin Canup, vice president of SwRI’s Solar System Science and Exploration Division, served as a co-chair for the survey. Learn more about SwRI Solar System Science & Exploration.
Read the article, “Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways?” in the Sept. 2026 issue of Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202660770.
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