Brown dwarfs that are doubly cool

Astronomers and citizen scientists have together discovered 95 of the coolest known brown dwarfs – objects more massive than planets but lighter than stars.

Several have temperatures approaching that of the Earth, making them cool enough to harbour water clouds. NASA’s Spitzer Space Telescope provided the temperature estimates.

The discoveries, made as part of the Backyard Worlds: Planet 9 project, bridge a gap in the range of low-temperature brown dwarfs, identifying a long-sought missing link, the team writes in a paper in the Astrophysical Journal.

“This collection of cool brown dwarfs also allows us to accurately estimate the number of free-floating worlds roaming interstellar space near the Sun,” says lead author Aaron Meisner from the National Science Foundation’s NOIRLab, US. Twenty citizen scientists from 10 countries are listed as co-authors.

The closest of the new discoveries is roughly 23 light-years from the Sun. Many more of are in the 30–60 light-year range.

Brown dwarfs are not big enough to power themselves like stars, but are still many times heavier than planets. Despite their name, they would actually appear magenta or orange-red to the human eye if seen close up.

Brown dwarfs with low temperatures are also small in diameter and therefore faint in visible light. Still, they give off heat in the form of infrared light, which is invisible to the human eye yet detectable by telescopes. For cold brown dwarfs like those in this study, the infrared signal is also faint, so they are easier to find the closer they are to our solar system.

And despite the abilities of machine learning and supercomputers, there’s no substitute for the human eye when it comes to scouring telescope images for moving objects.

As part of the Backyard Worlds project, a global network of more than 100,000 volunteers inspects trillions of pixels of images to identify the subtle movements of brown dwarfs and planets. They’ve found 1500 to date.

“This paper is evidence that the solar neighbourhood is still uncharted territory and citizen scientists are excellent astronomical cartographers,” says co-author Jackie Faherty of the American Museum of Natural History.

“Mapping the coldest brown dwarfs down to the lowest masses gives us key insights into the low-mass star formation process while providing a target list for detailed studies of the atmospheres of Jupiter analogues.”

In 2014, scientists discovered the coldest-known brown dwarf, called WISE 0855, using data from NASA’s WISE mission in infrared light. WISE 0855 is about minus 23 degrees Celsius. No other brown dwarf came close to this object’s low temperature.

Some researchers in fact wondered if 0855 was actually a rogue exoplanet – a planet that originated in a star system but was kicked out of its orbit.

This new batch of brown dwarfs, together with others recently discovered using NEOWISE and Spitzer, helps “connect the dots between 0855 and the other known brown dwarfs,” said Marc Kuchner, the principal investigator of Backyard Worlds.

Image details

The image above shows one of the new discoveries (circled) as seen in sky maps from the WISE (right) and Legacy Surveys DR8 (left). It appears orange in WISE and deep red in Legacy Surveys data. These colours are indicative of a very low temperature brown dwarf.

Another discovery was the oldest known wide-separation white dwarf plus cold brown dwarf pair. In the illustration below, the small white orb represents the white dwarf (the remnant of a long-dead Sun-like star), while the brown/orange foreground object is the newly discovered brown dwarf companion. This faint brown dwarf was previously overlooked because it lies right within the plane of the Milky Way.