Our Solar System serves as the setting for a vast array of celestial bodies, encompassing eight planets, five officially recognized dwarf planets, and nearly 1000 confirmed moons.
The eight planets are composed of the four rocky (terrestrial) planets in the inner Solar System and the four gas giants situated in the outer regions. Jupiter stands as the Solar System’s largest planet, possessing a radius and mass that exceed Earth’s by factors of 11 and 318, respectively. Nevertheless, the discovery of exoplanets has rapidly reshaped our comprehension of planetary scale, as numerous worlds have been found whose mass and radius dwarf even Jupiter’s. This prompts the question: what is the upper limit for planetary size?
A group of scientists from the US and Canada, spearheaded by the University of California San Diego, may have just taken a significant step toward answering this question. In their research, recently published in Nature Astronomy, they delved into the intricate geological and geochemical mechanisms responsible for the birth of gas giants. While established models posit that gas giants arise through the accumulation (accretion) of ice and rock, the precise nature of these processes remains inadequately understood.
Employing NASA’s powerful James Webb Space Telescope (JWST), the team focused on three gas giant exoplanets orbiting the star HR 8799, a system roughly 133 light-years away that hosts a total of four such worlds. These three observed planets possess masses between five and ten times that of Jupiter, orbiting their star at distances ranging from 15 to 70 astronomical units (AU). For context, one AU equals the Sun-Earth distance, and Jupiter orbits slightly beyond 5 AU from our Sun. The JWST’s advanced instruments analyzed these worlds’ atmospheres to determine their chemical and molecular makeup, aiming to shed light on their formation pathways.
Ultimately, the researchers verified the presence of water, carbon monoxide, carbon dioxide, methane, sulfur-bearing molecules, along with other compounds containing oxygen and carbon. The investigators concluded that this composition implies the planets contain heavier elements than their host star, confirming the existence of oxygen and carbon, and suggesting their formation processes paralleled those of Jupiter and Saturn. The team further suggests this points toward a far broader spectrum of planetary sizes and compositions, challenging long-held models of planetary formation and evolution.
“There are many planet formation models that need to be considered,” stated co-author Professor Quinn Konopacky from UC San Diego. “I believe this indicates that older core accretion models are outdated. Among the newer models, we are looking at those where gas giants can develop solid cores very far out from their stars. I suppose the question is, how massive can a planet get? Can a planet be 15, 20, or 30 times the mass of Jupiter and still form as a planet? Where is the dividing line between planet formation and brown dwarf formation?”
The detection of sulfur in these exoplanets has notably captured public attention due to this same research, marking the first time sulfur has been identified in an exoplanet. This confirmation of sulfur helped astronomers solidify that the four exoplanets in the HR 8799 system are indeed planets, rather than brown dwarfs—substellar objects that never ignited into stars and are typically much larger than Jupiter. Both findings stemming from this singular study illustrate how science can achieve multiple objectives simultaneously, advancing researchers’ understanding of planetary genesis and evolution while simultaneously establishing parameters for the search for extraterrestrial life.
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