Uranus and Neptune are often described as near-twins, but a new study suggests their chemistry, and possibly their histories, may differ more than expected.
The materials making up Neptune and Uranus likely came from different regions in the protoplanetary disk around the young Sun. Image credit: SwRI.
“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 researcher at the Southwest Research Institute.
“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.”
Both planets have been visited only once, by NASA’s Voyager 2 in the 1980s, and their deep composition is largely unknown.
The study authors therefore used carbon monoxide detected in the upper troposphere as an indirect probe.
This gas is thought to form deep inside the planets, where it is linked chemically to water, and then to be carried upward by vertical mixing.
By modeling that process, the researchers can work backward to estimate how much oxygen lies below.
“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,” Dr. Mousis said.
“Studying carbon monoxide is particularly useful because of its abundance in the upper troposphere.”
“Using these data, we can model the chemical evolution of the protoplanetary disk and the origins of its materials.”
For Uranus, the model used a fresh carbon monoxide measurement from ALMA, which the team put at about 5.6 parts per billion. Neptune’s level is roughly two orders of magnitude higher.
When vertical mixing was allowed to vary with latitude, the scientists estimated that Uranus has 62 to 177 times the Sun’s oxygen-to-hydrogen ratio, while Neptune has 222 to 342 times.
They also estimated carbon-to-oxygen ratios of about 0.06 to 0.52 for Uranus and 0.02 to 0.12 for Neptune.
Next, they compared those ratios with a model of the Sun’s protoplanetary disk.
Uranus’ upper range is close to the Sun’s value, while Neptune’s is clearly lower.
The results suggest Neptune’s building blocks may have formed near the water ice line closer to the Sun.
Uranus’ lower values could fit material that formed inside the methane ice line.
“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,” Dr. 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.”
“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.”
“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 results appear this week in the journal Astronomy & Astrophysics.
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T. Briand et al. 2026. Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways? A&A 714, A364; doi: 10.1051/0004-6361/202660770
