UNIVERSITY PARK, Pa. — Using NASA’s James Webb Space Telescope, astronomers have identified the new record-holder for the smallest object that forms like a star: a tiny, free-floating brown dwarf with only three to four times the mass of Jupiter. A paper describing the observations appeared today (Dec. 13) in the Astronomical Journal.
Brown dwarfs are objects that straddle the dividing line between stars and planets. They form like stars, growing dense enough to collapse under their own gravity, but they never become dense and hot enough to begin fusing hydrogen and turn into a star. Planets, on the other hand, form in the disk of gas and dust that surrounds a star. At the low end of the scale, some brown dwarfs are comparable with giant planets, weighing just a few times the mass of Jupiter.
“One basic question you’ll find in every astronomy textbook is, ‘what are the smallest stars?’” said Kevin Luhman, professor of astronomy and astrophysics in the Penn State Eberly College of Science, the lead author of the study. “That’s what we’re trying to answer.”
To locate this newfound brown dwarf, Luhman and his colleague, Catarina Alves de Oliveira of the European Space Agency, chose to study the star cluster IC 348, located about 1,000 light-years away in the Perseus star-forming region. This cluster is young, only about 5 million years old. As a result, any brown dwarfs would still be relatively bright in infrared light, glowing from the heat of their formation.
The team first imaged the center of the cluster using Webb’s near-infrared camera (NIRCam) to identify brown dwarf candidates from their brightness and colors. They followed up on the most promising targets using Webb’s near-infrared spectrograph (NIRSpec) microshutter array.
The researchers explained that Webb’s infrared sensitivity was crucial, allowing the team to detect fainter objects than ground-based telescopes. In addition, Webb’s sharp vision enabled them to determine which red objects were pinpoint brown dwarfs and which were blobby background galaxies.
This winnowing process led to three intriguing targets weighing three to eight times the mass of Jupiter, with surface temperatures ranging from 1,500 to 2,800 degrees Fahrenheit (830 to 1,500 degrees Celsius). The smallest of these weighs just three to four times Jupiter, according to computer models.
Explaining how such a small brown dwarf could form is theoretically challenging, according to the researchers. A heavy and dense cloud of gas has plenty of gravity to collapse and form a star. However, because smaller clouds have weaker gravity, it should be more difficult for them to collapse to form a brown dwarf, and that is especially true for brown dwarfs with the masses of giant planets.
“It’s pretty easy for current models to make giant planets in a disk around a star,” said Alves de Oliveira, principal investigator on the observing program. “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?”
In addition to giving clues about the star-formation process, tiny brown dwarfs also can help astronomers better understand exoplanets. The least massive brown dwarfs overlap with the largest exoplanets, so they would be expected to have some similar properties. However, a free-floating brown dwarf is easier to study than a giant exoplanet since the latter is hidden within the glare of its host star.
Two of the brown dwarfs identified in this survey show the spectral signature of an unidentified hydrocarbon, or 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, or gas between stars.
“This is the first time we’ve detected this molecule in the atmosphere of an object outside our solar system,” Alves 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.”
Since the objects are well within the mass range of giant planets, it raises the question of whether they are actually brown dwarfs, or if they’re really rogue planets that were ejected from planetary systems. While the team can’t rule out the latter, they argue that they are far more likely to be a brown dwarf than an ejected planet.
An ejected giant planet is unlikely for two reasons, according to the researchers. First, such planets are uncommon in general compared to planets with smaller masses. Second, most stars are low-mass stars, and giant planets are especially rare among those stars. As a result, it’s unlikely that most of the stars in IC 348 — which are low-mass stars — are capable of producing such massive planets. In addition, since the cluster is only 5 million years old, there probably hasn’t been enough time for giant planets to form and then be ejected from their systems.
The discovery of more such objects will help clarify their status, the researchers said. Theories suggest that rogue planets are more likely to be found in the outskirts of a star cluster, so expanding the search area may identify them if they exist.
Future work may also include longer surveys that can detect fainter, smaller objects. The short survey conducted by the team was expected to detect objects as small as twice the mass of Jupiter. Longer surveys could potentially reach objects as small as one Jupiter mass.
“If ejected giant planets do exist in young clusters like this, Webb should be able to find them,” Luhman said.
In addition to Luhman and Alves de Oliveira, the research team includes Isabell Baraffe and Gilles Chabrier at the Univeristy of Exeter in the U.K., Thomas R. Geballe at the NSF NOIRLab’s Gemini Observatory, Richard J. Parker at the University of Sheffield in the U.K., Yvonne J. Pendleton at the University of Central Florida, and Pascal Tremblin at Université Paris-Saclay in France. Luhman is a member of the Penn State Center for Exoplanets and Habitable Worlds, which is supported by Penn State, the Eberly College of Science and the Pennsylvania Space Grant Consortium.
These observations were taken as part of Guaranteed Time Observation program 1229. The James Webb Space Telescope is an international program led by NASA with its partners, the European Space Agency and the Canadian Space Agency.
Editor's note: A version of this story was originally published by NASA.