There is a long held mystery about Uranus and Neptune, the two planets of our Solar System farthest from the Sun: we don't know how they formed.

What makes these planets such a mystery is their composition. Both are nearly 90 percent ice, mostly made up of carbon. However, that proposes a problem. The outer edges of the Solar System, which is very far from the Sun, didn't contain the sort of materials, or building blocks, for such planet formation for very long, and definitely not long enough for these planets to have formed before the Universe reached its full mass. And after that, it would be impossible for Uranus and Neptune to develop their atmospheres.

That conundrum baffled scientists for decades. Some scientists even suggested that the two planets formed in the Oort Cloud, where most comets formed, before finding their current positions. However, a team of French and American researchers led by the UTINAM Institute suggest a different method of how Uranus and Neptune formed. Their new theory focuses on the positions of the planets in the Solar System today.

Using the Herschel Space Observatory, the researchers collected data about specific compositions on the two planets, particularly their deuterium-to-hydrogen ratios (D/H). This particular ratio would react to temperature changes as the Solar System reached its full mass. If the theory that the planets formed in the same region where comets originated is correct, then their D/H ratios would be high.

However, Herschel showed a different story: the D/H ratios of both planets were much lower than those measured in comets. This suggests that the planets formed exactly where they currently are within the Solar System.

With that information on hand, researchers ran simulations that demonstrated the distribution of elements such as water, carbon monoxide and nitrogen as the Solar System formed. The simulations showed "peaks" of solids where temperature was low enough for the condensation of gases (known as ice lines).

The results of the simulation show that both Uranus and Neptune formed on a specific gas line, the Carbon Monoxide line. This explains why their compositions are heavy with carbon-based solids, but lack nitrogen gas. Working backwards, this also explains why the D/H ratios of the planets are so low.

The simulation also showed a nitrogen ice line farther away, which explains why there is little nitrogen in their atmospheres.

However, most importantly, the final model suggests that both planets formed where they exist today.