Uranus and Neptune may have formed from different material, atmospheric study suggests

Uranus and Neptune may have formed from material in different parts of the early solar system, according to a study described on October 19 by Astronomy & Astrophysics and Southwest Research Institute. Researchers infer that Neptune’s deep atmosphere is more oxygen-rich than Uranus’s, a difference that could help explain how two similar-looking planets developed.

The international team combined models of atmospheric chemistry and transport with observations of carbon monoxide, including measurements from the Atacama Large Millimeter/Submillimeter Array. Carbon monoxide can move through cloud layers from deeper regions, allowing scientists to use its observed concentration to infer chemistry below the visible atmosphere, according to the Astronomy & Astrophysics account.

What the carbon-to-oxygen comparison suggests

One interpretation is that Uranus formed relatively far from the Sun, near its present orbit, while Neptune began closer to the Sun amid material richer in oxygen and later moved outward. Those proposed paths are inferences from atmospheric models, rather than a definitive account of either planet’s movements.

Olivier Mousis, a study co-author and program director at Southwest Research Institute, said the planets’ chemistry suggests they may have different histories despite their resemblance. The institute said the work was published in the September 2026 issue of Astronomy & Astrophysics. Its first author is Tom Briand.

Why the ice giants remain difficult to study

Voyager 2 is the only spacecraft to have flown past both planets, reaching Uranus in 1986 and Neptune in 1989, according to the Astronomy & Astrophysics account. Their limited exploration makes indirect measurements of the deep atmospheres important to understanding how the ice giants formed. Planets of comparable size account for more than half of the roughly 6,000 exoplanets discovered to date, the account said.

The researchers say circulation deep within the atmospheres remains a major uncertainty in translating observed carbon monoxide into formation histories. Further observations are needed to tighten estimates of the planets’ composition and test the proposed paths. Southwest Research Institute also pointed to the Uranus Orbiter and Probe, a mission concept recommended as a high-priority flagship mission by the 2023–2032 Planetary Science Decadal Survey.