Citizen Scientists Discover Nearly 100 Planet-Star Hybrids Near the Sun

When we gaze up at the night sky, we see a mixture of stars that are close to the Sun, making them visible even if their light is relatively faint, as well as stars that are far away from the Sun, which can be spotted because they are substantially brighter than the average star. 

What we can’t see, at least with the naked eye, are the multitudes of dim “failed stars,” known as brown dwarfs, that are lurking in the darkness around the Sun. These gassy objects are larger than planets but smaller than stars, and our cosmic backyard is absolutely filled with them, reports a forthcoming study in the Astrophysical Journal. 

Astronomers and volunteers teamed up to search for these in-between worlds, and ended up finding “95 candidate extremely cold brown dwarfs” that are “previously unrecognized substellar neighbors to the Sun,” according to a preprint of the study published on the arXIv preprint server. 

The new research was made possible by a citizen science project called Backyard Worlds, which enlisted 100,000 volunteers to meticulously scan through astronomical imagery to hunt for signs of elusive brown dwarfs.

“A complete census of the solar neighborhood provides the best way to identify and study the Galactic substellar population,” said the study’s authors, led by NOIRLab astronomer and Backyard Worlds co-founder Aaron Meisner. “The intrinsic faintness of the lowest temperature brown dwarfs means that we can only hope to directly image those which are nearby to the Sun.”

“Backyard Worlds crowdsources the visual vetting workload amongst thousands of volunteers who participate via the internet,” Meisner’s team added.

The Backyard Worlds team has already spotted some 1,500 faint objects near the Sun, but the new study pinpoints a handful of particularly cold and dim dwarfs located within a radius of about 65 light years. These objects, known as “Y dwarfs,” are the chilliest star-like entities known, according to NASA. Some are colder than the average human body temperature.

Brown dwarfs can be anywhere from 13 to 80 times as massive than Jupiter, but they are not big enough to initiate hydrogen fusion in their cores, which is the process that powers regular stars and makes them shine. The term “brown” is a bit of a misnomer as these objects would be closer to reddish or magenta if observed up close.

Given that they don’t emit starlight, scientists must use specialized observatories, such as NASA's Wide-field Infrared Survey Explorer (WISE) space telescope, to spot them. These telescopes are able to scan the skies for infrared light, which brown dwarfs emit.

Citizen scientists working with Backyard Worlds are trained to scan WISE images for the signature motion of brown dwarfs as they move through the telescope’s wide frames. Meisner’s team added a new filter to this process by following up on some of the coldest candidate objects with imagery collected by NASA’s recently retired Spitzer telescope and Keck Observatory's Near-Infrared Echellette Spectrometer (NIRES). 

The team confirmed that five objects previously seen in WISE pictures are bonafide Y dwarf candidates, as opposed to other cosmic phenomena or noise in the images. The researchers also identified dozens of “T dwarfs” which are slightly warmer and brighter. The study, which is co-authored both by professional astronomers and 20 citizen scientists from 10 countries, includes a comprehensive index of the objects.

Meisner and his colleagues hope to conduct even more follow-up observations to further resolve details about these curious worlds 

“While this work’s new brown dwarf candidates already demonstrate the power of citizen science for mapping the solar neighborhood, these objects make up only a small fraction of Backyard Worlds moving object discoveries to date,” the team concluded in the study. 

“Backyard Worlds will endeavor to search all of its newly delivered data for yet more cold and close neighbors to the Sun.”