If you could sniff a planet, what would it smell like? According to an international team of scientists working with the 8-meter Gemini North infrared telescope at the Mauna Kea Observatory in Hawaii, if you could get a whiff of the planet Uranus, it would have the pungent odor of rotten eggs. The icy planet is on the nose due to the presence of the noxious gas hydrogen sulfide, and the discovery could provide clues about the formation of the outer planets of our Solar System.

A glance at a chart of the Solar System shows that it's anything but homogenous. The inner system is made up of Earthlike rocky planets while the outer system is dominated by the gas giants. But it isn't as simple as that. For decades, scientists have suspected that the gas giants, Jupiter, Saturn, Uranus, and Neptune, might not be one group but two, with Jupiter and Saturn having developed in a different way to that of Uranus and Neptune.

The key to this is the composition of the upper atmosphere. According to Dr Leigh Fletcher, a member of the research team from the University of Leicester's Department of Physics and Astronomy, the different histories of the gas giants is reflected by the fact that the upper layers of Jupiter and Saturn are filled with ammonia, but that isn't the case with Uranus and Neptune, where it's been long suspected that hydrogen sulfide, the chemical that gives rotten eggs their stink, is present.

"During our Solar System's formation the balance between nitrogen and sulphur (and hence ammonia and Uranus's newly-detected hydrogen sulfide) was determined by the temperature and location of planet's formation," says Fletcher.

The evidence for hydrogen sulfide eluded scientists during decades of ground observations and even a 1986 flyby of the unmanned Voyager 2 deep space probe. But recently an international team that included Patrick Irwin from the University of Oxford managed to find the elusive molecule in the absorption spectra of the Uranian atmosphere collected by Gemini North from reflected sunlight. Since Uranus and Neptune are very similar in other respects, the scientists conclude that hydrogen sulfide should be present in the cloud tops of Neptune as well.

This difference may be significant clue as to the origin of the outer planets, because the presence of ammonia versus hydrogen sulfide is the product of the temperature and location of planet's formation.

"While the lines we were trying to detect were just barely there, we were able to detect them unambiguously thanks to the sensitivity of [the Near-Infrared Integral Field Spectrometer] on Gemini, combined with the exquisite conditions on Mauna Kea," says Irwin. "Although we knew these lines would be at the edge of detection, I decided to have a crack at looking for them in the Gemini data we had acquired."

If you're wondering if you could stand the stench of Uranus, the sort-of good news is that you wouldn't survive long enough to get a good whiff.

"Suffocation and exposure in the -200º Celsius (-328º F) atmosphere made of mostly hydrogen, helium, and methane would take its toll long before the smell," says Irwin.

The research was published in Nature Astronomy.

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