James Webb reveals why planet formation is a race against time

JWST reveals that newborn planets are racing against powerful winds and radiation before their supply of planet-building gas disappears.

Planets take shape inside disks of gas and dust surrounding young stars, but the supply of gas they depend on is temporary. New observations from NASA's James Webb Space Telescope (JWST) are giving astronomers a clearer picture of how that gas escapes and how the dominant escape mechanisms change as planetary systems mature.

The study, led by Naman Bajaj of the University of Arizona and coauthored by SETI Institute scientist Uma Gorti, examined 72 young, Sun-like stars and their protoplanetary disks. It is among the largest studies of planet formation conducted with JWST and suggests that different kinds of winds dominate at different stages of a system's early development.

The findings have been published in The Astronomical Journal.

Planet Formation Has a Deadline

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

Our solar system is now about 4.5 billion years old and consists mostly of empty space. During its first few million years, however, the young Sun was surrounded by a dense protoplanetary disk containing roughly 100 times more gas than dust. Nearly all of that gas eventually disappeared.

Understanding when and how this material is lost matters because gas is essential for forming giant planets such as Jupiter and Saturn. If a disk loses its gas too quickly, developing planets may not have enough time to accumulate the enormous atmospheres needed to become gas giants.

JWST Tracks Escaping Gas

Bajaj and his colleagues investigated this process using archival observations from JWST's Mid-Infrared Instrument (MIRI). The 72 systems span different stages of planetary development. Taken together, they provide something like a sequence of snapshots, allowing researchers to reconstruct how gas dispersal changes as systems grow older.

In 2020, LPL professor Ilaria Pascucci, second author of the paper and Bajaj's advisor, led a related study examining how jets and winds evolve. Before JWST, researchers could not directly observe molecular hydrogen, but the team predicted that molecular winds should exist and could be dense enough during the earliest stages to block X-ray photons. By directly tracing molecular hydrogen in the new observations, Bajaj's team confirmed those predictions with JWST images.

The researchers concentrated on two indicators of escaping material: molecular hydrogen, the most abundant molecule in protoplanetary disks, and ionized neon. JWST's sensitivity and resolution allowed them to distinguish broad winds containing molecular hydrogen from jets and winds traced by neon.

In the youngest systems, where material is still falling onto the central star, the researchers detected powerful jets along with broad winds containing both molecular and atomic gas. These outflows are consistent with winds generated by magnetic fields threading through the disk. Gas can follow those magnetic field lines outward, removing both material and angular momentum from the disk.

Stellar Radiation Takes Over

As planetary systems mature and the flow of material onto the star declines, the jets weaken and the escaping gas becomes increasingly atomic. At this stage, high energy radiation from the young star can penetrate the thinning material and heat gas in the disk until it escapes. This mechanism is known as photoevaporation.

Gorti has spent decades investigating the evolution and dispersal of protoplanetary disks, including the role of ultraviolet and X-ray radiation from young stars in driving photoevaporative winds. The latest JWST observations provide an observational connection to that theoretical work across dozens of systems. They indicate that photoevaporation becomes more important as disks age and magnetically driven jets and winds fade.

The findings suggest that there is no single mechanism responsible for clearing away a planet forming disk. Instead, young systems appear to begin with powerful magnetically driven jets and winds before transitioning toward a later stage in which atomic winds, including those produced through photoevaporation, become increasingly important.

That transition has major consequences for developing planets.

"Planet formation is therefore a race against time," Bajaj said. "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."

Researchers detected extended emissions from molecular hydrogen and ionized neon in 66 of the 72 disks. Conical molecular hydrogen winds appeared in 46 systems, while fast moving neon jets were found in 40. Every system containing a neon jet also showed evidence of a wind traced by molecular hydrogen or oxygen.

A Broader View of How Planetary Systems Evolve

The study expands on earlier observations from the same research group. In 2024, Bajaj, Gorti, and their colleagues used JWST to image gas being carried away from the planet forming disk surrounding the young star T Cha.

That earlier work demonstrated that JWST could directly investigate disk dispersal in an individual planetary system. The new study extends the approach to dozens of young stars, revealing how the balance between jets, molecular winds, and atomic winds changes as planetary systems age.

Researchers now want to determine exactly how much gas these winds remove over time and identify where within the disk the escaping material originates. Those measurements could reveal not only how quickly the window for planet formation closes, but also which regions of a disk are capable of producing different types of planets before their gas supply disappears.