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 gradual process where solid cores accumulate rocky and icy material until they attract surrounding gas, as seen with Jupiter and Saturn? Or did they form through gravitational instability, a dramatic collapse of gas clouds into massive objects akin to brown dwarfs?
A groundbreaking study led by the University of California San Diego, utilizing the James Webb Space Telescope (JWST), sheds new light on this debate. Published in Nature Astronomy (https://doi.org/10.1038/s41550-026-02783-z), their research focuses on the HR 8799 star system, a scaled-up version of our own solar system located 133 light-years away in the constellation Pegasus. This system hosts four gas giants, each five to ten times the mass of Jupiter, orbiting their star at astonishing distances – the closest planet is 15 times farther from its star than Earth is from the Sun.
And this is the part most people miss: traditional models of planet formation, based on our solar system, suggest planets wouldn’t have enough time to grow so massive before their star disperses the surrounding disk of gas and dust. So, how did these giants in HR 8799 form?
Enter the power of JWST. Astronomers employ spectroscopy, analyzing light waves to decipher the chemical composition and formation history of exoplanets. Previously, they focused on volatile molecules like water and carbon monoxide. However, these molecules don’t reveal the full story of planet formation. The key lies in refractory elements, like sulfur, which are only present in solid form within the protoplanetary disk. The detection of sulfur points towards core accretion as the likely formation mechanism.
“JWST’s unparalleled sensitivity allows us to study these planets’ atmospheres in unprecedented detail, offering clues to their origins,” explains Jean-Baptiste Ruffio, lead researcher at UC San Diego. “Detecting sulfur suggests the HR 8799 planets formed similarly to Jupiter, despite their immense size – a surprising finding.”
HR 8799, a relatively young system at 30 million years old (compared to our 4.6 billion-year-old solar system), provides an ideal target. Younger planets are brighter and easier to study spectroscopically. JWST’s high-resolution spectrograph, untainted by Earth’s atmospheric interference, revealed for the first time the presence of rare molecules, including hydrogen sulfide, in the atmospheres of the inner three HR 8799 gas giants.
This discovery wasn’t without challenges. These 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, created sophisticated atmospheric models to interpret the data. “The JWST data is truly revolutionary,” Xuan remarks, “requiring us to refine our models to fully understand what the data was telling us.”
The team found compelling evidence of sulfur in the third planet, HR 8799 c, and believes it’s likely present in all three inner planets. They also discovered these planets are enriched in heavy elements like carbon and oxygen compared to their star, further supporting their planetary origins.
“This challenges traditional core accretion models,” states Quinn Konopacky, UC San Diego Professor of Astronomy and Astrophysics. “We’re now looking at newer models where gas giants can form solid cores much farther from their stars.”
HR 8799, with its four massive gas giants, is unique among imaged systems. However, other systems with even larger companions exist, their formation mechanisms still shrouded in mystery. This raises a fundamental question: How big can a planet be before it becomes something else entirely? Where does the line between planet and brown dwarf truly lie?
The debate rages on, fueled by each new discovery. What do you think? Can a planet be 15, 20, or even 30 times the mass of Jupiter and still be considered a planet? Share your thoughts in the comments below!
This research was partially 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 author list, refer to the published paper in Nature Astronomy.
