Tiny brown dwarf is smallest found so far
Brown dwarfs are less massive than stars, but more massive than planets. They’re star-planet hybrids, typically only a bit bigger than Jupiter, but much more massive. Scientists have wondered, how small can a brown dwarf be, without being considered a planet? And on December 13, 2023, astronomers announced three newly discovered tiny brown dwarfs to add into their calculations. These astronomers used the Webb space telescope to make the discovery. They said one of the objects is the smallest brown dwarf yet seen. It’s only three to four times Jupiter’s mass. This tiny brown dwarf is located in a distant star cluster – IC 348 – about 1,000 light-years away.
The paper states:
Based on its luminosity and evolutionary models, the faintest new member of IC 348 has an estimated mass of 3–4 MJup [Jupiter masses], making it a strong contender for the least massive free-floating brown dwarf that has been directly imaged to date.
The researchers, led by Kevin Luhman at Pennsylvania State University, published their peer-reviewed findings in The Astronomical Journal on December 13.
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Brown dwarfs in the IC 348 star cluster
The IC 348 star cluster lies about 1,000 light-years from Earth, in the Perseus star-forming region. Luhman and his colleagues decided to search for brown dwarfs in IC 348 due to its young age. Since the star cluster is only about five million years old – practically a baby in cosmic terms – any brown dwarfs would still be hot from their formation. As a result, they would glow in infrared light.
The first region the research team searched was near the center of the cluster. To begin with, they used Webb’s Near-Infrared Camera (NIRCam) to try to identify candidate brown dwarfs based on their brightness and colors. After they found the most promising targets, the team used Webb’s Near-Infrared Spectrograph (NIRSpec) microshutter array. This allowed them to narrow down the candidates even further. The best candidates would appear red in color. Now, with Webb’s advanced instruments, researchers could determine which reddish objects were really brown dwarfs, if any, and which were instead more distant galaxies.
Ultimately, three intriguing candidates stood out. The objects ranged from three to eight Jupiter masses, with surface temperatures from about 1,500 to 2,800 degrees Fahrenheit (800 to 1,500 degrees Celsius). It was the smallest of these that was of the most interest to the researchers.
How did such a small brown dwarf form?
This little brown dwarf weighs only three to four times more than Jupiter. That makes it the smallest – as in the least massive – found to date. At least for free-floating brown dwarfs. The discovery is exciting, but scientists aren’t sure yet how to explain it.
Brown dwarfs form like stars, from clouds of gas and dust that collapse in on themselves due to gravity. If a gas and dust cloud is large and massive, it will form a star. But a lighter, smaller cloud has weaker gravity. Therefore, it can’t form a star, and current theoretical physics says it would even be difficult to form a brown dwarf. Especially as small as this one. So the way such small enigmatic objects can come to be is still a bit of a mystery.
Understanding how such small brown dwarfs form also helps scientists figure out the differences between them and the smallest stars. Luhman said:
One basic question you’ll find in every astronomy textbook is, what are the smallest stars? That’s what we’re trying to answer.
Co-author Catarina Alves de Oliveira at the European Space Agency (ESA) added:
It’s pretty easy for current models to make giant planets in a disk around a star. But in this cluster, it would be unlikely 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?
Brown dwarfs, exoplanets and a mystery molecule
As previously noted, brown dwarfs are not planets. But since the least massive ones can overlap with the most massive exoplanets, they can provide valuable clues about both kinds of objects. Scientists say they should share some characteristics with each other, even though they are still different kinds of objects.
Indeed, analysis of two of the newly discovered objects showed something interesting. Their atmospheres contained an unidentified hydrocarbon. That’s a molecule containing both hydrogen and carbon atoms. Interestingly, NASA’s Cassini spacecraft had previously found the same kind of hydrocarbon in the atmospheres of Saturn and its largest moon Titan. Astronomers have also detected it in interstellar space. But they didn’t expect to find it on brown dwarfs. As Oliveira noted:
This is the first time we’ve detected this molecule in the atmosphere of an object outside our solar system. Models for atmospheres of brown dwarfs didn’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.
What about rogue planets?
Is it possible these three brown dwarfs are really rogue planets? Many such worlds are now known to exist; they are planets not bound to any stars. Nomadic planets, if you will, ejected from their stars and drifting alone through the galaxy. But that’s unlikely in this case. First, because (again) they don’t fit the characteristics of planets. Also, they are too big. The most common known rogue planets are smaller and less massive than these newly discovered objects. Giant ejected planets, the size of these objects, are uncommon.
The researchers also said that the stars in the cluster are not massive enough to produce giant planets. And the cluster overall is too young. All of this makes it very unlikely that the three objects are giant planets that were ejected from stars in the cluster.
Bottom line: NASA’s Webb space telescope has found a tiny brown dwarf that is the smallest free-floating brown dwarf ever seen. Scientists aren’t quite sure how it formed.
Source: A JWST Survey for Planetary Mass Brown Dwarfs in IC 348*
Read more: What are brown dwarfs?
Read more: 95 new cool brown dwarfs in the sun’s neighborhood
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