Scientists have determined that the atmosphere above the planet Uranus is full of a gas that makes the distant planet smell like rotten eggs.

A new study, published Monday in the journal Nature Astronomy, established the chemical composition of Uranus’ upper cloud deck for the first time.

Using an instrument called the Near-Infrared Integral Field Spectrometer (NIFS) at the Gemini North telescope in Hawaii, researchers determined that Uranus’ cloud cover is composed of hydrogen sulfide (H2S) gas, which is what gives rotten eggs their distinctive, noxious scent.

“If an unfortunate human were ever to descend through Uranus’s clouds, they would be met with very unpleasant and odiferous conditions,” said study co-author Patrick Irwin of the University of Oxford, according to the BBC.

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Scientists had speculated that Uranus and Neptune’s clouds could also exhibit ammonia ice (NH₃), as on gas giants Jupiter and Saturn, but previous observations from both ground telescopes and the Voyager 2 spacecraft were inconclusive.

The new study revealed atmospheric differences between the gas and ice giants that could answer questions about the planets’ early history, scientists said.

But future space travelers would succumb to Uranus’ hostile elements long before noticing the odor, according to Irwin. “Suffocation and exposure in the negative 200 degrees Celsius [-392 Fahrenheit] atmosphere… would take its toll long before the smell,” he said.

Write to Eli Meixler at eli.meixler@time.com.</s>Hydrogen sulfide, the gas that gives rotten eggs their distinctive odor, permeates the upper atmosphere of the planet Uranus – as has been long debated, but never definitively proven. Based on sensitive spectroscopic observations with the Gemini North telescope, astronomers uncovered the noxious gas swirling high in the giant planet’s cloud tops. This result resolves a stubborn, long-standing mystery of one of our neighbors in space.

Even after decades of observations, and a visit by the Voyager 2 spacecraft, Uranus held on to one critical secret, the composition of its clouds. Now, one of the key components of the planet’s clouds has finally been verified.

Patrick Irwin from the University of Oxford, UK and global collaborators spectroscopically dissected the infrared light from Uranus captured by the 8-meter Gemini North telescope on Hawaii’s Maunakea. They found hydrogen sulfide, the odiferous gas that most people avoid, in Uranus’s cloud tops. The long-sought evidence is published in the April 23rd issue of the journal Nature Astronomy.

The Gemini data, obtained with the Near-Infrared Integral Field Spectrometer (NIFS), sampled reflected sunlight from a region immediately above the main visible cloud layer in Uranus’s atmosphere. “While the lines we were trying to detect were just barely there, we were able to detect them unambiguously thanks to the sensitivity of NIFS on Gemini, combined with the exquisite conditions on Maunakea,” said 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.”

“This work is a strikingly innovative use of an instrument originally designed to study the explosive environments around huge black holes at the centers of distant galaxies,” said Chris Davis of the United State’s National Science Foundation, a leading funder of the Gemini telescope. “To use NIFS to solve a longstanding mystery in our own Solar System is a powerful extension of its use.” Davis adds.

Astronomers have long debated the composition of Uranus’s clouds and whether hydrogen sulfide or ammonia dominate the cloud deck, but lacked definitive evidence either way. “Now, thanks to improved hydrogen sulfide absorption-line data and the wonderful Gemini spectra, we have the fingerprint which caught the culprit,” says Irwin. The spectroscopic absorption lines (where the gas absorbs some of the infrared light from reflected sunlight) are especially weak and challenging to detect according to Irwin.

The detection of hydrogen sulfide high in Uranus’s cloud deck (and presumably Neptune’s) contrasts sharply with the inner gas giant planets, Jupiter and Saturn, where no hydrogen sulfide is seen above the clouds, but instead ammonia is observed. The bulk of Jupiter and Saturn’s upper clouds are comprised of ammonia ice, but it seems this is not the case for Uranus. These differences in atmospheric composition shed light on questions about the planets’ formation and history.

Leigh Fletcher, a member of the research team from the University of Leicester in the UK, adds that the differences between the cloud decks of the gas giants (Jupiter and Saturn), and the ice giants (Uranus and Neptune), were likely imprinted way back during the birth of these worlds. “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.”

