The James Webb Space Telescope (JWST) has obtained the first direct spectrum of the planetary-mass companion GJ504b, revealing an atmosphere containing water vapor, methane, carbon dioxide, ammonia, hydrogen sulfide, and—for the first time—salt clouds as a key component.
The research, published in The Astronomical Journal on June 18, marks a significant milestone in understanding the chemistry of cold, planetary-mass objects beyond our solar system.
Object Characteristics
GJ504b, first directly imaged in 2013, orbits a Sun-like star 57 light-years from Earth. Its orbit is immense: over 40 times Earth's orbital distance from the Sun.
The object’s key characteristics are striking:
- Mass: Approximately 25 times the mass of Jupiter
- Radius: Roughly 10% smaller than Jupiter's
- Temperature: About 564 K (290°C / 550°F), making it one of the coldest directly imaged companions
- Estimated Age: Between 2.5 and 4 billion years
Its classification remains uncertain. It sits near the boundary between giant planets and brown dwarfs (failed stars). Astronomers refer to it as a "planetary-mass companion," and its formation mechanism—whether it formed like a planet from a protostellar disk or like a star through gravitational collapse—has not been determined.
Observations and Atmospheric Composition
Previous attempts to obtain a spectrum of GJ504b using ground-based telescopes failed due to the object's faintness. JWST captured its spectrum across 2.9–5.3 microns using the NIRSpec instrument in approximately two hours. Data were then processed to remove the overwhelming glare from the host star.
The spectrum revealed signatures of several molecules, including:
- Water vapor
- Carbon monoxide (including isotope-bearing forms)
- Carbon dioxide
- Methane
- Ammonia
- Hydrogen sulfide
The atmosphere appears enriched in heavier elements (those heavier than hydrogen and helium) relative to the host star. The data also indicate disequilibrium chemistry, suggesting active atmospheric mixing processes.
Evidence for Salt Clouds
Atmospheric modeling that did not include clouds required physically implausible conditions to fit the observed data. Models that included clouds produced a far better fit. Among the cloud types tested, salt clouds provided the best match.
The inferred salt clouds—potentially composed of compounds such as potassium chloride or zinc sulfide—are present at altitudes that mute molecular signatures below, a pattern consistent with the observed spectrum.
Important Note: The salt cloud interpretation is based on model fitting rather than direct imaging of the clouds. The study's title itself describes the finding as "possible."
Implications and Uncertainties
The detection of salt clouds in a cold object's atmosphere marks important progress in understanding cloud chemistry across planetary systems.
The heavy element enrichment of GJ504b relative to its star is similar to Jupiter's enrichment relative to the Sun. This tentatively supports a planetary formation scenario from a protostellar disk. However, the object's classification and formation mechanism remain unresolved.
The techniques used in this study, including angular differential imaging with NIRSpec's integral field unit, could now be applied to other faint, cold companions. The research was supported by NASA award 80NSSC20K0586.