Our Solar System contains eight major planets, and more than 400 known moons orbiting six of these planets. Where did they all come from? There are multiple formation mechanisms. The case for large moons, like the four Galilean satellites around Jupiter, is that they condensed out of a dust and gas disc encircling the planet when it formed. But that would have happened over four billion years ago, and there is scant forensic evidence today.

Webb has now provided the first direct view of material in a disc around a large exoplanet. An international team of astronomers have uncovered a carbon-rich disc encircling the world called CT Cha b, which is located 625 light-years away from Earth.

The young star the planet orbits is only two million years old and still accreting circumstellar material. However, the circumplanetary disc discovered by Webb is not part of the larger accretion disc around the central star. The two objects are 74 billion km apart.

Observing planet and moon formation is fundamental to understanding the evolution of planetary systems across our galaxy. Moons likely outnumber planets, and some might be habitats for life as we know it. But we are now only entering an era where we can witness their formation.

This discovery fosters a better understanding of planet and moon formation, say researchers. Webb’s data is invaluable for making comparisons to our Solar System’s birth over four billion years ago.

“We can see evidence of the disc 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,” said co-lead author Sierra Grant of the Carnegie Institution for Science in Washington, D.C., USA.

“We are seeing what material is accreting to build the planet and moons,” added main lead author Gabriele Cugno of the University of Zurich in Switzerland and member of the National Centre of Competence in Research PlanetS.