The NASA/ESA/CSA James Webb Space Telescope has delivered the first direct measurements of the chemical and physical properties of a potential moon-forming, circumplanetary disc around the massive planetary companion CT Cha b, 625 light-years from Earth. Webb’s mid-infrared spectroscopy reveals a carbon-rich environment in the disc, marking a key step toward understanding how moons around gas giants may assemble.
What Webb saw
Using the Mid-Infrared Instrument (MIRI) Medium Resolution Spectrograph, an international team isolated the faint signal of CT Cha b from the bright glare of its young host star. The observations identify seven carbon-bearing molecules in the disc, including acetylene (C2H2) and benzene (C6H6). No moons are detected, but the disc’s composition and structure point to conditions conducive to satellite formation.
A young system with room to grow
CT Cha b orbits a roughly two-million-year-old star that is still accreting material. The circumplanetary disc around the companion is distinct from the larger circumstellar disc; the objects are separated by about 74 billion kilometers. The results indicate rapid chemical evolution on million-year timescales and provide a comparative context for the early Solar System.
Chemistry in contrast
Webb’s spectra show a carbon-rich chemistry in the circumplanetary environment, while the disc around the host star exhibits signatures of water but lacks detectable carbon-bearing molecules in the same bands. This contrast underscores divergent pathways for material processing between circumstellar and circumplanetary discs.
How the data were obtained
The team applied high-contrast techniques to disentangle starlight from the planet’s signal in the mid-infrared. The approach enabled a direct view of gas and dust in the planet-encircling disc and a measurement-driven inventory of its key molecular constituents.
Why it matters
- First direct characterization of chemistry in a circumplanetary disc around a large exoplanet.
- Detection of multiple carbon-bearing molecules, including acetylene and benzene.
- Evidence for rapid chemical evolution within about 2 million years.
- Clear contrast between circumplanetary and circumstellar disc compositions.
- New constraints on conditions that may lead to the formation of large moons, analogous to the Galilean satellites.
What comes next
The team plans a broader Webb survey of similar young systems to map the diversity of circumplanetary discs. The findings complement ongoing exploration of moon-forming environments in our backyard as ESA’s Jupiter Icy Moons Explorer advances toward the Jovian system and as studies continue on concepts for future missions to ocean worlds such as Enceladus.
The study appears in The Astrophysical Journal Letters. For more details, see the ESA summary: ESA: Webb studies moon-forming disc around massive planet.
Webb is an international partnership between NASA, ESA and the Canadian Space Agency. ESA contributed the launch on Ariane 5 and major elements of MIRI and NIRSpec.
