Origins of Uranus and Neptune are surprisingly different
View larger. | NASA’s Voyager 2 spacecraft captured these views of Uranus and Neptune during its flybys of the planets in the late 1980s. A new study suggests both giant planets could have deep layers of water below their atmospheres. Image via NASA/ JPL-Caltech/ B. Jónsson/ NOIRLab.
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Uranus and Neptune look like twins. They are similar in size, color, and atmosphere. Both are blue or greenish-blue, with deep layers of methane, ammonia, and water. Neither has a solid surface beneath its atmosphere. But a new study says their origins are quite different.
An international team of researchers announced the finding on October 19, 2026. They published their peer-reviewed results in Astronomy & Astrophysics. The study suggests that Neptune’s building blocks first formed much closer to the sun than Uranus’s building blocks did. Over time, Neptune’s material moved outward to the region where Neptune is found today.
Similar worlds, different histories
Uranus and Neptune have long been called “twins.” They do look alike in many ways. But their chemistry tells a different story. Co-author Olivier Mousis is a program director at the Southwest Research Institute in Boulder, Colorado. He said that the two planets may resemble twins today, but their chemistry shows they are more like cousins with very different histories. Their atmospheres may have come from different parts of the protoplanetary disk, the cloud of dust and gas around the young sun where the planets formed. If so, their similarities are a remarkable coincidence.
To study the planets, researchers looked at their atmospheres. They used new measurements from the Atacama Large Millimeter/Submillimeter Array, or ALMA, in Chile. ALMA is a powerful telescope array in the Atacama Desert.
Neptune’s atmosphere has more oxygen than Uranus’s atmosphere. This difference creates a different carbon-to-oxygen ratio. The researchers used computer models to compare this ratio with the dust and gases in the protoplanetary disk billions of years ago.
Mousis explained that scientists cannot directly reach the deep atmospheres of Uranus and Neptune. However, chemical tracers can help them study conditions hidden below the visible clouds. Carbon monoxide, or CO, is especially useful because it is common in the upper troposphere of many planets. With this data, the team modeled how the protoplanetary disk changed over time and where its materials came from.
Moving outward from the sun
The most surprising result is that Neptune’s building blocks first formed much closer to the sun. This happened because volatile compounds, which evaporate easily, condensed into solids at different distances from the young sun. That process created separate chemical reservoirs of material at different distances.
The new models showed that the material that formed Neptune was originally much closer to the sun than Neptune is now. As a result, the origins of Uranus and Neptune split into two different paths. Later, that material migrated outward. It eventually reached the area where Neptune could collect it.
Mousis said Neptune’s chemistry is hard to explain if the planet formed where it is today and pulled from the same reservoir as Uranus. Instead, the models suggest that Neptune’s building blocks traveled a long way. In short, two worlds that look alike may have reached their present locations through two very different evolutionary paths.
Limits and wider meaning
The findings are strong, but the researchers agree that they are limited. At the time of the study, scientists could not study the planets’ atmospheres directly with a spacecraft. The results also matter for similar planets far beyond our solar system.
Mousis said the findings have effects beyond Neptune and Uranus, and even beyond our solar system. Planets of similar size are common around other stars. That is why Uranus and Neptune are important benchmarks for understanding how planets form and evolve across the galaxy. He said this is why missions like NASA’s planned Uranus Orbiter and Probe are so important. That mission is expected to launch in the next decade.
The true colors of Uranus and Neptune
In 2024, researchers at the University of Oxford in the United Kingdom said they had found the real colors of Uranus and Neptune. Uranus had always appeared light greenish-blue. Neptune looked like a much darker azure blue. But their study showed that Neptune was not as dark as people had thought. It was a lighter greenish-blue, closer to Uranus.
At the beginning of 2026, researchers from the University of Zurich and NCCR PlanetS in Switzerland said that Uranus and Neptune might be rocky worlds deep inside. That would mean they are not ice giants, as scientists had long believed. These ideas add new questions to the study of these distant planets.
A new study suggests that the origins of Uranus and Neptune are quite different, even though the two planets are similar in many ways.