Written by Isabel Swafford
July 23, 2024
A team of researchers and citizen scientists discovered 13 new red dwarf and brown dwarf binary systems in NASA's WISE and NEOWISE data that will help scientists better define the differences between stars and planets.
Previously, only about 20 confirmed pairs of this nature were known, so this 60% increase to the population provides researchers with more opportunities to theorize how these systems form. Federico Marocco, a staff scientist at Caltech IPAC, and his colleagues have published a paper in The Astrophysical Journal on May 29, 2024 that outlines the systems and their possible formation scenarios.
Unlike red dwarf stars, brown dwarfs are mysterious objects that are neither stars nor planets. While they are not massive enough to ignite nuclear fusion—the defining characteristic of stars—they are much bigger and denser than planets. Their masses range from 13 to 80 times the mass of Jupiter, with the upper boundary defining the mass at which nuclear fusion could ignite.
This gray area is of great interest to scientists studying planetary and stellar formation. Understanding the upper and lower limits of planets and stars would help astronomers define what exactly it means to be a planet, star, or something else entirely.
In the paper, Marocco studied brown dwarfs that exist in binary pairs with red dwarf stars. This means a brown dwarf is orbiting a red dwarf, the smallest, coolest classification of stars. Red dwarfs are the most common type of star in the Milky Way, but they glow very faintly in visible light. Brown dwarfs are even fainter objects, glowing primarily in the infrared, beyond what the human eye can see.
These newly discovered pairs orbit each other very weakly and very far apart, at 600 to 7000 AU, where Earth orbits the Sun at 1 AU and Neptune orbits at 30 AU. Since we are observing these pairs from so far away, the wide separation makes it easier for astronomers to distinguish each object.
The systems were identified from a catalog of almost two billion sources of WISE and NEOWISE survey data taken from 2010 to 2018. The team employed a combination of search techniques, including a machine learning tool and visual inspection by citizen scientists.
Called Backyard Worlds, the citizen science collaboration boasts over 80,000 registered volunteers who have collectively performed around 9,000,000 classifications. A passionate and dedicated subset of volunteers has arisen since the project's inception in 2017, with many of them contributing to and even leading scientific papers.
Marocco said it has been a fun and rewarding experience working with the Backyard Worlds volunteers. "It's a wonderful experience because you get to meet people that are completely outside of your bubble," said Marocco. "It's very nice to see their enthusiasm and how excited they get when the object that they pointed out in those images all those months back is now confirmed to actually be a brown dwarf."
Fifteen such hard-working citizen scientists contributed to Marocco's paper, independently discovering six out of the 13 systems, and assisting with the discovery of the rest.
Since brown dwarfs are difficult to observe, the researchers did not have direct distance measurements to most of the brown dwarfs they studied. To confirm that the brown dwarfs are truly gravitationally tied to their proposed red dwarf partners, the team used a computational model to estimate the probability of their companionship. The model used the coordinates, motion, and velocities of the systems to make its calculations.
Ten of the binary systems showed over 99.5% probability that they are physically bound, while three systems had insufficient information about the brown dwarfs to properly use the model.
Once the systems were confirmed to be binary pairs, Marocco and his colleagues used the better-understood red dwarf primaries to infer characteristics of their brown dwarf companions. Namely, they used the approximate ages of the red dwarfs to estimate the mass of the brown dwarfs, and they used the well-measured distances to the red dwarfs to calculate the projected separation between the two objects. The masses and separation of the paired objects are essential to understanding how these systems compare to stellar and planetary systems.
When compared to well-studied stellar binaries and exoplanetary systems, the researchers found that their brown dwarf systems blurred the boundaries between planets and stars.
"The primary question is how do these systems form? Are they extreme cases of stellar binaries, or are they extreme cases of planetary systems? Which one of the two populations do they belong to or are they a third population entirely?" said Marocco.
This study shows these systems could be one such "third population," with current theories of formation for binary stars and planetary systems struggling to explain their observations, and instead suggesting that the objects may have formed separately and then became gravitationally tied.
After scanning the sky more than 22 times in more than 11 years of survey operations, NEOWISE is slated for decommissioning on Aug. 8, 2024, though its extensive archival data hosted at the NASA Infrared Science Archive (IRSA) at IPAC will continue to serve as a rich treasure trove of potential discoveries.
"Since we are sadly approaching the end of the NEOWISE mission, it is powerful to have such a long, extended period of observations," Marocco said. "NEOWISE has been observing since 2010, and that allows you to, in my case, go and find very dim brown dwarfs that move. And you can measure their motion very precisely, because you have such a long baseline."
Marocco is looking forward to the upcoming NEO Surveyor mission, which he calls "a natural successor to NEOWISE." Although its primary mission is to search for potentially hazardous asteroids and comets, it will also provide a data set that will be even more sensitive to brown dwarfs than previous missions.
"With NEO Surveyor, we hope to find even colder, lower-mass companions to see where the limit is, or if there is a continuum between these systems and exoplanet systems," said Marocco.
The paper also suggests that these binary systems would be "prime" targets for JWST, since they could utilize the telescope's spectacular strength in the infrared wavelengths to observe the faint systems.
IPAC is the NEO Surveyor Survey Data Center, building upon its long heritage supporting NASA's major infrared sky surveys and detecting and characterizing small bodies in the solar system.
IPAC is responsible for the archival and distribution of WISE and NEOWISE data for the astronomical community through the NASA/IPAC Infrared Science Archive (IRSA).
NEOWISE is a project of the Jet Propulsion Laboratory/California Institute of Technology and the University of California, Los Angeles, supported by NASA’s Planetary Defense Coordination Office. Both NEOWISE and NEO Surveyor are managed by NASA's Jet Propulsion Laboratory (JPL). Caltech manages JPL for NASA.
Media Contact
Isabel Swafford
Caltech IPAC
The Near Earth Object Wide-field Infrared Survey Explorer (NEOWISE) Science Data Center is part of IPAC, located on the campus of the California Institute of Technology.