Another factor in the early formation of Uranus is the strong evidence that our Solar System’s giant planets likely migrated from where they initially formed. Therefore, confirming this composition information is invaluable in understanding Uranus’ birthplace, evolution and refining models of planetary migrations.

According to Fletcher, when a cloud deck forms by condensation, it locks away the cloud-forming gas in a deep internal reservoir, hidden away beneath the levels that we can usually see with our telescopes. “Only a tiny amount remains above the clouds as a saturated vapour,” said Fletcher. “And this is why it is so challenging to capture the signatures of ammonia and hydrogen sulfide above cloud decks of Uranus. The superior capabilities of Gemini finally gave us that lucky break,” concludes Fletcher.

Glenn Orton, of NASA’s Jet Propulsion Laboratory, and another member of the research team notes, “We’ve strongly suspected that hydrogen sulfide gas was influencing the millimeter and radio spectrum of Uranus for some time, but we were unable to attribute the absorption needed to identify it positively. Now, that part of the puzzle is falling into place as well.”

While the results set a lower limit to the amount of hydrogen sulfide around Uranus, it is interesting to speculate what the effects would be on humans even at these concentrations. “If an unfortunate human were ever to descend through Uranus’s clouds, they would be met with very unpleasant and odiferous conditions.” But the foul stench wouldn’t be the worst of it according to Irwin. “Suffocation and exposure in the negative 200 degrees Celsius atmosphere made of mostly hydrogen, helium, and methane would take its toll long before the smell,” concludes Irwin.

The new findings indicate that although the atmosphere might be unpleasant for humans, this far-flung world is fertile ground for probing the early history of our Solar System and perhaps understanding the physical conditions on other large, icy worlds orbiting the stars beyond our Sun.

Publication: Patrick G. J. Irwin, et al., “Detection of hydrogen sulfide above the clouds in Uranus’s atmosphere,” Nature Astronomy (2018) doi:10.1038/s41550-018-0432-1</s>University of Oxford researchers have discovered that Uranus smells bad.

According to an article in USA Today, scientists examining the planet’s infrared light through the Gemini North telescope found clouds of hydrogen sulfide spreading into the distant planet’s upper atmosphere.

Hydrogen sulfide is a toxic gas that smells like rotten eggs, or like someone passed gas.

As if the name of the planet wasn’t enough, scientists recently discovered Uranus is home to fart-smelling clouds.

The research team’s findings were published Monday in Nature Astronomy, an online, peer-reviewed scientific journal.

“If an unfortunate human were ever to descend through Uranus’s clouds, they would be met with very unpleasant and odiferous conditions,” lead author Patrick Irwin said in a release.

But the chances of that actually happening are slim.

Irwin said atmospheric conditions on Uranus, which include temperatures of negative 200 degrees Celsius and a mixture of hydrogen, helium, and methane, would almost certainly be fatal to humans — long before the bad odor would be a factor.</s>Scientists solve a "longstanding mystery", detecting hydrogen sulphide gas — which smells like rotten eggs — swirling in Uranus's cloud tops.

Scientists spotted the noxious gas swirling above the clouds in Uranus's atmosphere. (Credit: ABC)

They discovered hydrogen sulphide gas — also known as rotten egg gas — swirling in the planet's cloud tops.

"If an unfortunate human were ever to descend through Uranus's clouds, they would be met with very unpleasant and odiferous conditions," lead author of a study, published in Nature Astronomy, Professor Patrick Irwin said in a statement.

The composition of Uranus's clouds, and whether their main component was hydrogen sulphide or ammonia, has long been debated.

The scientists found the answer by looking at Uranus through an eight-metre-diameter telescope at the Gemini Observatory in Hawaii, and spotted the noxious gas swirling above its clouds.

"We were able to detect the 'fingerprint' of [hydrogen sulphide] in the light reflected from Uranus's clouds," Professor Irwin said.

"From the strength of the fingerprint, we were able to figure out how much [hydrogen sulphide] is present at the cloud tops."

But Professor Irwin noted Uranus's bad smell would be far from the biggest concern for a human visitor.

"Suffocation and exposure in the minus-200-degrees-Celsius atmosphere — made of mostly hydrogen, helium and methane — would take its toll long before the smell," Professor Irwin said.

