A team of professional astronomers and citizen scientists from the Backyard Worlds: Planet 9 project has spotted 95 new brown dwarfs near the Sun.
An artist’s impression of one of the team’s discoveries — the oldest known wide-separation white dwarf plus cold brown dwarf pair. Image credit: NOIRLab / NSF / AURA / P. Marenfeld / William Pendrill.
Brown dwarfs are cool, dim objects that have a size between that of a gas giant planet, such as Jupiter or Saturn, and that of a Sun-like star.
Sometimes called failed stars, these objects are too small to sustain hydrogen fusion reactions at their cores, yet they have star-like attributes.
Typically, they have masses between 11-16 Jupiters (the approximate mass at which deuterium fusion can be sustained) and 75-80 Jupiters (the approximate mass to sustain hydrogen fusion).
Brown dwarfs are classified spectrally into M-, L-, T- and Y-type dwarfs.
Despite their name, they are of different colors. Many brown dwarfs would likely appear magenta or orange-red to the human eye.
Their low mass, low temperature, and lack of internal nuclear reactions make them extremely faint — and therefore extremely difficult to detect.
Still, they give off heat in the form of infrared light, which is invisible to the human eye yet detectable by telescopes such as NASA’s NEOWISE and Spitzer observatories.
To help find our Sun’s coldest, nearest neighbors, astronomers with the Backyard Worlds project turned to a worldwide network of more than 100,000 citizen scientists.
These volunteers diligently inspect trillions of pixels of telescope images to identify the subtle movements of nearby brown dwarfs and planets. Despite the advances of machine learning and supercomputers, there’s still no substitute for the human eye when it comes to finding faint, moving objects.
“These cool worlds offer the opportunity for new insights into the formation and atmospheres of planets beyond the Solar System,” said Dr. Aaron Meisner, an astronomer with NSF’s National Optical-Infrared Astronomy Research Laboratory.
“This collection of cool brown dwarfs also allows us to accurately estimate the number of free-floating worlds roaming interstellar space near the Sun.”
To identify several of the faintest and coolest of the new brown dwarfs, Dr. Meisner and colleagues used the Near-Infrared Echellette Spectrometer (NIRES) instrument at Keck Observatory.
“We used the NIRES spectra to measure the temperature and gases present in their atmospheres,” said Professor Adam Burgasser, an astrophysicist in the Center for Astrophysics and Space Science at the University of California San Diego.
“Each spectrum is essentially a fingerprint that allows us to distinguish a cool brown dwarf from other kinds of stars.”
Follow-up observations using NASA’s Spitzer Space Telescope, Mont Megantic Observatory, and Las Campanas Observatory also contributed to the brown dwarf temperature estimates.
WISEA J025805.29-321917.4, one of the new brown dwarf discoveries, as seen in sky maps from WISE (right) and Legacy Surveys DR8 (left). Image credit: A. Meisner / NOIRLab / NSF / AURA.
Among the team’s most exciting discoveries are several brown dwarfs within 10 parsec (33 light-years) from the Sun, two fast moving objects, three T-type subdwarfs, five Y-type dwarfs, and a new T8 plus white dwarf co-moving system.
The astronomers also discovered a T8-type dwarf co-moving with a white dwarf star called LSPM J0055+5948, the fourth such system to be found, and a T-type dwarf companion to the white dwarf LSR J0002+6357.
“It’s exciting these could be spotted first by a citizen scientist,” Dr. Meisner said.
“The Backyard Worlds discoveries show that members of the public can play an important role in reshaping our scientific understanding of our solar neighborhood.”
In 2014, astronomers discovered the coldest-known brown dwarf, called WISE J085510.83-071442.5 (WISE 0855 for short).
This Y2-type brown dwarf has a temperature of about minus 23 degrees Celsius (minus 10 degrees Fahrenheit) — no other brown dwarf came close to its low temperature. Some researchers wondered if WISE 0855 was actually a rogue exoplanet.
The newly-discovered brown dwarfs, together with others recently discovered using NEOWISE and Spitzer, puts WISE 0855 in context.
“Our new discoveries help connect the dots between WISE 0855 and the other known brown dwarfs,” said Dr. Marc Kuchner, the principal investigator of Backyard Worlds and the Citizen Science Officer for NASA’s Science Mission Directorate and an astrophysicist at NASA’s Goddard Space Flight Center.
The team’s paper will be published in the Astrophysical Journal.
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Aaron M. Meisner et al. 2020. Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project. ApJ, in press; arXiv: 2008.06396
