Carbon Monoxide Data Contradicts Twin Planet Theory
Uranus and Neptune may look like identical blue ice giants, but their deep atmospheric chemistry reveals they formed in entirely different regions of the solar system, according to a new study published in Astronomy & Astrophysics. While decades of observation relied on their visual similarities to group them together, planetary scientists now argue that those traits are merely a coincidence.
Both planets were visited only once by NASA’s Voyager 2 in the 1980s, leaving their deep interiors largely unknown. To bypass this limitation, researchers turned to carbon monoxide gas detected in the upper troposphere to act as an indirect probe of the planets’ hidden chemistry.
This gas originates deep within the planets, where it links chemically to water before vertical mixing carries it upward. By modeling that atmospheric process, scientists can work backward to calculate the underlying abundance of oxygen.
Contrasting Oxygen Ratios and Protoplanetary Origins
Using fresh carbon monoxide measurements from the Atacama Large Millimeter/submillimeter Array (ALMA), the research team set Uranus’s level at about 5.6 parts per billion. Neptune’s carbon monoxide concentration proved to be about two orders of magnitude higher.
When factoring in variable vertical mixing across latitudes, the scientists estimated that Uranus contains 62 to 177 times the Sun’s oxygen-to-hydrogen ratio. Neptune, by contrast, holds 222 to 342 times that solar ratio.
The results showed that Neptune’s building blocks likely formed near the water ice line closer to the Sun, while Uranus’s 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,” said Dr. Olivier Mousis, a researcher at the Southwest Research Institute.
The team concluded that Neptune’s primordial material traveled a long way to where Neptune could accrete it. Uranus, meanwhile, matches materials closer to its current orbital neighborhood, meaning the two worlds evolved along entirely separate paths.
Implications for Exoplanet Research and Future Exploration
Because planets comparable in size to Uranus and Neptune are common around other stars, understanding our solar system’s ice giants remains critical. Researchers rely on them as essential benchmarks for understanding how planets form and evolve throughout the galaxy.
However, scientists remain fundamentally limited by the lack of in situ atmospheric access. The research team emphasized that resolving these lingering questions about planetary formation requires dedicated missions, such as NASA’s planned Uranus Orbiter and Probe, which is expected to launch sometime in the next decade.
Did you know?
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.
Frequently Asked Questions About Ice Giant Chemistry
Why do Uranus and Neptune look so similar if their chemistry differs?
The study suggests their similarities are a noteworthy coincidence, as their atmospheric compositions may be connected to different regions of the protoplanetary disk where the planets formed.
How do scientists measure oxygen levels deep inside these planets?
Researchers cannot directly access the deep atmospheres of Uranus and Neptune, so they use chemical tracers like carbon monoxide in the upper troposphere—which is carried upward from deep interiors via vertical mixing—to calculate underlying oxygen ratios.
What was the spacecraft that originally visited Uranus and Neptune?
NASA’s Voyager 2 visited both planets in the 1980s.
When is the next dedicated mission to the ice giants scheduled?
NASA is currently planning the Uranus Orbiter and Probe mission, with an anticipated launch timeframe sometime in the next decade.
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