New research utilizing spectral data from the James Webb Space Telescope (JWST) on the HR 8799 star system suggests its distant, massive gas giants formed through core accretion, a process similar to Jupiter's formation.
The study, published in Nature Astronomy, identified hydrogen sulfide in the atmosphere of at least one of these exoplanets, with its presence inferred across the system's inner three planets. This detection of sulfur, a refractory element, provides direct evidence supporting the core accretion model for these planets.
The HR 8799 System: A Glimpse at Distant Super-Jupiters
A research team, with contributions from the University of California San Diego and UCLA, employed JWST data to investigate the formation mechanisms of gas giants within the HR 8799 system. Located approximately 133 light-years away in the constellation Pegasus, the HR 8799 system is estimated to be 30 million years old and hosts four exoplanets.
These planets are classified as "super Jupiters," possessing masses ranging from 5 to 10 times that of Jupiter. They orbit their star at distances between 15 and 70 astronomical units. HR 8799 is currently the only imaged system known to contain four massive gas giants.
Understanding Gas Giant Formation Theories
Two primary theories explain the formation of gas giants, which are large planets primarily composed of helium and/or hydrogen:
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Core Accretion: In this model, solid cores form within a protoplanetary disk by accumulating rocky and icy material. Once sufficiently massive, these cores then attract surrounding gas. This process is generally understood to be how planets like Jupiter and Saturn in our solar system formed.
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Gravitational Instability: This alternative hypothesis suggests that a cloud of gas within the protoplanetary disk rapidly collapses to form a massive object. This theory has often been considered for more massive, distantly orbiting exoplanets, where earlier core accretion models indicated insufficient time for growth.
JWST's Unprecedented View and Analytical Breakthroughs
Astronomers utilized spectroscopy with JWST's high-resolution spectrograph to infer the physical properties and formation pathways of the exoplanets. Traditional methods relying on "volatile" molecules like water and carbon monoxide had yielded inconclusive results regarding planet origins.
The current study focused on "refractory" elements, such as sulfur, which exist as solids within a planet-forming disk. The presence of sulfur is considered evidence for core accretion.
Observing the HR 8799 planets presented challenges due to their faintness, being approximately 10,000 times dimmer than their host star. The JWST spectrograph was also not originally designed for such observations. Research scientist Jean-Baptiste Ruffio developed new data analysis techniques to extract the faint signals from the inner three gas giants. Jerry Xuan created detailed atmospheric models to compare with the JWST spectra, leading to the identification of sulfur. The data, collected from the younger, brighter planets, was free from Earth's atmospheric contamination, facilitating detailed atmospheric studies.
Key Discoveries and Future Implications
The research confirmed the detection of hydrogen sulfide in the atmosphere of HR 8799 c, the third planet in the system, and inferred its likely presence across all three innermost planets. This detection of sulfur suggests that these HR 8799 planets likely formed through core accretion, a process comparable to the formation of Jupiter, despite their significantly larger masses and wider orbital distances.
Furthermore, the study found that the inner three planets exhibited higher enrichment in heavy elements, including carbon and oxygen, compared to their star. This elemental composition further supports a planetary formation mechanism.
Co-authors of the study indicated that these findings necessitate updates to existing core accretion models, particularly those concerning the formation of gas giants at considerable distances from their host star. The results align with newer models that allow for the formation of solid cores at greater stellar distances. Researchers also noted that these observations establish a new benchmark for understanding the extent of core accretion in gas giant formation.
Ongoing research aims to further investigate the maximum size a celestial body can attain through planetary formation processes and to delineate the transition between planet and brown dwarf formation. This work received support from the National Aeronautics and Space Administration (NASA).