Astronomers have succeeded in obtaining an infrared spectrum of the recently discovered brown dwarf WISE J085510.83-071442.5 (WISE 0855 for short), providing details of the object’s composition and chemistry. Among the findings is strong evidence for the existence of clouds of water or water ice.
This artist’s conception shows WISE 0855, the coldest known brown dwarf. Image credit: NASA / JPL-Caltech / Penn State University.
At a distance of about 7.2 light-years away from Earth, WISE 0855 is the fourth closest system to us, after Alpha Centauri AB – Proxima Centauri, Barnard’s Star and WISE 1049-5319.
WISE 0855 is the nearest known planetary mass object and the coldest known compact object outside of our Solar System.
It was discovered in 2014 by Pennsylvania State University astronomer Kevin Luhman using NASA’s Wide-field Infrared Survey Explorer and Spitzer Space Telescope.
With about five times the mass of Jupiter, WISE 0855 resembles a gas giant planet in many respects.
Its temperature is about minus 10 degrees Fahrenheit (250 degrees Kelvin, minus 23 degrees Celsius), making it nearly as cold as our Solar System’s gas giants.
Previous observations of this brown dwarf, published in 2014, provided tentative indications of water clouds based on very limited photometric data.
“Obtaining a spectrum is the only way to detect an object’s molecular composition,” said Dr. Andrew Skemer from the University of California, Santa Cruz.
“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.”
Dr. Skemer and his colleagues 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,” Dr. Skemer said.
“Our spectrum shows that WISE 0855 is dominated by water vapor and clouds, with an overall appearance that is strikingly similar to Jupiter.”
The team developed atmospheric models of the equilibrium chemistry for a brown dwarf at 250 degrees Kelvin and calculated the resulting spectra under different assumptions, including cloudy and cloud-free models.
The models predicted a spectrum dominated by features resulting from water vapor, and the cloudy model yielded the best fit to the features in the spectrum of this object.
Comparing WISE 0855 to Jupiter, the astronomers found that their spectra are strikingly similar with respect to water absorption features.
One significant difference is the abundance of phosphine in Jupiter’s atmosphere.
According to planetary researchers, phosphine forms in the hot interior of the planet and reacts to form other compounds in the cooler outer atmosphere, so its appearance in the spectrum is evidence of turbulent mixing in Jupiter’s atmosphere.
The absence of a strong phosphine signal in the spectrum of WISE 0855 implies that it has a less turbulent atmosphere.
A scientific report of the team’s findings has been submitted to the Astrophysical Journal Letters. The article is also publicly available at arXiv.org.
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Andrew Skemer et al. 2016. The First Spectrum of the Coldest Brown Dwarf. ApJL, submitted for publication; arXiv: 1605.04902
