James Webb Telescope Captures 72 Planet-Forming Disks

A team of researchers at the University of Arizona published, on August 25 in The Astronomical Journal, the results of one of the largest measurements yet of gas loss from protoplanetary disks, the clouds of gas and dust that surround young stars and give rise to planets. The university announced the findings publicly on August 26 and 27. The study draws on 72 images of young, Sun-like star systems, captured with the MIRI instrument on the James Webb Space Telescope, and shows that gas giants like Jupiter have only a limited window to build up their atmospheres before winds and gas jets carry that material off into space.

The lead author is Naman Bajaj, a PhD student at the University of Arizona’s Lunar and Planetary Laboratory. His work builds on an earlier study from 2020, when professor Ilaria Pascucci, his advisor and a co-author of this new paper as well, predicted with her team the existence of so-called magnetic winds, even though at the time, before Webb’s launch, molecular hydrogen could not yet be observed directly. In 2024, Bajaj and colleagues used James Webb to directly image, for the first time, gas escaping the disk around the young star T Cha. The new study extends that method to dozens of systems at once, building a picture of how the process changes over time.

The team tracked two gas tracers across images spanning a field roughly 750 astronomical units wide: molecular hydrogen, which makes up most of the mass in these disks, and ionized neon, visible only when excited by high energy radiation. Extended emission from both gases was measured in 66 of the 72 disks observed. Cone shaped winds of molecular hydrogen, launched above and below the plane of the disk, were seen in 46 systems, while fast neon jets were recorded in 40. The winds’ speeds range from 10 to 100 miles per second, or roughly 16 to 160 kilometers per second.

The data show that powerful magnetic winds, dense enough to block the star’s X-ray radiation, dominate during the first few million years of a system’s life. Later, as those magnetic winds weaken, so called photoevaporative winds take over, driven by the star’s own ultraviolet and X-ray radiation. After several million years, according to the study, the jets disappear, molecular winds fade, and only gentler atomic winds remain, slowly breaking down whatever is left of the disk. By comparison, the early solar system had a gas to dust ratio of 100 to 1, while today’s solar system, 4.5 billion years old, is, according to the University of Arizona, mostly empty space.

“Planet formation is therefore a race against time. Gas giants like Jupiter must assemble their massive atmospheres while the disk is still substantial enough to supply them, before winds and jets carry that raw material away into space,” Bajaj said in a University of Arizona statement.

Co-author Uma Gorti, a scientist at the SETI Institute and NASA, added in a statement on the research:

“Disk dispersal sets a fundamental clock for planet formation: once the gas is gone, the opportunity to build gas-rich planets is essentially over.”

Other co-authors of the paper, titled “JWST/MIRI Reveals the Evolution from Molecular to Atomic Disk Winds” and carrying the identifier 10.3847/1538-3881/ae9089, include Sylvie Cabrit of France’s National Center for Scientific Research, Suzan Edwards of Smith College, and Tracy Beck of the Space Telescope Science Institute.

The study does not measure the total mass of gas lost over time, nor does it pin down exactly what distance from the star the gas escapes the disk, which the authors describe as the next step for the research. Its conclusions also rest on comparing 72 different systems at different stages of development, rather than tracking a single disk over multiple years, so the timeline of planet formation has been reconstructed rather than directly observed in any one system.