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 universe holds far more. Some gas giants, known as exoplanets, dwarf even Jupiter, raising a fascinating question: Where does a planet end and a star begin?
This question lies at the heart of a groundbreaking study led by the University of California San Diego. Utilizing the James Webb Space Telescope (JWST), researchers peered into the HR 8799 system, a young stellar family located 133 light-years away in the constellation Pegasus. Here, four gas giants, each five to ten times Jupiter's mass, orbit their star at astonishing distances – the closest being 15 times farther from its star than Earth is from the Sun.
But here's where it gets controversial: Traditional theories of planet formation, based on our own solar system, suggest these giants couldn't have formed through the gradual accumulation of dust and gas known as core accretion. The process, they argue, would take too long before the star's radiation blows away the surrounding material. So, did these giants form through a different mechanism, perhaps a rapid collapse of gas clouds like brown dwarfs, those enigmatic objects sometimes called 'failed stars'?
The JWST, with its unparalleled sensitivity, provided a surprising answer. By analyzing the light from these distant worlds, astronomers detected sulfur, a telltale sign of core accretion. This discovery, published in Nature Astronomy (https://doi.org/10.1038/s41550-026-02783-z), challenges conventional wisdom. It suggests that even these colossal planets, despite their size, likely formed in a similar manner to Jupiter, a process previously thought to be limited to smaller worlds.
And this is the part most people miss: The key to this discovery lay in shifting focus from volatile molecules like water and carbon monoxide to more stable elements like sulfur. These 'refractory' elements, found only in solid form within the protoplanetary disk, provide a clearer picture of a planet's formation history.
The HR 8799 system, a mere 30 million years old compared to our solar system's 4.6 billion, offered another advantage. Younger planets are brighter, making them easier to study spectroscopically. JWST's high-resolution spectrograph, untainted by Earth's atmosphere, revealed intricate details of the planets' atmospheres, including the presence of hydrogen sulfide, previously undetected.
This wasn't an easy feat. The planets are incredibly faint compared to their star, requiring innovative data analysis techniques developed by Jean-Baptiste Ruffio, the study's lead author. Jerry Xuan, a UCLA researcher, created sophisticated atmospheric models to interpret the JWST data, iteratively refining them to match the observed spectra.
The findings are compelling. Sulfur was clearly detected in the third planet, HR 8799 c, and likely exists on the other inner planets. Furthermore, the planets are enriched in heavy elements like carbon and oxygen compared to their star, further supporting their planetary origins.
This study, as Quinn Konopacky, a UC San Diego professor and co-author, points out, challenges older core accretion models. It suggests that gas giants can form solid cores much farther from their stars than previously thought.
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 tantalizing question: How big can a planet get before it becomes something else entirely? Where does the line between planet and brown dwarf truly lie?
The JWST has opened a new era in exoplanet exploration, allowing us to probe the atmospheres of these distant worlds with unprecedented detail. As we continue to study systems like HR 8799, we inch closer to understanding the diverse pathways of planet formation and the boundaries that define these celestial bodies.
What do you think? Does this discovery challenge your understanding of what constitutes a planet? Share your thoughts in the comments below.
This research was partially funded by the National Aeronautics and Space Administration (80NSSC25K7300 and FINESST Fellowship award 80NSSC23K1434). The views expressed herein are those of the authors and do not necessarily reflect those of NASA.
For a full list of authors, please refer to the original paper in Nature Astronomy.
