The Tricky Observations That Revealed the First Water Clouds Outside the Solar System
WISE 0855 concept

7.2 light-years away from Earth sits the coldest known object outside of our solar system. The brown dwarf WISE 0855. The largest ground-based telescopes can barely make out the dim, failed star.

Last week, a team of astronomers announced an amazing discovery. Using the Gemini North telescope atop Mauna Kea in Hawaii, astronomers at UC Santa Cruz gathered an infrared spectrum of the brown dwarf. Within this spectrum, they found strong evidence of clouds of water or water ice.

It’s the first instance of water clouds being detected outside our solar system. But, the discovery itself isn’t unexpected. “We would expect an object that cold to have water clouds, and this is the best evidence that it does,” said Andrew Skemer, an assistant professor of astronomy and astrophysics at UC Santa Cruz. Skemer is also first author of a paper detailing the new findings.

How cold are we talking? The team estimates the temperature of WISE 0855 is about 250 Kelvin, or minus 10 degrees Fahrenheit.

WISE 0855 formed just like a star. But during the process, it couldn’t gain enough mass to spark the nuclear fusions that make a star a star. Instead, WISE 0855 is known as a failed star and clocks in at about five times the mass of Jupiter.

Observing WISE 0855

Getting a hold of a spectrum for WISE 0855 was no easy task. “It’s five times fainter than any other object detected with ground-based spectroscopy at this wavelength,” says Skemer.

The failed star is too dim for typical spectroscopy at optical or near-infrared wavelengths. The team of astronomers had to focus on thermal emissions from the deep atmosphere in an extremely narrow range. It was “challenging but not impossible,” said Skemer.

Using the Gemini Near-Infrared Spectrograph, the telescope observed WISE 0855 for about an hour a night for 13 nights. The result?

“Our spectrum shows that WISE 0855 is dominated by water vapor and clouds, with an overall appearance that is strikingly similar to Jupiter.”’

If your eyes could see infrared light, the data would look like a “rainbow of colors,” according to Skemer. “The relative brightness of each color gives us a glimpse into the environment of the object’s atmosphere.”

Once the team had their spectrum, they developed atmospheric models to better understand what WISE 0855 may look like. The models pointed to a cloudy model as the best fit for the spectrum they observed. They also noted water absorption features were extremely similar to Jupiter.

There was one major difference, though. Phosphine is seen in abundance in Jupiter’s atmosphere, but not WISE 0855’s. Its presence in Jupiter’s atmosphere is a result of a more turbulent atmosphere. It appears WISE 0855’s atmosphere is calmer.

Pushing Gemini North to its limits

gemini north telescope

The team of astronomers didn’t think they could even get a spectrum of the brown dwarf at first. “I thought we’d have to wait until the James Webb Space Telescope was operating to do this,” said Skemer.
A combination of a fantastic telescope and the perfect location made this discovery possible.

“We pushed the boundary of what could be done with a telescope here on Earth. And the result is spectacular,” said Jacqueline Faherty of the Carnegie Department of Terrestrial Magnetism.

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