Two planets that look like close relatives may have assembled from different chemical reservoirs. Research highlighted by Southwest Research Institute on October 19 links Uranus and Neptune to distinct supplies of planet-building material.
The difference matters because size and appearance do not fully describe a planet's history. Understanding these nearby worlds also helps researchers interpret similarly sized planets orbiting other stars.
The study combines atmospheric chemistry models with carbon monoxide observations, including measurements from the ALMA observatory. Carbon monoxide serves as a chemical tracer, helping researchers infer conditions below the visible clouds.
The team's models indicate that Neptune's deep atmosphere contains substantially more oxygen than Uranus's. That produces different inferred carbon-to-oxygen ratios. It is an interpretation of observations through chemistry models, not a direct sample from either deep atmosphere.
The researchers then compared those ratios with models of the young Solar System's disk of gas and dust. Different compounds condensed into solids at different distances from the Sun, creating chemically distinct reservoirs.
Their reconstruction points to Neptune's ingredients originating closer to the young Sun before moving outward. It challenges the simple picture of both planets gathering similar material where they are today.
The paper appeared in the September issue of Astronomy & Astrophysics. The authors emphasize that limited access to the planets' interiors constrains the conclusion. Atmospheric measurements do not provide a complete inventory of either world.
Their similar appearance conceals a possible difference in chemical ancestry.
