Free Astronomy Magazine September-October 2021

17 SEPTEMBER-OCTOBER 2021 ASTRO PUBLISHING “Our ALMA observations were ob- tained at such exquisite resolution that we could clearly identify that the disc is associated with the planet and we are able to constrain its size for the first time,” she adds. The disc in question, called a circumplanetary disc, surrounds the exoplanet PDS 70c, one of two giant, Jupiter-like planets orbiting a star nearly 400 light-years away. Astronomers had found hints of a “moon-forming” disc around this exoplanet before but, since they could not clearly tell the disc apart from its surrounding environment, they could not con- firm its detection — until now. In addition, with the help of ALMA, Benisty and her team found that the disc has about the same diameter as the distance from our Sun to the Earth and enough mass to form up to three satellites the size of the Moon. But the results are not only key to finding out how moons arise. “These new observations are also extremely important to prove theo- ries of planet formation that could not be tested until now,” says Jae- han Bae, a researcher from the Earth and Planets Laboratory of the Carnegie Institution for Science, USA, and author on the study. Planets form in dusty discs around young stars, carving out cavities as they gobble up material from this circumstellar disc to grow. In this process, a planet can acquire its own circumplanetary disc, which con- tributes to the growth of the planet by regulating the amount of mate- rial falling onto it. At the same time, the gas and dust in the circumplan- etary disc can come together into progressively larger bodies through multiple collisions, ultimately lead- ing to the birth of moons. But as- tronomers do not yet fully under- stand the details of these processes. “In short, it is still unclear when,