Webb finds ammonia on HATS-6 b, with a temperature puzzle
A giant planet orbiting a small star appears cooler than standard calculations suggest. Reflective clouds could help explain the finding, but that interpretation remains unconfirmed.
Astronomers using the James Webb Space Telescope report evidence of water, methane and ammonia in HATS-6 b’s atmosphere, alongside a surprisingly low inferred temperature. The University of Maryland’s September 22 account describes findings that could help test how astronomers interpret giant planets orbiting small stars.
The team, led by Maryland astronomy doctoral candidate Giannina Guzmán Caloca, inferred the gases from starlight passing through the planet’s atmosphere. Maryland says the study appeared in The Astronomical Journal on September 8; the manuscript was posted on August 17.
Reading the atmosphere
The researchers observed two passages of the planet across its star using Webb’s Near Infrared Spectrograph. Their study abstract reports strong evidence for the three molecules from modelling the resulting spectrum, covering wavelengths of 0.6–5.3 micrometres.
Independent discovery research established the planet’s unusual proportions: roughly Jupiter’s radius but only 0.32 times its mass. It circles a star with about 0.57 times the Sun’s mass every 3.3253 days.
Could clouds explain the chill?
The discovery study calculated an equilibrium temperature of about 713 kelvin assuming no reflected starlight. That provides a theoretical baseline, rather than a direct atmospheric temperature measurement.
Maryland reports early temperature estimates near 250°F, far below the commonly cited calculation of roughly 800°F. The university describes reflective clouds or haze as a possible explanation: they could send starlight back into space before it warms the planet.
Independent modelling by Christiane Helling and colleagues, first posted in 2022, placed HATS-6 b among planets expected to have broadly uniform cloud coverage. That prediction supplies relevant context, but does not confirm clouds or their cooling effect in the new observations.
The low temperature remains an inference from atmospheric models. Maryland says longer-wavelength observations could test the cloud explanation. The new study also leaves a spectral feature near three micrometres unidentified because several molecules could produce overlapping signals there.
Sources and context
- New Study Detects Ammonia and Unexpected Chill on a Distant Giant PlanetUniversity of Maryland, Department of Astronomy
- GEMS JWST: Hold on to your HATS(-6 b), a sub-solar metallicity giant planet with water, methane and ammonia in its atmospherearXiv; Giannina Guzmán Caloca and colleagues
- HATS-6b: A Warm Saturn Transiting an Early M Dwarf Star, and a Set of Empirical Relations for Characterizing K and M Dwarf Planet HostsarXiv; J. D. Hartman and the HATSouth discovery team
- Exoplanet weather and climate regimes with clouds and thermal ionospheres: A model grid study in support of large-scale observational campaignsarXiv; Christiane Helling and colleagues
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