When it comes to planets, bigger is not always easier to understand. The largest planets in the universe are gas giants—massive worlds made mostly of hydrogen and helium, with no solid surface.
In our own solar system, Jupiter and Saturn are the classic examples.
But beyond our neighborhood, astronomers have discovered gas giants that are many times larger than Jupiter, pushing the limits of what scientists consider a true planet.
At the extreme end, the line between planets and brown dwarfs becomes blurry.
Brown dwarfs are sometimes called “failed stars” because they are too small to ignite sustained hydrogen fusion. So how do scientists tell whether a massive object formed as a planet or more like a star?
That question has taken center stage in a new study led by researchers at University of California San Diego, using data from the powerful James Webb Space Telescope.
Their findings, published in Nature Astronomy, focus on the unusual planetary system around the star HR 8799.
HR 8799 lies about 133 light-years from Earth in the constellation Pegasus. What makes this system remarkable is that it hosts four enormous gas giants, each between five and ten times the mass of Jupiter.
These planets orbit far from their star, at distances ranging from 15 to 70 times the Earth–Sun distance. In many ways, the system resembles a scaled-up version of our own solar system, with multiple large planets spread across wide orbits.
For years, astronomers debated how such massive planets could form so far from their star. One leading theory, called core accretion, suggests planets begin as solid cores that slowly grow by collecting rock and ice before pulling in surrounding gas. This process explains Jupiter and Saturn, but it was thought to be too slow to create extremely large planets at such great distances. Another idea, known as gravitational instability, proposes that giant planets form quickly when parts of the gas disk around a young star collapse directly, similar to how stars form.
To investigate which process shaped HR 8799, the research team turned to spectroscopy, a technique that breaks light into its component wavelengths to reveal the chemical makeup of planetary atmospheres. Using JWST’s unmatched sensitivity, scientists examined the atmospheres of the three inner gas giants in the system.
Instead of focusing on common molecules like water or carbon monoxide, which can be difficult to trace back to a planet’s origin, the team looked for sulfur-bearing molecules. Sulfur is considered a “refractory” element, meaning it tends to form solid materials early in a planet’s history. Detecting sulfur in a gas giant’s atmosphere strongly points to formation through core accretion.
To the researchers’ surprise, they found clear evidence of sulfur, including hydrogen sulfide, especially in one planet known as HR 8799 c. The planets were also enriched in heavy elements compared to their host star, another sign that they formed like planets rather than stars.
These results suggest that even planets five to ten times more massive than Jupiter can form through core accretion, challenging long-standing models of planet formation. The findings imply that planets can grow larger, and farther from their stars, than previously thought.
HR 8799 is still a young system, only about 30 million years old, which makes its planets brighter and easier to study. Even so, detecting their faint light next to a bright star required new data analysis techniques and advanced atmospheric modeling.
The study raises a bigger question that scientists are now eager to answer: how massive can a planet become before it crosses the line into brown dwarf territory? Could a planet 15, 20, or even 30 times the mass of Jupiter still form like Jupiter did?
For now, HR 8799 offers a powerful reminder that the universe is more creative than our theories—and that planet formation may come in far more varieties than once imagined.
