Our Solar System is home to a vast array of planetary bodies, encompassing eight planets, five officially recognized dwarf planets, and nearly 1000 confirmed moons.
The eight planets consist of four rocky (terrestrial) worlds in the inner Solar System and four gas giants situated further out. The most massive planet in our Solar System is Jupiter, whose radius and mass are 11 and 318 times those of Earth, respectively. However, the discovery of exoplanets has rapidly reshaped our comprehension of planetary sizes, as several planets have been found with masses and radii several times that of Jupiter. This leads to the question: how large can planets grow, and are there limits to their dimensions?
A team of scientists from the United States and Canada, spearheaded by the University of California, San Diego, may have just moved closer to answering this. In their research, recently published in Nature Astronomy, they investigated the intricate geological and geochemical mechanisms driving the formation of gas giants. While long-standing models suggest these planets originate through the accumulation (accretion) of ice and rock, the precise mechanisms remain under-investigated.
Employing NASA’s powerful James Webb Space Telescope (JWST), the team observed three gas giant exoplanets orbiting the star HR 8799, a system approximately 133 light-years distant that hosts four gas giant exoplanets in total. These three observed planets boast masses 5 to 10 times that of Jupiter and orbit their star at distances ranging from 15 to 70 astronomical units (AU). For context, 1 AU is the distance between the Sun and Earth, and Jupiter orbits at just over 5 AU from our Sun. JWST’s sophisticated instruments analyzed their atmospheres to ascertain their chemical and molecular composition, aiming for a better grasp of their formation processes.
The researchers ultimately confirmed the detection of water, carbon monoxide, carbon dioxide, methane, sulfur-bearing molecules, alongside other oxygen- and carbon-bearing species. The investigators concluded that this points to the planets harboring heavier elements than their parent star, indicating the presence of oxygen and carbon, and suggesting their formation pathways mirrored those of Jupiter and Saturn. The team notes this points towards a far broader spectrum of planetary sizes and compositions, casting doubt on established models of planet formation and evolution.
“There are many planet formation models that need to be explored,” remarked study co-author Professor Quinn Konopacky from UC San Diego. “I think this suggests that the old core accretion models are outdated. Among the new models, we are considering those where gas giants can develop solid cores very far out from their host star. I think the question is, how massive can a planet get? Can a planet be 15, 20, 30 times Jupiter’s mass and still form as a planet? Where does the boundary lie between planet formation and brown dwarf formation?”
The sulfur detected in these exoplanets recently garnered public attention due to this same research, marking the first-ever identification of sulfur in exoplanets. This verification of sulfur’s presence aided astronomers in confirming that the four exoplanets in the HR 8799 system are indeed planets, rather than brown dwarfs—substellar objects that never ignited as stars and are typically much heftier than Jupiter. Both discoveries stemming from the same investigation showcase how science can achieve multiple objectives concurrently, providing researchers with deeper insights into planetary formation and evolution processes while simultaneously setting constraints for the search for extraterrestrial life.
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