Imagine planets so massive they blur the line between world and star. These are the gas giants, behemoths composed primarily of hydrogen and helium, lacking solid surfaces despite their dense cores. Our solar system boasts two such giants, Jupiter and Saturn, but beyond our cosmic neighborhood, even larger gas giants lurk, some dwarfing Jupiter in size. But here's where it gets controversial: could these colossal worlds be more than just planets? Could they be failed stars, teetering on the edge of stellar ignition?
The formation of these giants has long puzzled astronomers. Did they arise through core accretion, a process where solid cores gradually accumulate rocky and icy material until they’re massive enough to attract surrounding gas, as seen with Jupiter and Saturn? Or did they form through gravitational instability, where vast clouds of gas collapse directly into massive objects, akin to brown dwarfs? And this is the part most people miss: the answer might not be so straightforward, and it could challenge our understanding of planetary formation.
A groundbreaking study led by researchers at the University of California San Diego, published in Nature Astronomy (https://doi.org/10.1038/s41550-026-02783-z), sheds new light on this mystery. Using spectral data from the James Webb Space Telescope (JWST), the team probed the HR 8799 star system, located a mere 133 light-years away in the constellation Pegasus. This system is a scaled-up version of our own solar system, hosting four gas giants, each five to ten times the mass of Jupiter. These planets orbit their star at astonishing distances—15 to 70 times farther than Earth is from the Sun. Such extreme orbits and masses have led astronomers to question whether core accretion could account for their formation.
Traditionally, astronomers relied on spectroscopy to study exoplanets, analyzing light waves to uncover their physical properties. Before JWST, ground-based telescopes focused on detecting volatile molecules like water and carbon monoxide. However, these molecules proved unreliable tracers of planetary formation, as their origins remain ambiguous. Enter refractory elements like sulfur, which are only present in solid form within protoplanetary disks. The detection of sulfur in a gas giant’s atmosphere is a smoking gun, indicating core accretion as the likely formation mechanism.
Here’s the game-changer: JWST’s unprecedented sensitivity allowed researchers to detect sulfur in the atmosphere of HR 8799 c, the third planet in the system. This discovery suggests that despite their immense size, these planets likely formed through core accretion, much like Jupiter. “With the detection of sulfur, we are able to infer that the HR 8799 planets likely formed in a similar way to Jupiter, which was unexpected,” explained Jean-Baptiste Ruffio, lead researcher and co-author of the paper. This finding challenges older models of planet formation, which predicted that such massive planets couldn’t form through core accretion before their star’s disk dissipated.
But this discovery wasn’t without its hurdles. The HR 8799 planets are 10,000 times fainter than their star, pushing JWST’s capabilities to the limit. Ruffio developed innovative data analysis techniques to extract the faint signals, while Jerry Xuan, a 51 Pegasi b Fellow at UCLA, refined atmospheric models to accurately interpret the data. “The quality of the JWST data is truly revolutionary,” Xuan noted. “Existing models were simply not adequate, so I iteratively refined the chemistry and physics to capture what the data were telling us.”
The team also found that the planets are enriched in heavy elements like carbon and oxygen compared to their star, further supporting their planetary origins. Yet, questions remain. How large can a planet grow before it becomes a brown dwarf? Where does the transition lie between planet and star? Ruffio ponders, “Can a planet be 15, 20, or even 30 times the mass of Jupiter and still form like a planet? This is a question that continues to intrigue us.”
Now, we turn to you: Do you think there’s a clear boundary between planets and brown dwarfs, or is it a spectrum? Could our definition of a planet need rethinking? Share your thoughts in the comments below!
This research was supported by the National Aeronautics and Space Administration (80NSSC25K7300 and FINESST Fellowship award 80NSSC23K1434). The views expressed are those of the authors and do not necessarily reflect those of NASA. For the full list of authors, refer to the original paper.
This material has been edited for clarity, style, and length. Mirage.News does not take institutional positions or sides, and all views expressed are solely those of the author(s). View the original article here: https://www.miragenews.com/giant-gas-planets-measuring-their-massive-size-1616173/
