Scientists spot water clouds on mysterious cold brown dwarf

Scientists have found evidence for clouds of water in a mysterious cold brown dwarf located 7.2 light-years from Earth, the first such clouds detected outside of our solar system.

Since its detection in 2014, the brown dwarf known as WISE 0855 has fascinated astronomers. It is the coldest known object outside our solar system and is just barely visible at infrared wavelengths with the largest ground-based telescopes.

Scientists at University of California, Santa Cruz have succeeded in obtaining an infrared spectrum of WISE 0855 using the Gemini North telescope in Hawaii, providing the first details of the object's composition and chemistry.

Among the findings is strong evidence for the existence of clouds of water or water ice, the first such clouds detected outside of our solar system.

"We would expect an object that cold to have water clouds, and this is the best evidence that it does," said Andrew Skemer, assistant professor at UC Santa Cruz.

A brown dwarf is essentially a failed star, having formed the way stars do through the gravitational collapse of a cloud of gas and dust, but without gaining enough mass to spark the nuclear fusion reactions that make stars shine.

With about five times the mass of Jupiter, WISE 0855 resembles that gas giant planet in many respects. Its temperature is about minus 23 degrees Celsius, making it nearly as cold as Jupiter.

"WISE 0855 is our first opportunity to study an extrasolar planetary-mass object that is nearly as cold as our own gas giants," Skemer said.

Previous observations of the brown dwarf provided tentative indications of water clouds based on very limited photometric data.

Researchers said obtaining a spectrum (which separates the light from an object into its component wavelengths) is the only way to detect an object's molecular composition.

WISE 0855 is too faint for conventional spectroscopy at optical or near-infrared wavelengths, but thermal emission from the deep atmosphere at wavelengths in a narrow window around 5 microns offered an opportunity where spectroscopy would be "challenging but not impossible," Skemer said.

The team used the Gemini-North telescope in Hawaii and the Gemini Near Infrared Spectrograph to observe WISE 0855 over 13 nights for a total of about 14 hours.

"It's five times fainter than any other object detected with ground-based spectroscopy at this wavelength," Skemer said.

"Our spectrum shows that WISE 0855 is dominated by water vapour and clouds, with an overall appearance that is strikingly similar to Jupiter," he said.

Comparing the brown dwarf to Jupiter, the team found that their spectra are strikingly similar with respect to water absorption features.

The research was published in the Astrophysical Journal Letters.