Uranus and Neptune look like a matched pair, but their chemistry suggests they were built from different material in different places.
A study published in Astronomy & Astrophysics estimates the carbon-to-oxygen ratio deep inside both planets, using models of atmospheric chemistry and transport together with carbon monoxide observations, many of them from the ALMA array in Chile. That ratio works as a chemical fingerprint of the material a planet formed from.
A hidden ratio, measured indirectly
Nobody can sample the deep atmosphere of either planet. Successive cloud layers block direct views, and only Voyager 2 has ever flown past them, in 1986 and 1989.
Carbon monoxide offers a way in. It forms or sits in deep layers and gets carried up through the clouds to heights that telescopes can see, so its concentration carries information about regions far below.
The team found Neptune appears substantially richer in oxygen than Uranus. Combined with carbon estimates, that gives Neptune a lower carbon-to-oxygen ratio. The ranges of possible values still overlap, but the difference is hard to explain if both planets grew from the same mix in the same region of the disk that surrounded the young Sun.
Cousins, not twins
Comparing the results with a model of how the early solar system evolved chemically, the authors suggest separate histories. Uranus may have formed relatively far out, near its present orbit. Neptune may have started closer to the Sun, in a zone where solids held more oxygen, and then moved outward.
"Uranus and Neptune may resemble twins today, but their chemistry is telling us that they are more like cousins with very different histories," said Olivier Mousis of the Southwest Research Institute, a co-author.
The result matters beyond two planets. Worlds of this size make up more than half of the roughly 6,000 exoplanets found so far, so how ice giants assemble shapes thinking about a large share of known planets.
The open questions
The authors say the picture is not settled. Vertical mixing in the deep atmospheres remains a large source of uncertainty, and better measurements are needed. A dedicated mission such as the Uranus Orbiter and Probe, ranked the top priority for planetary science in 2023 by the US National Academy of Sciences, would help settle it.