The newly-discovered brown dwarf has an estimated mass of 3-4 Jupiter masses, making it a strong contender for the least massive free-floating brown dwarf that has been directly imaged to date.
This image from Webb’s NIRCam instrument shows the central portion of the star cluster IC 348. Image credit: NASA / ESA / CSA / STScI / K. Luhman, Penn State University / C. Alves de Oliveira, ESA.
Brown dwarfs are cool, dim objects that have a size between that of a gas-giant planet 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 (approximate mass at which deuterium fusion can be sustained) and 75-80 Jupiters (approximate mass to sustain hydrogen fusion).
“One basic question you’ll find in every astronomy textbook is, what are the smallest stars? That’s what we’re trying to answer,” said Dr. Kevin Luhman, an astronomer at Pennsylvania State University.
The newly-discovered brown dwarf resides in IC 348, a star cluster located 1,000 light-years away in the constellation of Perseus.
Also known as Collinder 41, Gingrich 1 and Theia 17, this cluster contains nearly 400 stars and is around 5 million years old.
IC 348 is part of the larger Perseus star-forming region, which is generally invisible to the naked eye, but shines brightly when viewed in infrared wavelengths.
Dr. Luhman and his colleagues imaged the center of the cluster using Webb’s NIRCam instrument to identify brown dwarf candidates from their brightness and colors.
They followed up on the most promising targets using Webb’s NIRSpec microshutter array.
This process led to three intriguing targets weighing 3-8 Jupiter masses, with surface temperatures ranging from 830 to 1,500 degrees Celsius.
The smallest of these weighs just 3-4 times Jupiter, according to computer models.
“It’s pretty easy for current models to make giant planets in a disc around a star,” said Dr. Catarina Alves de Oliveira, an astronomer at ESA.
“But in this cluster, it would be unlikely that this object formed in a disk, instead forming like a star, and three Jupiter masses is 300 times smaller than our Sun.”
“So we have to ask, how does the star formation process operate at such very, very small masses?”
Two of the brown dwarfs identified by the team show the spectral signature of an unidentified hydrocarbon, a molecule containing both hydrogen and carbon atoms.
The same infrared signature was detected by NASA’s Cassini mission in the atmospheres of Saturn and its moon Titan.
It has also been seen in the interstellar medium, the gas between stars.
“This is the first time we’ve detected this molecule in the atmosphere of an object outside our Solar System,” Dr. de Oliveira said.
“Models for brown dwarf atmospheres don’t predict its existence. We’re looking at objects with younger ages and lower masses than we ever have before, and we’re seeing something new and unexpected.”
A paper on the findings was published in the Astronomical Journal.
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K.L. Luhman et al. 2023. A JWST Survey for Planetary Mass Brown Dwarfs in IC 348. AJ 167, 19; doi: 10.3847/1538-3881/ad00b7
