By taking advantage of the dust-penetrating capabilities of the James Webb Space Telescope's infrared instruments, designed and built in part by University of Arizona scientists, astronomers have obtained the first direct observations with the new space telescope of gas and dust feeding a nascent planetary system with raw material for planet formation.

How planets form from a roiling maelstrom of gas and dust swirling around a young star is one of astronomy's most active fields of research. Much of the action during the early stages of planet formation remains shrouded in mystery – quite literally, as telescopes have historically struggled to peer through the dense clouds of dust surrounding planetary systems when they are in their infancy.

A team led by Jarron Leisenring at the UArizona Steward Observatory has obtained the deepest look yet into such planetary nurseries.

By combining JWST's images with prior observations by the Hubble Space Telescope and the Atacama Large Millimeter Array, or ALMA, in Chile, the researchers were able to piece together previously unseen interactions between the planet-forming disk and the envelope of gas and dust surrounding the young stars. The team presents its findings in three papers accepted in The Astrophysical Journal and two others in preparation.

Nascent planetary systems, also known as protoplanetary disks, are important targets of astronomical observation because they offer glimpses into how our own solar system came to be 4.6 billion years ago. Protoplanetary disks form when a vast cloud of interstellar gas and dust condenses under the effect of gravity before collapsing into a swirling "pancake" of matter. At its center shines a young star – only a few million years old.

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As microscopic dust particles coalesce into larger grains that stick together to form pebbles, and pebbles pile up to become planetesimals – "seedlings" that ultimately grow into planets – a planetary system is born. On astronomical time scales, protoplanetary disks are very short-lived. Typically, after 10 million years – at most – the material dissipates, clearing out from the disk. How that happens is not fully understood.

In the most likely scenario, much of the disk's material gets accreted onto the star, some is blown away by stellar radiation and the rest goes into forming planets, asteroids and comets. Although protoplanetary disks have been observed in various levels of detail, it is still extremely difficult to make out any planets that may be forming within. Rather, researchers have relied on features such as gaps and rings to infer the presence of planets as they plow through the disk. 

JWST made its most striking observations in a protoplanetary disk around HL Tauri, or HL Tau for short, a young, sun-like star in the Taurus star forming region, about 457 light-years from Earth. HL Tau is hidden in visible light behind a massive envelope of dust and gas and surrounded by a protoplanetary disk with multiple rings and gaps. ALMA images reveal the presence of several gaps in the disk, hinting at the possibility that several planets the size of Jupiter or smaller might be plowing through the disk material on their orbits. 

Based on the ALMA observations, the team set out to observe HL Tau along with two other protoplanetary disk systems, SAO 206462 and MWC 758, with JWST in hopes of detecting any planets that might be forming. Previous observations by the UArizona-led team revealed spiral arms forming in the protoplanetary disk of MWC 758, hinting at a massive planet orbiting its host star.

While no new planets were detected in the disk systems during the most recent observations, the sensitivity is groundbreaking, the researchers say, as it allows them to place the most stringent constraints yet on the suspected planets. For one, the results rule out the existence of additional planets in the outer regions of the MWC 758, consistent with a single giant planet driving the spiral arms.

"The lack of planets detected in HL Tau, and really in all three systems, tells us that the planets causing the gaps and spiral arms either are too close to their host stars or too faint to be seen with JWST," said Kevin Wagner, a NASA Hubble/Sagan Fellow at Steward Observatory who is a co-author on the HL Tau paper and lead author on the MWC 758 paper. "If the latter is true, it tells us that they're of relatively low mass, low temperature, enshrouded in dust, or some combination of the three – as is likely the case in MWC 758."

"While there is a ton of evidence for ongoing planet formation, HL Tau is too young with too much intervening dust to see the planets directly," Leisenring said. "We have already begun looking at other young systems with known planets to help form a more complete picture."

To the team's surprise, JWST revealed unexpected details of a different feature: the proto-stellar envelope – essentially a dense inflow of dust and gas surrounding the young star that is just beginning to coalesce, according to Leisenring, assistant research professor and principal investigator of the project. Under the influence of gravity, material from the interstellar medium falls inward onto the star and the disk, where it serves as the raw material for planets and their precursors.

"When I saw the JWST images of HL Tau, they just blew my mind," Wagner said. "I was expecting to see the disk or the rings, or maybe some planets in the rings, but instead, what we see are these features of the proto-stellar envelope resembling streams, clearly showing material flowing into the protoplanetary disk."

"We see a very complex and dynamic system with 'streamers' feeding material from the outer envelope into the inner regions of the disk, where we expect planets to be forming," Leisenring said.

The observations help scientists who study how planetary systems are being born refine their theories about the processes involved and shed light on what our sun did when it was very young. With the ongoing refinement of observational techniques and technological capabilities, and more advanced instruments coming online in the foreseeable future, astronomers hope to probe protoplanetary disks in greater detail and learn more about the conditions needed to build planets around stars other than our sun.