In contrast, no hydrogen sulphide was seen above the clouds of inner gas giant planets Jupiter and Saturn.

Instead, most of their upper-clouds were made of ammonia ice.

The Gemini Observatory said the planets' differences in atmospheric composition "shed light on questions about the planets' formation and history".

The differences were likely imprinted in their early history, determined by the conditions where they formed, study co-author Dr Leigh Fletcher said.

"This composition information is invaluable in understanding Uranus' birthplace, evolution and refining models of planetary migrations," the Gemini Observatory said.

Professor Irwin said there was now another mystery about Uranus to solve.

"We should also see a very thick cloud of methane about the [hydrogen sulphide] cloud, but actually only see clear methane clouds in small, occasional storm cloud systems," he said.</s>If humans ever make it to Uranus, they’re in for an unpleasant experience.

Scientists have confirmed that the seventh planet from the sun is swathed by clouds that carry the distinctly foul odor of rotten eggs, thanks to the presence of hydrogen sulfide.

In addition to the smell, the ice giant planet can hit temperatures as low as minus 200 degrees Celsius.

While it’s long been speculated that Uranus is home to this noxious gas, the composition of its clouds has eluded scientists for decades until now.

By analyzing infrared light from Uranus using the 8-meter Gemini North telescope on Hawaii’s Maunakea, an international team of researchers has finally verified a major component of Uranus’ upper atmosphere.

‘We’ve strongly suspected that hydrogen sulfide gas was influencing the millimeter and radio spectrum of Urnaus for some time, but we were unable to attribute the absorption needed to identify it positively,’ said Glenn Orton, of NASA’s Jet Propulsion Laboratory.

‘Now, that part of the puzzle is falling into place as well.’

The team used data from Gemini’s Near-Infrared Integral Field Spectrometer, which sampled reflected sunlight from just above Uranus’ main visible cloud layer.

The composition of Uranus’ clouds has long been the subject of debate.

Without definitive evidence, it’s remained unclear for decades whether the region is predominantly composed of hydrogen sulfide, or of ammonia.

With the Gemini data, the researchers were able to see the spectroscopic absorption lines despite their faint appearance.

‘While the lines we were trying to detect were just barely there, we were able to detect them unambiguously thanks to the sensitivity of NIFS on Gemini, combined with the exquisite conditions on Maunakea,’ said 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.’

‘Now, thanks to improved hydrogen sulfide absorption-line data and the wonderful Gemini spectra, we have the fingerprint which caught the culprit,’ says Irwin.

The planets in our solar system can have dramatically different cloud decks based on how they were formed, the researchers explain.

As it’s thought that the planets migrated from where they formed, the latest findings add additional insight on the origin and evolution of Uranus.

‘During our solar system’s formation the balance between nitrogen and sulfur (and hence ammonia and Uranus’ newly-detected hydrogen sulfide) was determined by the temperature and location of planet’s formation,’ Leigh Fletcher, a member of the research team from the University of Leicester in the UK.

With the data, the researchers have refined the limits of hydrogen sulfide that likely resides around Uranus.

And, while it’s lower than previously expected, they say the unsavory smell wouldn’t be the worst of it.

‘If an unfortunate human were ever to descend through Uranus’ clouds, they would be met with very unpleasant and odiferous conditions,’ Irwin.

‘Suffocation and exposure in the negative 200 degrees Celsius atmosphere made of mostly hydrogen, helium, and methane would take its toll long before the smell.’</s>Researchers confirmed Monday the seventh planet from the sun has an upper atmosphere full of one of the smelliest chemicals known to humans, hydrogen sulfide, according to a study published by Nature Astronomy.

The odorous gas is what gives rotten eggs — and human flatulence — their distinctive and unpleasant smell. According to the Environmental Protection Agency, people can smell the gas when it makes up as little as three out of every billion molecules in the air, so imagine what being surrounded by clouds of the stuff would smell like.

“If an unfortunate human were ever to descend through Uranus’s clouds, they would be met with very unpleasant and odoriferous conditions,” Patrick Irwin a physicist at the University of Oxford, who led the study, said in a statement.

