What is the maximum size of a planet? After discovering huge gas giants, it's hard to know

The James Webb Space Telescope has detected hydrogen sulfide around massive gas giants, and a study in Nature Astronomy strengthens the formation scenario around the core – and sharpens the boundary with brown dwarfs

Gas giants are planets made mostly of hydrogen and helium, with no solid “ground” to stand on. In the solar system, these are Jupiter and Saturn, but outside of it there are much larger gas giants. As mass soars, the line between “planet” and “brown dwarf” begins to blur: is this a world that slowly builds around a core, or a body formed by the rapid collapse of gas – a process more akin to star formation?

A new study, published in Nature Astronomy on February 9, 2026, offers a surprising answer to this question – with the help of a “chemical clue” that is not always center stage: sulfur.

What are you looking for when you ask “How did a giant planet form?”

There are two super-scenarios for the formation of gas giants. The first is Core accretion: Initially, a core of rock and ice is built, “sucking” tiny grains and pebbles into the disk around the young star. When the core grows large enough, it begins to rapidly draw in gas, forming a gas giant. This is how, according to classical models, Jupiter and Saturn were formed.

The second scenario is Gravitational instability: Part of the gaseous disk becomes unstable and collapses rapidly into a giant body – a process that can also lead to brown dwarfs. This scenario seems especially tempting when it comes to very massive gas giants and in distant orbits, because it is difficult to have time to “build” a core and pull in gas before the disk disperses.

This is where a relatively young star system comes into the picture, about 133 light-years away, in which several very massive gas giants have been found: five to ten times the mass of Jupiter, and at distances reaching about 15 to 70 astronomical units – that is, about 2.2 to 10.5 billion km from the star. For years, these data cast doubt on whether accretion around a core is even possible there.

Why is sulfur a “fingerprint” for the formation of

Until a few years ago, researchers relied heavily on “volatile” molecules like water and carbon monoxide to learn about planetary atmospheres. But it turns out that such molecules don’t always tell us where the material came from, making it difficult to infer anything about the formation process.

In the current study, the researchers turned to more stable molecules, which represent materials that are beginning their journey. As solids On the disk: RefractoriesSulfur is an important example, because it is “locked” in solid grains, and its presence in the atmosphere can indicate a trajectory in which a solid core was built first, and only then did the gas arrive – just as would be expected from an accumulation around a core.

To do this, the team led by the University of California, San Diego used spectroscopy data from the James Webb Space Telescope. The instrument allows chemical “fingerprints” to be read in light, unobstructed by Earth’s atmosphere.

The challenge was extreme: The planets in this system are very faint relative to their star—about 10,000 times dimmer. In addition, Webb's spectrograph was not designed for such a demanding task. Jean-Baptiste Ruffio led the development of new data analysis methods to extract the faint signal, and Jerry W. Xuan built detailed atmospheric models to compare with the data. In the end, the researchers identified, among other things, Hydrogen sulfide (H₂S) – including a particularly clear signature on one of the planets. (today.ucsd.edu)

Here comes the main result: Sulfur in the atmospheres of gas giants (This phrase will come up again) is seen as strong evidence that at least some of the gas giants in the system formed in a Jupiter-like orbit, that is, from the bottom up around its core.
The team also found that these planets are richer in “heavy” elements relative to their star, such as carbon and oxygen – another sign that these are planets built from a solid-rich disk rather than by “stellar” collapse.

And what does this say about the boundary between a planet and a brown dwarf?

This study not only answers “how did these giants form,” but also brings the big question back to the table: How big can a planet be and still be considered a “planet” in terms of its formation?

There is a popular definition that places a threshold around About 13 essays on justice: Above it, a body may begin to burn deuterium (heavy hydrogen) and therefore be considered a brown dwarf, and below it – a “regular” gas giant. But this is not a sharp line either, because the threshold depends on the composition and models.
In any case, the new findings suggest that nature does not always adhere to convenient limits: some very massive bodies may still be building up like planets, especially if new models allow solid cores to form even very far from the star.

Quinn Konopacky said in the university's announcement that the finding "shows that old models of accretion around a core are no longer sufficient," and that it is worth focusing on new models in which gas giants can build a solid core even at great distances.

Sulfur in the atmospheres of gas giants It's not just an interesting chemical detail. It could become a tool to help map the "life paths" of giant planets, and understand when we're looking at a planet that formed in a disk—and when we're looking at a body that was born from a rapid collapse, closer to the world of brown dwarfs.

for the scientific article

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