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 the galaxy teems with countless others, some dwarfing our familiar giants in size. But here's where it gets controversial: could these colossal worlds be formed in the same way as our own gas giants, or do they represent a different, more extreme process? And this is the part most people miss: the answer might lie in the faint glow of distant planetary atmospheres, a glow now decipherable thanks to the James Webb Space Telescope (JWST).
The traditional view holds that gas giants like Jupiter and Saturn formed through core accretion. Imagine a dusty disk swirling around a young star, where rocky and icy pebbles gradually coalesce into a solid core. Once this core reaches a critical mass, it begins to attract the surrounding gas, eventually becoming the gas giant we see today. But what about those gas giants many times larger than Jupiter? Could they have formed in the same way, or did they arise through a different mechanism, like gravitational instability, where vast clouds of gas collapse directly into massive objects, potentially even brown dwarfs – those 'failed stars' that never ignite nuclear fusion?
A team led by the University of California San Diego, armed with JWST's unprecedented power, has shed new light on this debate. They turned their gaze towards the HR 8799 system, a young stellar family located 133 light-years away in the constellation Pegasus. This system is remarkable: its four gas giants are each five to ten times the mass of Jupiter, orbiting their star at distances 15 to 70 times farther than Earth is from the Sun. Think of it as a scaled-up version of our own solar system, with its outer giants stretched to colossal proportions.
The traditional models of planet formation, based on our solar system, struggle to explain HR 8799. These models suggest that planets wouldn't have enough time to grow so massive before the star's radiation blows away the surrounding disk. So, how did these giants form? Enter JWST and its revolutionary spectroscopy capabilities.
Astronomers have long used spectroscopy to analyze the light from exoplanets, revealing their chemical composition and hinting at their formation history. Before JWST, they focused on molecules like water and carbon monoxide. However, these 'volatile' molecules, prone to change, aren't reliable indicators of a planet's origins. The UC San Diego team shifted their focus to 'refractory' elements, like sulfur, which are only found in solid form within the protoplanetary disk. The presence of sulfur in a gas giant's atmosphere is a telltale sign of core accretion.
And this is where JWST's power truly shines. Its ultra-sensitive spectrograph allowed researchers to detect sulfur in the atmosphere of HR 8799c, the third planet in the system, and likely in its siblings as well. This discovery, led by Jean-Baptiste Ruffio, was no easy feat. These planets are incredibly faint compared to their star, requiring Ruffio to develop innovative data analysis techniques. Jerry Xuan, a key collaborator, created intricate atmospheric models to interpret the JWST data, revealing not only sulfur but also other molecules, some detected for the first time in exoplanets.
The findings are groundbreaking. Despite their immense size, the HR 8799 planets appear to have formed through core accretion, challenging our previous understanding. 'Older core accretion models are outdated,' states Quinn Konopacky, a co-author of the study. 'We're now looking at models where gas giants can form solid cores much farther away from their stars.'
But the questions don't end here. HR 8799, with its four massive gas giants, is unique among imaged systems. Yet, there are other systems with even larger companions, their formation stories still shrouded in mystery. How big can a planet truly be before it becomes something else entirely? Where does the line between planet and brown dwarf lie? The JWST has opened a new chapter in our exploration of these colossal worlds, but the story is far from over. Each new discovery, each star system studied, brings us closer to understanding the incredible diversity of planets in our universe.
This research, published in Nature Astronomy (https://doi.org/10.1038/s41550-026-02783-z), was made possible by the National Aeronautics and Space Administration (NASA) through grants 80NSSC25K7300 and the FINESST Fellowship award 80NSSC23K1434. The views expressed herein are those of the authors and do not necessarily reflect those of NASA. The full list of authors, including Jean-Baptiste Ruffio, Eve J. Lee, Quinn Konopacky, Jerry W. Xuan, Dimitri Mawet, Aurora Kesseli, Charles Beichman, Geoffrey Bryden, Thomas P. Greene, and Yayaati Chachan, can be found in the published paper.
What do you think? Does the discovery of sulfur in HR 8799's planets settle the debate on their formation? Or does it raise even more questions about the diversity of planetary systems in our galaxy? Share your thoughts in the comments below!
