The James Webb Space Telescope (JWST) has achieved a groundbreaking milestone by providing the first direct measurements of the chemical and physical properties of a potential moon-forming disk encircling a large exoplanet. This significant discovery revolves around a carbon-rich disk surrounding an exoplanet known as CT Cha b, which is located approximately 625 light-years away from Earth. Although no moons have been detected in the Webb data, the findings suggest that this disk could serve as a construction yard for future moons. The results of this research were published today in The Astrophysical Journal Letters.
CT Cha b orbits a young star that is only 2 million years old and is still in the process of accreting circumstellar material. It is important to note that the circumplanetary disk discovered by the James Webb Space Telescope is distinct from the larger accretion disk surrounding the central star, with the two objects being separated by a staggering distance of 46 billion miles. Observing the formation of planets and moons is crucial for understanding the evolution of planetary systems throughout our galaxy. Researchers believe that moons likely outnumber planets, and some may even harbor the potential for life as we know it.
This discovery enhances our understanding of both planet and moon formation, according to researchers involved in the study. The data gathered by Webb is invaluable for making comparisons with the birth of our solar system over 4 billion years ago. “We can see evidence of the disk around the companion, and we can study the chemistry for the first time. We're not just witnessing moon formation — we're also witnessing this planet’s formation,” stated co-lead author Sierra Grant from the Carnegie Institution for Science in Washington. Gabriele Cugno, the main lead author from the University of Zürich and a member of the National Center of Competence in Research PlanetS, added, “We are seeing what material is accreting to build the planet and moons.”
The infrared observations of CT Cha b were conducted using Webb’s MIRI (Mid-Infrared Instrument) with its medium resolution spectrograph. An initial examination of Webb’s archival data unveiled signs of molecules within the circumplanetary disk, prompting researchers to delve deeper into the data. Due to the planet’s faint signal being obscured by the brightness of its host star, the research team employed high-contrast methods to disentangle the light from the star and the planet. “We saw molecules at the location of the planet, and so we knew that there was stuff in there worth digging for and spending a year trying to tease out of the data. It really took a lot of perseverance,” said Grant.
Ultimately, the research team identified seven carbon-bearing molecules within the planet’s disk, including acetylene (C2H2) and benzene (C6H6). This carbon-rich chemistry starkly contrasts with the composition of the disk surrounding the host star, where only water was detected, with no carbon found. The notable difference between the two disks provides evidence of their rapid chemical evolution over just 2 million years.
A circumplanetary disk has long been theorized as the birthplace of major moons, such as Jupiter’s four Galilean satellites. These moons are believed to have condensed from a flattened disk billions of years ago, as evidenced by their co-planar orbits around Jupiter. The two outermost Galilean moons, Ganymede and Callisto, are composed of approximately 50% water ice and are thought to possess rocky cores, potentially made of carbon or silicon. “We want to learn more about how our solar system formed moons. This means we need to look at other systems that are still under construction. We’re trying to understand how it all works,” said Cugno. “How do these moons come to be? What are their ingredients? What physical processes are at play, and over what timescales? Webb allows us to witness the drama of moon formation and investigate these questions observationally for the first time.”
In the coming year, the team plans to utilize the James Webb Space Telescope for a comprehensive survey of similar objects to gain a deeper understanding of the diversity of physical and chemical properties in disks surrounding young planets. The James Webb Space Telescope stands as the world’s premier space science observatory, addressing mysteries in our solar system, exploring distant worlds around other stars, and probing the enigmatic structures and origins of our universe.
Webb is an international program led by NASA in collaboration with its partners, the European Space Agency (ESA) and the Canadian Space Agency (CSA). To learn more about the capabilities and findings of Webb, visit: NASA’s Webb Space Telescope.