Scientists discovered evidence of “the noxious gas swirling high in the giant planet’s cloud tops” after observing how sunlight bounced off Uranus’ atmosphere, according to a news release from the Gemini Observatory, a high-power telescope located on top of Hawaii’s Mauna Kea volcano.

The new findings come after decades of observations and even a visit by the Voyager 2 spacecraft to the blue-green ice giant, the release said. Before making the discovery, scientists had long inferred hydrogen sulfide existed in the planet’s atmosphere, but never “conclusively detected” the gas before, according to Science News.

Using a 26-foot Gemini North telescope, the team of scientists studied the reflected sunlight in infrared and determined what types of molecules made up the planet’s atmosphere, the release said. While evidence of the molecular compounds was “barely there,” Irwin said scientists were still “able to detect them unambiguously” given the sensitivity of their instruments and the “exquisite conditions” on Mauna Kea.

The Uranus’ atmospheric composition was so difficult to nail down because when a cloud deck forms by condensation, it hides the gas responsible for forming the clouds beneath levels that can be usually seen with telescopes, Leigh Fletcher, a member of the research team, said in the release.

“Only a tiny amount remains above the clouds as a saturated vapour,” said Fletcher, who is a planetary scientist at the University of Leicester in the United Kingdom. “The superior capabilities of Gemini finally gave us that lucky break.”

Aside from lending credence to Uranus jokes, the hydrogen sulfide discovery sheds light (or maybe smell) on how planets and the solar system formed, the release said.

Being able to confirm the composition information is “invaluable in understanding Uranus’ birthplace, evolution and refining models of planetary migrations,” the release said.

Knowing what makes up distant planets, such as Uranus, could help scientists determine where in the solar system the planets first formed and how far they moved from the sun over time, Business Insider reported.

Glenn Orton, a co-author of the new study and a planetary scientist at NASA’s Jet Propulsion Laboratory, told Business Insider the new research points to “evidence of a big shakeup early on in the solar system’s formation.”

“There was definitely a migration taking place,” Orton said.

While the planet’s smell may be more than enough to repel most from wanting to visit, Orton said researchers are working on a proposal for a new Uranus spacecraft, which they hope will help them learn more about where the outer planets actually formed and how the solar system came to be.

Hopefully, the proposed spacecraft will be unmanned like its predecessor, Voyager 2, because Orton said the probe will be expected to plunge through Uranus’ pungent clouds.</s>It’s a smell that make most people gag, but it seems that Uranus has the distinct aroma of rotten eggs.

Scientists from the University of Oxford have discovered a cloud of hydrogen sulphide - the gas that gives rotten eggs their smell - surrounding the planet.

In the study, the researchers analysed the infrared light from Uranus, captured by the Gemini North telescope on Hawaii’s Maunakea.

While astronomers have previously theorised that Uranus’ clouds contain hydrogen sulphide, this is the first study to prove it.

Patrick Irwin, who led the study, said: “Now, thanks to improved hydrogen sulfide absorption-line data and the wonderful Gemini spectra, we have the fingerprint which caught the culprit.”

Unlike Uranus, Jupiter and Saturn have no hydrogen sulphide int their clouds, and instead have ammonia.

These differences could provide key insights into how the planets formed.

Leigh Fletcher, who also worked on the study, said: “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.”

The researchers also speculated about what the effects of the levels of hydrogen sulphide around Uranus would be on humans.

Dr Irwin said: “If an unfortunate human were ever to descend through Uranus’s clouds, they would be met with very unpleasant and odiferous conditions.

“Suffocation and exposure in the negative 200 degrees Celsius atmosphere made of mostly hydrogen, helium, and methane would take its toll long before the smell.”</s>
• Uranus is one of the solar system's most mysterious planets.
• Scientists had long believed that the "ice giant" world has clouds of hydrogen sulfide, a compound that smells bad to people, but they couldn't be certain.
• New telescope observations confirm the planet is clouded by the chemical.
• The discovery may help astronomers further unravel the twisted history of the solar system's formation.

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."

The journal Nature Astronomy published the findings on Monday.

Why it took so long to detect Uranus's stinky clouds

The reason most people avoid hydrogen sulfide is because the compound is a signature ingredient in the scent of rotten eggs — and farts.

