JWST Study of 72 Stars Shows Planet Formation Is a Race Against Time

Astronomers using the James Webb Space Telescope have studied 72 young, sun-like stars and found that forming planets is a genuine race against time. Gas and dust in the swirling "protoplanetary disks" that surround infant stars — the building blocks of planets — are constantly escaping into space, and the study suggests some types of planets may have their formation window close sooner than others.

The research, led by Naman Bajaj of the University of Arizona, is the most in-depth investigation yet into how matter escapes these disks and how that escape shapes different stages of planet formation around sun-like stars. "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.

Using the telescope's Mid-Infrared Instrument, the team tracked matter loss by following molecular hydrogen, one of the most common molecules in protoplanetary disks. Because each of the 72 stars represented a different stage in a star system's early life, the snapshots could be combined into something like a "movie" of how a planetary system develops. The researchers found that the dominant process removing gas from a disk changes as the star ages: early on, powerful jets and winds driven by magnetic fields dominate the mass loss; later, as the disk thins, these weaken and give way to photoevaporation, in which high-energy radiation from the star ionizes gas and blows it into space.

"What is exciting about this study is that we can now see, across a large sample of young systems, how the mechanisms that remove gas from planet-forming disks change with time," said team member Uma Gorti of the SETI Institute. "Disk dispersal sets a fundamental clock for planet formation: once the gas is gone, the opportunity to build gas-rich planets is essentially over." Gas giants such as Jupiter and Saturn need far more raw material to build their vast atmospheres than smaller, rocky worlds like Earth. The study was published August 25 in The Astronomical Journal.