Gas giants are large planets composed mainly of hydrogen and helium with dense cores but no solid surfaces. Astronomers have long debated how the most massive of these worlds form and where the boundary lies between true planets and brown dwarfs. A new investigation of the HR 8799 planetary system using the James Webb Space Telescope provides key evidence that even very massive gas giants can gr Los Angeles CA (SPX) Feb 10, 2026
Gas giants are large planets composed mainly of hydrogen and helium with dense cores but no solid surfaces. Astronomers have long debated how the most massive of these worlds form and where the boundary lies between true planets and brown dwarfs. A new investigation of the HR 8799 planetary system using the James Webb Space Telescope provides key evidence that even very massive gas giants can grow like Jupiter rather than forming like failed stars.
HR 8799 lies about 133 light years away in the constellation Pegasus and hosts four giant planets, each between five and ten times the mass of Jupiter and orbiting between 15 and 70 astronomical units from their star. The wide orbits and large masses of these planets previously challenged traditional models of core accretion, which suggested there would not be enough time for solid cores to grow and capture gas before the surrounding disk dispersed. Their properties led some researchers to suggest that they might instead have formed through gravitational instability, where clumps in the protoplanetary disk rapidly collapse into massive objects.
The new study, led by scientists at the University of California San Diego, uses JWST's high resolution spectroscopy to examine the atmospheres of three of the HR 8799 planets in unprecedented detail. Earlier observations of exoplanets focused on volatile molecules such as water and carbon monoxide, but these species can originate from different parts of the disk and are not ideal tracers of how planets assembled. The team instead targeted refractory elements like sulfur, which are locked in solid grains in the protoplanetary disk and are carried into growing planets via pebbles and planetesimals.
With JWST's sensitivity, the researchers detected clear signatures of sulfur-bearing molecules, including hydrogen sulfide, in the atmosphere of HR 8799 c and likely in the other inner giants as well. Because sulfur resides in solids in the disk, its enrichment in the planets supports a picture in which solid cores formed and then accreted gas, matching the core accretion scenario. The planets also show higher abundances of heavy elements such as carbon and oxygen compared to their host star, another hallmark of growth by swallowing solid material.
The HR 8799 system is relatively young at around 30 million years old, so its planets are still hot and bright enough to study effectively with infrared spectroscopy. Even so, the planets are about ten thousand times fainter than their star, and JWST's spectrograph was not originally designed for these difficult high contrast observations. Lead analyst Jean-Baptiste Ruffio developed new data processing methods to extract the faint planetary spectra from the overwhelming starlight, while Jerry Xuan built detailed atmospheric models to match the observed molecular features and verify the presence of sulfur.
The quality of the JWST data revealed fine spectral structures from several rare molecules that were previously undetectable in these planets. By iteratively refining the chemistry and physics in their models, the team was able to interpret the rich spectra and confirm that multiple molecules, some seen for the first time in this system, contribute to the observed signatures. Their analysis indicates that the HR 8799 planets share a uniform pattern of metal enrichment similar to Jupiter, despite being much more massive.
Co-author Quinn Konopacky argues that the findings show older core accretion models are insufficient and that newer versions, in which solid cores can grow far from their star, are better suited to explain systems like HR 8799. In these updated models, icy pebbles and planetesimals can efficiently feed distant cores, allowing them to reach the threshold mass needed to pull in large quantities of gas before the disk dissipates. The results therefore extend the range over which core accretion is thought to operate and suggest that very massive gas giants need not form like brown dwarfs.
Ruffio notes that HR 8799 remains unique as the only directly imaged system with four massive gas giants, but other systems are known with one or two even larger companions whose origins are still unclear. The team's work demonstrates that careful measurements of refractory elements and heavy element enrichment can distinguish between planetary and brown dwarf formation pathways. They now aim to apply the same techniques to additional systems to map out how planet formation proceeds across a wide range of masses and orbital distances.
Research Report:Jupiter-like uniform metal enrichment in a system of multiple giant exoplanets
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