Humans can smell hydrogen sulfide when it makes up as little as three out of every billion molecules in the air, the EPA says. At higher concentrations, such as near volcanic areas, it can be poisonous. Breathing a concentration of a few hundred parts per million can kill a person in about half an hour, according to the Centers for Disease Control.

"If an unfortunate human were ever to descend through Uranus's clouds, they would be met with very unpleasant and odiferous conditions," Patrick Irwin, a physicist at the University of Oxford who led the new study, said in the press release. But he added that "suffocation and exposure" to Uranus' -200 degree Celsius temperatures "would take its toll long before the smell."

Researchers had long suspected that Uranus's atmosphere was laced with hydrogen sulfide, and in concentrations dozens of times higher than at Saturn or Jupiter.

They couldn't be certain, though, since Uranus orbits the sun from 1.85 billion miles away.

The vast distance, aside from making the planet distant and difficult to study, leads to blisteringly cold temperatures that freezes hydrogen sulfide. Hydrogen-sulfide ice can form clouds, but the solid crystals are hard for chemical-analyzing instruments called spectrometers to study. (The method works far better with liquids and gases.)

Irwin and others suspected there were at least whiffs of hydrogen sulfide gas drifting above the clouds. And now, thanks to an extremely sensitive Gemini instrument that can see light invisible to humans, Irwin said scientists "have the fingerprint which caught the culprit."

Why the discovery might 'shake up' ideas about the solar system's evolution

Astronomers like Irwin have an interest in hydrogen sulfide on Uranus that goes far beyond the gas' smell.

The discovery of hydrogen sulfide may help piece together the story of how the solar system formed and arranged itself some 4.6 billion years ago. Figuring out the exact makeup of distant planets could help determine where in the solar system they first formed — and how far they migrated away from the sun afterward.

Like all planets, Uranus and Neptune formed from a giant disk of gas and dust that shrouded the sun some 4.6 billion years ago. The planets are mostly made of heavier elements, and didn't balloon with lightweight hydrogen and helium gases.

"Uranus and Neptune never had the time to grow into gas giants like Jupiter and Saturn," Imke de Pater, an astronomer at the University of California Berkeley who wasn't involved in the study, wrote in an accompanying article in Nature Astronomy. "The composition of a celestial body is a fundamental parameter in determining its formation and evolutionary history."

The two planets failed to become gas giants (and instead became "ice giants") for two main reasons.

First, the solar system's early disk of dust and gas grew more diffuse farther out from the sun. With less material available, it took Uranus and Neptune longer to form.

Second, this slower formation gave the sun more time to blow hydrogen and helium out of the solar system with its stellar winds — before Uranus and Neptune could grow massive enough to capture it with their gravity.

"Giant planets form really fast, in a few million years," Kevin Walsh, who studies planet formation at the Southwest Research Institute in Colorado, told Space.com in March. "That creates a time limit because the gas disk around the sun only lasts 4 to 5 million years."

Scientists agree on this much, but Orton said they have "too many theories" about the migration of Uranus and Neptune, which came next.

One leading idea says the planets coalesced millions of miles closer to the sun, then quickly migrated outward.

But Orton said the newly detected hydrogen sulfide — and a strange lack of ammonia in the Gemini Observatory readings — suggest the planets actually formed farther out, then moved inward. (The ratio of the two molecules suggests the worlds were once even colder than they are today.)

To solve the question once and for all, Orton says researchers need to send spacecraft to plunge through the clouds of Uranus, not unlike how the Cassini probe dove into Saturn. The goal: figure out the exact abundances of hydrogen sulfide and ammonia, among other gases, and use the readings to pin the exact birth locations of Uranus and Neptune.

"We're working on that now," Orton said, referring to a proposal for a new Uranus probe.</s>Uranus stinks. And we're not joking.

The enigmatic outer solar system planet has long had a credibility problem, what with it being the butt of countless immature jokes. Now, astronomers have just discovered a gas in Uranus' clouds that does nothing to curtail the giggles. At all.

The new study, published in the journal Nature Astronomy, has discovered the chemical signature of hydrogen sulfide, a compound that gives rotten eggs their distinctive stench, in the planet's clouds. Besides launching a thousand new smelly planet puns, this finding could transform our understanding of how our solar system evolved. It also may help us to understand the atmospheres of massive planets orbiting other stars.

