A team using the James Webb Space Telescope (JWST) spotted the smallest free-floating brown dwarf star ever recorded and two other “failed stars.” They are located in a star cluster that’s only 1,000 light-years from Earth and is not associated with a parent star. The findings were published December 13 in the Astronomical Journal and may help astronomers better determine the boundaries between stars and planets.
Brown dwarfs are celestial bodies that are more massive than planets, but not quite as large as stars. They form the way stars do, growing dense enough to collapse under the weight of their own gravity, but they never become dense and hot enough to start fusing the hydrogen needed to turn into a star. This is why they get the nickname “failed stars.”
The brown dwarf JWST spotted has a mass around eight times that of the planet Jupiter. Meanwhile, the smallest of these stars has a mass around three times that of Jupiter, which challenges current theories about how these types of celestial bodies are formed. Astronomers are using JWST to try and determine what the smallest celestial objects that can form in a star-like manner are.
“One basic question you’ll find in every astronomy textbook is, what are the smallest stars? That’s what we’re trying to answer,” study co-author and Pennsylvania State University astronomer Kevin Luhman said in a statement.
Scouring the skies
Luhman and his colleague Catarina Alves de Oliveira began their search with star cluster IC 348. This grouping is only about 1,000 light-years away in the Perseus star-forming region. Star cluster IC 348 is relatively young, at only about 5 million years old. Due to its age, any brown dwarfs present would still be relatively bright in infrared light and be glowing from the heat of their formation.
They imaged the center of the star cluster with JWST’s Near-Infrared Camera (NIRCam) to identify any brown dwarf candidates from their brightness and colors. They then used the microshutter array on the telescope’s Near-Infrared Spectrograph (NIRSpec) to look at the most promising targets. The JWST’s sensitivity to infrared light allowed the team to detect fainter objects than other ground-based telescopes.
They narrowed the star cluster down to three possible targets. All of the stars weighed three to eight Jupiter masses and had surface temperatures ranging from 1,500 to 2,800 degrees Fahrenheit. According to the team’s computer models, the smallest target was only three to four times the size of Jupiter and can offer clues to the star formation process.
“It’s pretty easy for current models to make giant planets in a disk around a star,” study co-author and European Space Agency (ESA) astronomer Catarina Alves de Oliveira of ESA said in a statement. “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?”
A strange molecule
Tiny brown dwarfs can also help astronomers better understand exoplanets because the smallest brown dwarfs overlap with the largest known exoplanets. While they would generally be expected to have some similar properties, a free-floating brown dwarf is easier to study than a giant exoplanet. The glare of its host star generally hides giant exoplanets, making them more difficult to observe.
Two of the brown dwarfs in this study also have evidence of an unidentified hydrocarbon, a molecule made up of both hydrogen and carbon atoms. NASA’s Cassini mission detected the same infrared signature in the atmosphere of Saturn and its moon Titan and in the gas between stars.
“This is the first time we’ve detected this molecule in the atmosphere of an object outside our solar system,” said Alves de Oliveira. “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.”
The star or planet identity crisis
The question remains whether brown dwarfs are considered stars or rogue planets that were ejected from planetary systems. This team argues that the brown dwarfs in this study are most likely brown dwarf stars, and not an ejected planet.
While the rogue planet theory couldn’t be completely ruled out, it is unlikely. Most of the stars in cluster IC 348 are low-mass and the team believes that it’s unlikely that they are capable of producing massive planets. The cluster also may not have had enough time during its 5 million years of existence for gas giants to form and be ejected from their planetary systems.
Finding more objects like these brown dwarfs could help clarify their status as stars or planets. Some theories suggest that rogue planets are more likely to be spotted on the outskirts of a star cluster. Expanding the search area may reveal if they exist within IC 348. Future research could also take longer surveys that can pick up fainter and smaller objects.