The James Webb Space Telescope (JWST) has observed the presence of planet-forming disks emitting a frigid “steam,” providing critical support for a prevailing theory explaining the process of planetary formation.
This additional water vapor was identified by the JWST within two dense disks composed of gas and dust encircling youthful stars, which are a mere 2 to 3 million years old — a remarkably youthful age on the cosmic timescale. These disks reside within the Taurus star-forming region, positioned approximately 430 light-years away.
James Webb
Astrophysicists hypothesize that the genesis of planets stems from a phenomenon referred to as “pebble accretion.” This mechanism involves the aggregation of small pieces of silicate rock, ranging from centimeters to around a meter in size, coated with ice. Initially, these pebbles are believed to form in the frozen outer sections of a planet-forming disk, typically where comets originate. Over time, they undergo interactions with the gas in the disk, resulting in a gradual loss of orbital energy and their migration towards the disk’s inner regions.
As these pebbles congregate in the inner regions, they are thought to collide and amalgamate, gradually forming larger structures until they evolve into protoplanets. The enhanced gravitational force of these protoplanets enables them to accumulate pebbles at an accelerated pace, hastening their growth. This represents the prevailing concept of planetary formation.
The detection of water vapor by the JWST’s Mid-Infrared Instrument (MIRI) provides evidence in support of this theory, as this water vapor is expected to originate from the migrating icy pebbles.
It is believed that as icy pebbles migrate inwards, they pass a critical boundary referred to as the “snow line.” In our solar system, the snow line was positioned just within the present orbit of Jupiter during the formation of the planets. Within this boundary, the temperature within the disk is presumed to be too high for water to exist in its icy state. Consequently, the icy coating on the pebbles would vaporize, leading to the injection of cold water vapor into the inner part of the disk.
This phenomenon was captured by MIRI.
“Webb has finally illuminated the link between water vapor in the inner disk and the migration of icy pebbles from the outer disk,” stated Andrea Banzatti of Texas State University, the lead author of a new paper detailing the JWST’s observations.
In total, the JWST examined four planet-forming disks, with water vapor only detected in the two compact disks. Nonetheless, certain questions remain unanswered. For instance, the two extended disks, associated with systems named CI Tau and IQ Tau, appear to exhibit ring-like structures in images captured by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The formation of these rings remains a topic of debate, with one theory proposing that when migrating pebbles encounter regions of heightened pressure, their inward movement decelerates.
This James Webb deceleration is believed to trap the pebbles within these pressure regions, giving rise to the formation of rings. However, the implications of these rings on planetary formation through pebble accretion within them are not yet clear. Additionally, it is intriguing that the two compact disks, known as GK Tau and HP Tau, do not display any signs of ring-like structures.
Another open query concerns the process of accretion itself — the specific conditions necessary for pebbles to adhere to each other upon collision and accumulate into larger entities without disintegrating.
The JWST’s observations endorsing the inward migration of pebbles suggest that solutions to these puzzles exist, awaiting further exploration. With any luck, the JWST will prove instrumental in deciphering these enigmas, ultimately refining our contemporary understanding of planetary formation.
“In the past, we held a rather static view of planet formation, almost as if there were isolated regions where planets formed,” remarked Colette Salyk of Vassar College in New York, a co-author of the paper discussing the JWST’s findings. “Now, we have tangible evidence that these regions can interact with each other.”
