JWST Study Finds Giant Planets Face a Race Against Time to Form

A new study using NASA's James Webb Space Telescope (JWST) has tracked how young star systems lose the gas needed to build giant planets. Led by Naman Bajaj of the University of Arizona, with SETI Institute scientist Uma Gorti as a co-author, the research examined 72 young, Sun-like stars. It is one of the largest JWST studies of planet formation to date, and was published in The Astronomical Journal.

Around a young star, a disc of gas and dust supplies the material for planets. Gorti said this loss of gas "sets a fundamental clock for planet formation," since giant planets like Jupiter and Saturn need that gas to build their massive atmospheres before it disappears. The young Sun's disc once held about 100 times more gas than dust.

Using archival observations from JWST's Mid-Infrared Instrument (MIRI), the team traced two signs of escaping gas: molecular hydrogen, the most abundant molecule in these discs, and ionized neon. In the youngest systems, where material is still falling onto the central star, JWST showed strong jets and broad outflows powered by magnetic fields threading through the disc, carrying gas away.

As systems age and less material falls onto the star, the jets weaken and radiation from the star penetrates deeper into the disc, heating gas until it escapes — a process called photoevaporation. A 2020 study led by Ilaria Pascucci at the University of Arizona had predicted such winds should exist; the new JWST observations confirm them by directly detecting the molecular hydrogen.

Extended hydrogen and neon emission appeared in 66 of the 72 discs studied; conical molecular hydrogen winds were seen in 46 systems and fast neon jets in 40. "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, describing planet formation as "a race against time."