Star formation is happening all around us in the Universe. However, there is still plenty we don’t know about it, including, as a recent press release points out, something that every astronomy textbook points out – we don’t know the size of the smallest star. Most current answers in those textbooks refer to an object known as a brown dwarf, a cross between a star and a giant planet. Recently, the James Webb Space Telescope (JWST) found what is believed to be the smallest brown dwarf ever discovered – and it weighs in at only 3-4 times the weight of Jupiter.
When searching for brown dwarfs, knowing where to look is important. Kevin Luhman, the lead author of a paper describing the findings and a professor at Penn State University, had a good idea of where. Star cluster IC 348 is found in the constellation Perseus, about 1,000 light years away. It’s only about 5 million years old and has been known as an excellent place to find low-mass brown dwarfs since at least 2009, when another paper described three brown dwarfs with a mass lower than ten times that of Jupiter.
JWST changed the brown dwarf hunting game, though. Its highly sensitive infrared images made it possible to track objects that would be invisible to other telescopes, including larger ground-based ones. In IC 348, many of the brown dwarfs are relatively hot since they had just formed several million years ago. That heat shows up very clearly in JWST’s infrared instrumentation.
The researchers leveraged both the Near-Infrared Camera and the Near-Infrared Spectrograph onboard the space telescope to peer directly into the star-forming region and to differentiate possible brown dwarf candidates from background galaxies that were blurred by the region’s mass.
Three seems to be the magical number for these kinds of studies, as Dr. Luhman and his co-authors found three more brown dwarf candidates in the area, including one that was small at three times the size of Jupiter. But interestingly, two of the three candidates also have an unknown hydrocarbon in their atmosphere according to a spectrographic analysis. The same signature has been found on Jupiter and Titan and in the space between stars, but no one has yet identified it.
Given the spectrographic similarities to a known planet and even a moon in our own solar system, what’s to say the objects identified in the studies were actually brown dwarfs and not just a rogue planet that happened to be passing through the IC 348 as JWST looked at it? Dr. Luhman and his team think that brown dwarfs are the most likely answer because of the low likelihood that giant planets would be created around the low-mass stars that populate star-forming regions like IC 348. Combine that with its relatively short lifespan, and it’s unlikely that a giant rogue planet would be making its way through that part of the galaxy.
But a better way to get a definitive answer is by collecting more data, as with all science. JWST has plenty of demands on its time, so the observational window on which this study was based was less than needed to do a more interesting study. With more time on the ultra-sensitive infrared instruments, researchers could find brown dwarfs down to the same mass as Jupiter. If they can find such an object, it would prove enlightening for the field of star formation theory, even if the object itself doesn’t give off much light of its own.
ESA – Webb identifies tiniest free-floating brown dwarf
K.L. Luhman et al – A JWST Survey for Planetary Mass Brown Dwarfs in IC 348*
UT – JWST Accidentally Found 21 Brown Dwarfs
UT – Brown Dwarfs are Probably Much More Common in the Milky Way Than Previously Believed
Star Cluster IC 348 seen by the MIRI instrument of JWST. Image credit: NASA, ESA, CSA, STScI, Kevin Luhman (PSU), Catarina Alves de Oliveira (ESA). Brown dwarf image credit: NASA