Planetary ‘twins’ Uranus and Neptune may have different origin stories

By indirectly probing the depths of Uranus and Neptune’s atmospheres, an international team of scientists has revealed a significant contrast in their chemical composition. Neptune appears to be more enriched in oxygen than Uranus. These results suggest that the two ice giants may have formed in different regions of the primitive solar system before reaching their current orbits, calling into question the idea of a common origin for these two planets, often considered twins.

Uranus and Neptune are the least explored planets in the solar system. Only the Voyager 2 spacecraft has flown by them, in 1986 and 1989, respectively. Yet understanding their origin is an important challenge for planetary science: Planets of comparable size account for more than half of the roughly 6,000 exoplanets discovered to date.

To trace the history of these two worlds, scientists seek to determine the composition of their deep layers, which are inaccessible to direct observation due in part to the many successive cloud layers. They must therefore identify tracers that can indirectly reveal their composition. Carbon monoxide (CO) is one such tracer. Produced or present in the deep layers of the atmosphere, it can be transported through multiple cloud layers to altitudes accessible to observation. Its concentration thus provides a window into regions located far below the visible clouds.

In this new study published in Astronomy & Astrophysics, an international team of researchers combined numerical models describing atmospheric chemistry and transport with recent observations of carbon monoxide, notably obtained with the ALMA interferometer. This approach made it possible to constrain the carbon-to-oxygen abundance ratio (C/O) in the depths of Uranus and Neptune. This ratio constitutes a chemical fingerprint of the materials from which these planets formed.

As suggested in previous work, Neptune appears to be substantially richer in oxygen than Uranus. When combined with carbon abundances, this difference leads to lower C/O ratios on Neptune, although the ranges of possible values overlap between the two planets. This difference is difficult to explain if the two planets formed from materials of the same composition in a similar region of the gas and dust disk that surrounded the young sun.

By comparing these results with a model describing the chemical evolution of the primitive solar system, the scientists show that Uranus and Neptune may have had distinct formation histories. Uranus may have formed relatively far from the sun, near its current orbit, while Neptune may have initially emerged closer to the sun, in a region where solid materials were more enriched in oxygen, before migrating to its current orbit. However, the joint formation and movements of these planets within the primitive solar system remain to be clarified.

These new results do not yet allow a definitive reconstruction of the two planets’ history. In particular, vertical circulation in their deep atmospheres remains a major source of uncertainty. New observations will be essential to better constrain the deep composition of these planets and test these scenarios. An exploration mission dedicated to the ice giants, such as the Uranus Orbiter and Probe (UOP) concept, designated in 2023 as the top priority mission for the coming decade by the U.S. National Academy of Sciences, would lift the veil further on the origin of these still largely unknown worlds.

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Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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