Uranus, the seventh planet from the sun, has held a vital (and smelly) secret of the solar system for decades.

An international team of researchers reported on Monday that they’ve discovered evidence that Uranus holds one of the most unpleasant-smelling chemicals known to humankind.

“They found hydrogen sulfide, the odiferous gas that most people avoid, in Uranus’s cloud tops,” according to a press release from Gemini Observatory, a high-power telescope atop a Hawaiian volcano.

Voyager 2 was the only spacecraft ever to visit the chilly, blue-green “ice giant.” The probe tried to see which chemicals were in Uranus’ clouds during its 1986 flyby, but it couldn’t tell scientists for certain.

Now, however, astronomers have used an instrument at the Gemini Observatory to “sniff” the planet’s gases from Earth. Their discovery could help write the book on when and where the planets of the solar system formed — and if they ever switched places.

“This is evidence of a big shakeup early on in the solar system’s formation,” Glenn Orton, a co-author of the new study and a planetary scientist at NASA’s Jet Propulsion Laboratory, told Business Insider. “There was definitely a migration taking place.”

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