First, a bit of background: Uranus has only been visited by a spacecraft once, when NASA's Voyager 2 zipped past the planet in 1986. The flyby produced many beautiful and iconic views of the almost featureless, light-blue world. Astronomers have made countless ground-based observations of Uranus, too, in the hopes of better understanding the composition of its atmosphere. Despite these efforts, however, we know surprisingly little about this enigmatic planet. But the discovery of hydrogen sulfide is a big step forward, and it could only be done using one of the planet's most powerful observatories.

Using the Near-Infrared Integral Field Spectrometer (NIFS) that is attached to the Gemini North telescope in Hawaii, astronomers were able to detect the very slight spectroscopic signature of hydrogen sulfide in the uppermost layers of Uranus' clouds. This whiff of hydrogen sulfide is only the tip of the odoriferous iceberg, however; the presence of this gas is indicative of a huge reservoir below the obscuring cloud deck.

"Only a tiny amount [of hydrogen sulfide] remains above the clouds as a saturated vapour," said co-investigator Leigh Fletcher, of the University of Leicester, U.K., in a Gemini North statement. "And this is why it is so challenging to capture the signatures of ammonia and hydrogen sulfide above cloud decks of Uranus. The superior capabilities of Gemini finally gave us that lucky break."

Astronomers have long argued over whether hydrogen sulfide or ammonia dominate Uranus' clouds. It is well-known that the inner massive planets, Jupiter and Saturn, have atmospheres dominated by ammonia ice, whereas Uranus (and presumably Neptune) do not. It's those very differences in atmospheric compositions that place Jupiter and Saturn in the "gas giant" category and Uranus and Neptune in the "ice giant" category, and these differences reveal an insight as to where the planets formed.

"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," said Fletcher.

The thought is that early in our solar system's history, the massive planets migrated from where they initially formed, eventually settling into the stable orbits we see them in today. Through the analysis of the chemicals in their clouds, astronomers can now formulate theories as to how far away from the sun these giant worlds formed and where they migrated from. With this information in mind, astronomers can then look to other stars and gain an insight as to how and where giant exoplanets formed.

This is all very interesting, but the biggest question scientists are likely answering right now is: If we could smell Uranus' atmosphere, would it kill us?

"If an unfortunate human were ever to descend through Uranus's clouds, they would be met with very unpleasant and odiferous conditions," said lead author Patrick Irwin, of the University of Oxford, U.K., also in the accompanying release. But it's not the stench that will kill you.

"Suffocation and exposure in the negative 200 degrees Celsius atmosphere made of mostly hydrogen, helium, and methane would take its toll long before the smell," he concluded.</s>They discovered hydrogen sulphide gas — also known as rotten egg gas — swirling in the planet's cloud tops.

"If an unfortunate human were ever to descend through Uranus's clouds, they would be met with very unpleasant and odiferous conditions," lead author of a study, published in Nature Astronomy, Professor Patrick Irwin said in a statement.

The composition of Uranus's clouds, and whether their main component was hydrogen sulphide or ammonia, has long been debated.

The scientists found the answer by looking at Uranus through an eight-metre-diametre telescope at the Gemini Observatory in Hawaii, and spotted the noxious gas swirling above its clouds.

But Professor Irwin noted Uranus's bad smell would be far from the biggest concern for a human visitor.

"Suffocation and exposure in the minus-200-degrees-Celsius atmosphere — made of mostly hydrogen, helium and methane — would take its toll long before the smell," Professor Irwin said.

In contrast, no hydrogen sulphide was seen above the clouds of inner gas giant planets Jupiter and Saturn.

Instead, most of their upper-clouds were made of ammonia ice.

The Gemini Observatory said the planets' differences in atmospheric composition "shed light on questions about the planets' formation and history".

The differences were likely imprinted in their early history, determined by the conditions where they formed, study co-author Dr Leigh Fletcher said.

"This composition information is invaluable in understanding Uranus' birthplace, evolution and refining models of planetary migrations," the Gemini Observatory said.
Scientists using infrared spectroscopy data from the Gemini North telescope have established the presence of hydrogen sulfide in the clouds of Uranus.