The intriguing overlap of massive exoplanets has raised fundamental questions in astronomy regarding their formation processes. One widely considered theory is core accretion, which is thought to have led to the creation of Jupiter and Saturn. In this model, a solid core gradually accumulates within a disk composed of dust and ice, gathering rocky and icy materials until it becomes substantial enough to attract surrounding gases. Alternatively, gravitational instability may also play a role, where a swirling gas cloud around a young star collapses under its own gravity, resulting in the formation of a large object akin to a brown dwarf.
A research team from the University of California San Diego embarked on a mission to unravel this mystery, utilizing data from the James Webb Space Telescope (JWST). Their investigation into the HR 8799 star system yielded unexpected findings, which were detailed in a recent publication in Nature Astronomy.
The HR 8799 System and Its Super Jupiters
Located approximately 133 light years away in the constellation Pegasus, HR 8799 is home to four colossal planets, each boasting a mass between five and ten times that of Jupiter. These planets orbit at distances ranging from 15 to 70 astronomical units, meaning even the closest one is situated 15 times farther from its star than Earth is from the sun. The mass of these planets ranges from 5−10 MJup, indicating that even the lightest among them is five times heavier than Jupiter.
In many respects, HR 8799 resembles an enlarged version of our solar system, which also features four outer giant planets extending from Jupiter to Neptune. However, the significant size of the planets in HR 8799 and their expansive orbits have left scientists perplexed. Previous models based on our solar system suggested that planets formed through core accretion would not have sufficient time to achieve such massive sizes before the young star dispersed the surrounding gas disk.
JWST Spectroscopy Reveals Sulfur Clues
To delve deeper, astronomers employed spectroscopy, a method that analyzes light to determine the chemical composition and physical characteristics of distant planets. Prior to JWST, researchers relied on ground-based telescopes to identify molecules like water and carbon monoxide in the atmospheres of exoplanets. Over time, it became evident that carbon and oxygen-based molecules were not ideal for tracing planetary formation due to their ambiguous origins.
Shifting focus, the team concentrated on more stable materials known as refractory elements. These elements, such as sulfur, exist in solid form within the protoplanetary disk where planets develop. The detection of sulfur in a gas giant's atmosphere strongly indicates formation via core accretion.
"With its unparalleled sensitivity, JWST is facilitating the most comprehensive study of these planets' atmospheres, providing insights into their formation pathways. The detection of sulfur suggests that the HR 8799 planets likely formed similarly to Jupiter, despite their greater mass, which was an unexpected finding," remarked Jean-Baptiste Ruffio, a research scientist at UC San Diego and the paper's lead co-author.
HR 8799 is relatively young, estimated to be around 30 million years old (in contrast, our solar system is about 4.6 billion years old). Younger planets retain heat from their formation, making them brighter and easier to analyze using spectroscopy.
The high-resolution spectrograph of JWST enables scientists to scrutinize exoplanet light without interference from Earth's atmospheric molecules. For the first time, astronomers have detected detailed signatures of several rare molecules in the atmospheres of the system's three inner gas giants that were previously unobserved.
Detecting Hydrogen Sulfide on Distant Worlds
Extracting this information posed challenges, as the planets are approximately 10,000 times fainter than their host star, and JWST was not initially designed for such extreme contrasts. Ruffio developed innovative data analysis techniques to isolate the faint signals from the planets. Jerry Xuan, a 51 Pegasi b Fellow at UCLA, created advanced atmospheric models to compare with the telescope's spectra in order to identify the presence of sulfur.
"The quality of the JWST data is genuinely groundbreaking, and existing atmospheric model grids were insufficient. To fully interpret the data, I iteratively refined the chemistry and physics in the models," he explained. "Ultimately, we detected several molecules in these planets -- some for the first time, including hydrogen sulfide."
Clear indications of sulfur were identified on the third planet, HR 8799 c, and researchers believe it likely exists on the other two inner planets as well. The team also found that these planets possess higher concentrations of heavy elements like carbon and oxygen compared to their star, further supporting the notion that they formed as planets rather than brown dwarf-like objects.
Rethinking Planet Formation Models
"There are numerous models of planet formation to consider. This evidence suggests that older core accretion models may be outdated," stated UC San Diego Professor of Astronomy and Astrophysics Quinn Konopacky, another co-author of the study. "Among the newer models, we are investigating those where gas giants can develop solid cores at significant distances from their star."
Currently, HR 8799 remains the only directly imaged system known to host four massive gas giants. However, other systems have been discovered with one or two even larger companions, the origins of which remain uncertain.
"The question remains: how large can a planet become?" Ruffio pondered. "Is it possible for a planet to reach 15, 20, or even 30 times the mass of Jupiter and still form like a planet? Where is the boundary between planet formation and brown dwarf formation?"
Researchers continue to investigate these compelling questions, examining one star system at a time.
A partial list of authors includes Jean-Baptiste Ruffio, Eve J. Lee, and Quinn Konopacky (all from UC San Diego); Jerry W. Xuan (California Institute of Technology and UCLA); Dimitri Mawet, Aurora Kesseli, Charles Beichman, Geoffrey Bryden, and Thomas P. Greene (all from California Institute of Technology); and Yayaati Chachan (UC Santa Cruz). The full list of authors is available in the published paper.
This research was partially supported by the National Aeronautics and Space Administration (grants 80NSSC25K7300 and FINESST Fellowship award 80NSSC23K1434). The opinions, findings, conclusions, or recommendations expressed in this work are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration.