A research breakthrough by the James Webb Space Telescope opens a new chapter in understanding how moons form around young, massive planets – and in the process, takes us back to the time when our Solar System was being born.

The focus of attention is the young system CT Chamaeleontis in the constellation Chameleon, approximately 625 light-years away from Earth. Within this system lies CT Cha b, a planet-like object in a wide orbit surrounded by a dense disk of gas and dust – a ribbon of material that, according to leading theories, condenses into moons while the planet is still forming. The latest observations by Webb in MIRI/MRS mode provide for the first time directly measurable chemical and physical characteristics of this circumplanetary disk, separating the planet's light from the glare of the young star.

Why this disk is special

Unlike protoplanetary disks that surround stars and from which planets are born, circumplanetary disks are local reservoirs of material within the gravitational domain of a young gas giant. It was precisely such an environment that served as the cradle of Jupiter's Galilean moons – Io, Europa, Ganymede, and Callisto – which grew more than four billion years ago from a gaseous-dusty ring around a young Jupiter. To observe a present-day example of such a "moon construction site" is to get a window into the early history of the Solar System.

Webb's analysis shows that the disk around CT Cha b is extremely rich in carbon molecules. In the mid-infrared region, spectral "fingerprints" of acetylene (C₂H₂), benzene (C₆H₆), and several other carbon compounds were identified, while at the same time, in the larger, stellar disk around the young star, water lines were recorded without clear indications of similar carbon chemistry. This contrast suggests a very rapid and locally distinct chemical evolution within the first ~2 million years of the system's life.

System geometry and light separation

The young star CT Cha is only about 2 million years old and is still accreting matter from its own, extensive disk. However, CT Cha b orbits outside this bustling construction site – at an estimated spatial distance of approximately 74 billion kilometers (which corresponds to about ~495 astronomical units). On this scale, the planetary companion and its disk form a separate chemical laboratory. The fact that Webb managed to isolate the subtle signal of the disk from the intense background light of the young star emphasizes the power of combining high angular resolution and medium-resolution spectroscopy.

To extract the light of CT Cha b from the ambiguous glare, researchers used high-contrast methods and careful point spread function decomposition (PSF subtraction). This way, a pure infrared spectrum was reconstructed, from which the chemical signatures of the disk can be read.

What the chemistry reveals: building blocks of future moons

The presence of acetylene and benzene is not just an exotic footnote: they are precursors to more complex organic molecules. In environments where ultraviolet radiation, thermal gradients, and collisions of dust grains are present, such hydrocarbons can participate in polymerization and the formation of PAHs (polycyclic aromatic hydrocarbons), but also trigger carbonation and ice restructuring on the surfaces of grains. All this changes the stickiness and aerodynamics of the particles, accelerates growth-inducing collisions, and helps in the formation of larger bodies – from millimeter-sized grains to kilometer-sized moonlets.

Conversely, the water-rich spectrum of material in the stellar disk indicates a different thermochemical zone and source of gas supply. This implies that the moons around CT Cha b, if they do indeed form, may develop a different ice-to-rock ratio than we would expect from the conditions in the larger, stellar disk alone. The two most distant Galilean moons, Ganymede and Callisto, today contain up to 50% water in the form of ice – an example of how the chemistry of the disk defines the internal layers and long-term evolution of moons.

MIRI/MRS: how Webb "hears" molecules

The MIRI (Mid-Infrared Instrument) on the Webb telescope covers a range of wavelengths where vibrational-rotational excitations of molecules leave recognizable "lines." In Medium Resolution Spectrograph mode, MIRI offers a balance between spectral sharpness and sensitivity, enabling the detection of faint lines even when they are next to much stronger sources. For CT Cha b, this meant that the characteristic absorption/emission signatures of hydrocarbons were isolated from the star's background light, followed by a comparison with spectral libraries and computer models that describe gaseous disks and protoplanetary atmospheres.

Such "chemical tomography" tells us not only what is present, but also where certain molecules are located within the disk. For example, warmer inner belts are more favorable for acetylene and benzene, while water vapor and CO₂ are more easily retained in slightly cooler zones or on the surfaces of particles in the form of ice. This internal distribution is directly related to the temperature profile, accretion rates, and turbulent mixing in the disk.

Rapid evolution at an early system age

The system's age of about 2 million years highlights how much chemistry changes in short cosmic intervals. During this period, dust clumps together, planetesimals form, and the disk slowly loses gas due to photoevaporation and accretion onto the central object. The fact that at the same time the stellar disk shows pronounced traces of water, while the disk around the planet is hydrocarbon-dominant, points to different sources of material supply and different UV environments that "cook" the chemistry in opposite ways.

What distance and arrangement tell us

CT Cha b and its star are separated by about 74,000,000,000 km – a figure that, when converted to astronomical units, is approximately 495 AU. For comparison, Pluto orbits the Sun at an average of about 39 AU. Such a large separation simplifies observation, but also raises questions about its formation: did CT Cha b form as a planet in a wide orbit within the stellar disk, or is it closer to a brown dwarf or a "failed star" scenario? The answer lies precisely in the chemical composition and kinematics of the disk that surrounds it.

Implications for the habitability of moons

If moons do indeed form around CT Cha b, their initial chemical inventory will be colored by an abundance of carbon. This could mean an abundance of organic precursors which, when they arrive on surfaces in the form of ice or in subsurface oceans, could serve as an energetic and chemical currency for prebiotic processes. In the Solar System, missions to Europa and Enceladus are motivated precisely by the search for the chemical building blocks of life in the moons that were formed.