The sizes that gas giant planets can reach could be much larger than previously thought.
The HR 8799 system, located about 133 light-years from Earth, hosts four gas giants with masses ranging from five to ten times that of Jupiter. These objects reside in the blurred boundary between planets and brown dwarfs, the latter often called "failed stars." To analyze these worlds, researchers used the James Webb Space Telescope.
Example of a brown dwarf (SIMP-0136).
Credit: Evert Nasedkin/Trinity College Dublin
For years, scientists have debated the formation of these giants. The primary mechanism, called core accretion, involves solid materials first clumping together into a dense core, which then attracts gas. However, at large orbital distances where matter is sparse, it is widely accepted that this process should be too slow to produce such massive planets.
Thanks to JWST's infrared observations, the chemical composition of the atmospheres could be studied. The team focused not on common gases, but on molecules containing sulfur, such as hydrogen sulfide. The presence of this element in the atmosphere of HR 8799 c strongly suggests that the planet formed via core accretion, as sulfur typically originates from solid grains in protoplanetary disks.
This discovery demonstrates that core accretion can operate even for extremely massive and distant planets, thereby challenging the traditional distinction between gas giants and brown dwarfs. Another clue supports this idea: the planets of HR 8799 also show enrichment in heavy elements, such as carbon and oxygen, compared to their star, which corroborates this type of formation.
For Jean-Baptiste Ruffio, lead author of the study, the detection of sulfur suggests that these planets formed similarly to Jupiter, despite their much greater mass. These results, published in
Nature Astronomy, could lead to revising planetary formation models and redefining the limits of what is considered a planet.
The three inner planets orbiting the star HR 8799, captured by the JWST in 2023. Spectral analysis detected hydrogen sulfide in the atmosphere of HR 8799 c, indicating that the massive planet formed by core accretion.
Credit: Jean-Baptiste Ruffio, Jerry Xuan et al. Brown Dwarfs and Deuterium Fusion
Brown dwarfs are substellar objects whose mass is too low to sustain hydrogen fusion, the process that powers stars like the Sun. Nevertheless, they can fuse deuterium, an isotope of hydrogen, which distinguishes them from gas giant planets. This fusion occurs at masses between approximately 13 and 80 times that of Jupiter.
Unlike planets, brown dwarfs often form by the direct collapse of a gas cloud, a process similar to star birth. This gives them intermediate properties, with internal heat coming from gravitational contraction and limited deuterium fusion, and temperature and luminosity much lower than those of stars.
The boundary between brown dwarfs and massive gas giant planets is blurred, as some planets can reach comparable masses. Chemical observations, like those from JWST, help differentiate them by revealing signatures of their formation. For example, enrichment in heavy elements can indicate an origin via core accretion.