JWST Finds Planet Formation Is a Race Against the Loss of Disk Gas

The raw material for a gas giant is also temporary. Young stars are surrounded by rotating protoplanetary disks containing the gas and dust from which planets form. But those disks do not last forever: jets, winds, accretion onto the star and stellar radiation progressively remove their material.

A James Webb Space Telescope survey led by Naman Bajaj of the University of Arizona turns that process into a population-level timeline. Using archival observations from Webb’s Mid-Infrared Instrument (MIRI), the researchers examined 72 inclined, mostly Class II disks around young stars similar to the Sun. The results, reported in The Astronomical Journal, show that the way disks lose gas changes as they evolve. 12

The central finding: cores must beat disk dispersal

A rocky planetary core can become a gas giant only if it captures a large hydrogen–helium envelope while the surrounding disk still contains enough gas. If the disk is cleared first, the core may remain rocky or become a smaller, gas-poor planet instead. The study therefore frames giant-planet formation as a race between rapid envelope growth and the dispersal of the gas reservoir. 12

This is especially important for planets such as Jupiter. The survey cannot establish Jupiter’s exact formation date, but it reinforces the basic requirement that Jupiter’s core and massive atmosphere had to assemble while the young Solar System still retained a substantial gas disk. 12

What Webb detected in the 72 disks

The researchers used infrared emission from molecules and atoms as tracers of material moving away from the disks. Extended emission from molecular hydrogen or ionized neon was detected in 66 of the 72 systems. The team identified:

• 46 conical molecular-hydrogen winds, which trace broader outflows of disk material.
• 40 high-velocity ionized-neon jets, which trace faster, more focused flows.

Every system with an ionized-neon jet also showed evidence of a broader wind. In most of those cases, the broader outflow was traced by molecular hydrogen; in the remainder, it was traced by atomic oxygen. That pairing links the narrow, fast jet to a wider process capable of removing gas from the planet-forming environment. 2

The sample is not a single system observed continuously from birth to maturity. Instead, the 72 disks represent systems at different stages of early development. Comparing them provides an evolutionary sequence—a set of still images that researchers use to reconstruct how disk clearing changes over time. 12

Disk clearing appears to change with age

The survey’s pattern is consistent with two broad clearing regimes that operate in succession during the first several million years of a planetary system’s life. 1

Early stage: magnetic jets and molecular winds

Systems with higher stellar accretion rates were more likely to show jets and molecular winds. That association is consistent with an early phase in which magnetohydrodynamic—magnetically driven—jets and winds dominate the removal of disk material. The outflows can carry gas away while the disk is still actively feeding the young star. 12

For a growing planetary core, this is the dangerous part of the race. The disk may still contain abundant gas, but the same active environment is beginning to drain it.

Later stage: weaker jets and radiation-driven loss

As accretion declines, the jets weaken and the hotter molecular-wind tracers fade first. The remaining outflows become increasingly atomic. The researchers interpret this later state as one in which stellar radiation, including photoevaporation, becomes more important in clearing the thinner disk. 12

The implication is not that one mechanism instantly replaces another. Rather, the relative importance of the clearing mechanisms shifts as the star–disk system evolves. The observed transition helps constrain when that change occurs, although the current detection statistics do not yet provide the complete mass budget of each wind. 12

Why some systems may form gas giants while others do not

The findings offer a physical explanation for why similar young systems can develop very different planetary architectures. A system may produce a gas giant if a core becomes sufficiently massive early enough to begin rapid envelope capture. Another system may lose much of its disk gas before that threshold is reached.

Several factors can influence the outcome, including the disk’s mass, the speed of core growth, the star’s accretion history, the strength of magnetic outflows and the level of high-energy radiation. The survey supports wind-driven disk dispersal as an important part of this variation, not as the sole factor determining whether a gas giant forms. 2

This also puts the Solar System’s architecture in context. The Solar System is now about 4.6 billion years old, and its original planet-forming disk has long disappeared. Its surviving planets therefore reflect an early, short-lived competition between planetary growth, material accreting onto the young Sun and gas escaping through jets, winds and radiation. 12

What the researchers need to measure next

Identifying which outflows occur at different evolutionary stages is an important first step, but occurrence alone does not reveal whether those outflows removed enough gas to control planet formation. The team plans to measure the mass-loss rate of each wind and determine the disk radii from which the winds are launched. 12

Those measurements could connect the infrared signatures to a quantitative question: can the observed outflows remove disk gas quickly enough to change the probability of forming a Jupiter-like planet? Answering that will help distinguish a compelling evolutionary sequence from a direct measurement of how disk dispersal sets the final architecture of planetary systems.