Groundbreaking research has shed light on the formation of “super Jupiters,” massive exoplanets that orbit distant stars. Using data from the James Webb Space Telescope (JWST), astronomers have studied the HR 8799 star system, located approximately 133 light-years away in the constellation Pegasus. This system contains four super Jupiters, each with a mass five to ten times that of Jupiter.
Traditionally, planetary formation is understood through the core accretion model, where planets develop from a disk of material surrounding a star. This process involves tiny grains coalescing into larger bodies, eventually forming full-fledged planets. However, previous theories suggested that super Jupiters, due to their size and distance from their host star, might form in a different manner known as gravitational instability.
The recent findings, published in the journal Nature Astronomy, indicate that the third planet in the HR 8799 system, designated HR 8799 c, contains sulfur. This discovery is significant because sulfur, unlike carbon and oxygen compounds, remains solid in the cooler conditions of a planet-forming disk, suggesting that HR 8799 c formed through core accretion rather than gravitational instability.
The research team, led by Jean-Baptiste Ruffio, a research scientist at UC San Diego and former postdoctoral scholar at Caltech, posits that sulfur may also be present on the other two innermost planets in the system. Observations revealed that these planets are enriched in heavy elements like carbon and oxygen, reinforcing the idea that they formed similarly to Jupiter.
New Insights into Planetary Formation
According to co-author Charles Beichman, a senior faculty associate at IPAC, a science and data center at Caltech, the findings set a new benchmark for understanding where gas giants can form rocky cores. “It was not previously clear how far out a gas giant could be from its star and still form a rocky core,” he explained.
The study’s success hinged on isolating spectral data from the planets, which are approximately 10,000 times fainter than their host star. The JWST’s spectrograph was not initially designed for such observations, prompting Ruffio to devise new techniques to extract the necessary faint signals. This innovative approach allowed the team to identify several molecules in the atmospheres of these planets, including hydrogen sulfide—some detected for the first time.
Co-lead author Jerry Xuan, a 51 Pegasi b Fellow at UCLA, emphasized the revolutionary quality of the JWST data, explaining that existing atmospheric model grids were insufficient for the task. “To fully capture what the data were telling us, I iteratively refined the chemistry and physics in the models,” Xuan noted.
Beichman highlighted that these observations will challenge theorists to refine their models of planetary formation. He remarked, “Astronomy is driven by observations, and then the theorists have to explain it. This is how we expand our knowledge.”
Collaborative Efforts and Future Research
The research drew contributions from several notable figures at Caltech, including Dimitri Mawet, the David Morrisroe Professor of Astronomy; Heather Knutson, a professor of planetary science; Geoffrey Bryden of JPL; and Thomas Greene, executive director of IPAC. This collaborative effort was supported by NASA, underscoring the importance of inter-institutional partnerships in advancing our understanding of the cosmos.
As astronomers continue to analyze data from the JWST and other telescopes around the world, the implications of these findings may reshape our understanding of planetary formation not only in the HR 8799 system but also in other distant star systems. The research reinforces the notion that even the most massive planets can form through processes similar to those of their smaller counterparts, challenging previous assumptions about planetary development in the universe.
