Our solar system is home to a wide diversity of planetary bodies, boasting eight planets, five officially recognized dwarf planets, and almost 1,000 confirmed moons. The eight planets consist of the four rocky (terrestrial) planets of the inner solar system and the four gas giant planets of the outer solar system. The largest planet in our solar system is Jupiter, measuring a radius and mass of 11 and 318 times of Earth, respectively. However, the discovery of exoplanets quickly altered our understanding of planetary sizes, as several have been discovered to have masses and radii several times that of Jupiter. So, how big can planet get, and are there limits to their sizes?
Now, a team of scientists from the United States and Canada and led by the University of California, San Diego might be one step closer to answering that question. For their study, which was recently published in *Nature Astronomy*, they investigated the complex geological and geochemical processes responsible for gas giant planet formation. While longstanding models have postulated gas giants form from the accumulation (accretion) of ice and rock, the exact processes are poorly understood.
Using NASA’s powerful James Webb Space Telescope (JWST), the team observed three gas giant exoplanets in the HR 8799 system, which is located approximately 133 light-years from Earth and contains four total gas giant exoplanets. The three planets range between 5 to 10 times the mass of Jupiter and orbit their star between 15 to 70 astronomical units (AU). For context, 1 AU is the distance from the Sun to the Earth, and Jupiter orbits just over 5 AU from our Sun. JWST’s powerful instruments analyzed their atmospheres to ascertain their chemical and molecular compositions with the goal of better understanding their formation processes.
In the end, the researchers confirmed detections of water, carbon monoxide, carbon dioxide, methane, molecules containing sulfur, and other molecules containing oxygen and carbon. The researchers concluded this indicates the planets contain heavier elements than their star, indicative of the oxygen and carbon, and experienced similar formation processes as Jupiter and Saturn. The team notes this indicates a much wider range of planetary sizes and compositions that challenge longstanding models of planetary formation and evolution.
“There are many models of planet formation to consider,” said Dr. Quinn Konopacky, who is a UC San Diego Professor of Astronomy and Astrophysics and a co-author on the study. “I think this shows that older core accretion models are outdated. And of the newer models, we are looking at ones where gas giants can form solid cores really far away from their star. I think the question is, how big can a planet be? Can a planet be 15, 20, 30 times the mass of Jupiter and still have formed like a planet? Where is the transition between planet formation and brown dwarf formation?”
The sulfur detected in these exoplanets recently made headlines from this same study as a first-time detection of sulfur in exoplanets. This confirmation of sulfur helped the astronomers confirm the four exoplanets in the HR 8799 system were actually planets instead of brown dwarfs, the latter of which are known as substellar objects that never became stars and are typically much larger than Jupiter. Both discoveries from the same study demonstrate how science can knock out two birds with one stone while helping scientists gain greater insight into planetary formation and evolution and offer constraints for the search for life beyond Earth.
What new insight into planetary sizes will researchers make in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!