Imagine a planet so massive it blurs the line between world and star. How big can a planet truly get before it becomes something else entirely? This question lies at the heart of a groundbreaking discovery that’s shaking up our understanding of planet formation. But here’s where it gets controversial: could planets far larger than Jupiter—some even 30 times its mass—still form the same way as the gas giants in our own solar system? A recent study using the James Webb Space Telescope (JWST) has uncovered surprising clues, but it’s sparking debates among astronomers.
On February 9, 2026, researchers revealed that the detection of sulfur in the distant HR 8799 star system, located 133 light-years away in the constellation Pegasus, is offering unprecedented insights into how colossal planets form. Gas giants, like Jupiter and Saturn, are primarily composed of hydrogen and helium, with dense cores but no solid surfaces. However, the HR 8799 system takes this to an extreme: its four planets 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. This system is like a scaled-up version of our solar system, but its sheer scale challenges traditional theories of planet formation.
And this is the part most people miss: the debate over how these giants form. There are two leading theories. The first is core accretion, where solid cores gradually accumulate rocky and icy material until they’re massive enough to pull in surrounding gas—much like how Jupiter formed. The second is gravitational instability, where clouds of gas collapse rapidly into massive objects, similar to brown dwarfs, which are sometimes called “failed stars” because they lack the mass to sustain hydrogen fusion. For years, astronomers thought planets as large as those in HR 8799 couldn’t form through core accretion, as there wouldn’t be enough time before the star’s disk of material dissipates.
Enter the JWST, a game-changer in this cosmic mystery. By analyzing spectral data from the telescope, a team led by the University of California San Diego discovered sulfur in the atmosphere of HR 8799’s third planet. Sulfur is a refractory element—one that only exists in solid form in the protoplanetary disk where planets form. Its presence strongly suggests that these massive planets formed through core accretion, not gravitational instability. This finding, published in Nature Astronomy, challenges older models and hints that even the largest gas giants might share a formation process with Jupiter.
But the journey to this discovery wasn’t easy. The planets in HR 8799 are 10,000 times fainter than their star, making observations incredibly difficult. Lead researcher Jean-Baptiste Ruffio had to develop new data analysis techniques to extract the faint signals, while Jerry Xuan created detailed atmospheric models to confirm the presence of sulfur and other molecules like hydrogen sulfide. Their work not only confirms core accretion as a viable pathway for these giants but also raises new questions about the limits of planet formation.
Here’s the controversial part: If planets can be 15, 20, or even 30 times the mass of Jupiter and still form like traditional planets, where do we draw the line between a planet and a brown dwarf? The HR 8799 system is unique in that it’s the only imaged system with four massive gas giants, but other systems with even larger companions remain unexplained. This discovery forces us to rethink the boundaries of what we call a planet and how they come to be.
As astronomers continue to study one star system at a time, the question lingers: how big can a planet truly be? And what does this mean for our understanding of the universe? Let us know your thoughts in the comments—do you think there’s a hard limit to planet size, or is the line between planet and brown dwarf more fluid than we’ve imagined?
