The James Webb Space Telescope (JWST) has spied planet-forming disks letting off a cold “steam,” providing crucial evidence for a leading theory describing how planets are built.
This excess water vapor was detected by the James Webb Space Telescope in two compact disks of gas and dust surrounding young stars that are just 2 million to 3 million years old — which is incredibly young in the scope of our universe’s timeline. The disks are located in the Taurus star-forming region, which sits about 430 light years away.
Astronomers believe planets form through a process that begins with what is termed “pebble accretion.” This involves small chunks of silicate rock, ranging in size from centimeters to about a meter, that are coated in ice. They’re thought to begin their lives in the freezing outer parts of a planet-forming disk, typically home to comets, and eventually start to experience friction with gas in the disk. This friction presumably robs the pebbles of their orbital energy and causes them to migrate into the disk’s inner realm.
As they congregate in the inner realm, the pebbles are thought to start bumping into one another and sticking together, slowly building up into larger and larger objects until they become protoplanets. From there, the much stronger gravity of these protoplanets allows them to sweep up pebbles at even faster rates, accelerating their growth. Such is the longstanding theory of planetary formation.
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The water vapor detected by the JWST’s Mid-Infrared Instrument (MIRI) is evidence for that process because this kind of water is expected to come from the migrating icy pebbles.
As icy pebbles drift inwards, they’re believed to pass a boundary called the “snow line.” In our solar system, the snow line was just inside the current orbit of Jupiter when the planets were forming. Inside this boundary, the temperature within the disk is thought to be too great for water to exist as ice. The icy coating on the pebbles would thus vaporize, leading to an injection of cold water vapor into the inner part of the disk.
That’s what MIRI could detect.
“Webb finally revealed the connection between water vapor in the inner disk and the drift of icy pebbles from the outer disk,” Andrea Banzatti of Texas State University, who is the lead author of a new paper describing the JWST observations, said in a statement.
The JWST observed four planet-forming disks in total — two disks that are quite compact, and two that are extended and haven’t experienced as much inward migration. Water vapor was only found in the two compact disks.
Still, there are unanswered questions. For example, the two extended disks, belonging to systems named CI Tau and IQ Tau, seem to sport rings in images taken by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. How these rings form is still debated, but one theory is that when migrating pebbles encounter a region of higher pressure, their inward drift slows. That slowing is thought to contain the pebbles within these sorts of pressure traps, forcing them to manifest as rings. How these rings affect planet formation from pebble accretion within them is unclear, however. It’s also intriguing that the two compact disks, called GK Tau and HP Tau, show no evidence of ring-like structures.
Another open question concerns the process of accretion in the first place — what conditions are required to cause pebbles to stick together when they collide and build up into larger objects without smashing apart?
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The JWST observations supporting the inward migration of pebbles suggest there are answers to these conundrums, but we just have to find out what they are. Hopefully the JWST will be the key to this — if nothing else, the observatory can help solidify a more modern picture of how planets form.
“In the past, we had this very static picture of planet formation, almost like there were these isolated zones that planets formed out of,” Colette Salyk of Vassar College in New York, who is a co-author on the paper describing the JWST results, said in the statement. “Now we actually have evidence that these zones can interact with each other.”
Salyk also points out that pebble accretion has been proposed to have occurred 4.5 billion years ago in our solar system, meaning the JWST observations won’t just tell us about how exoplanets form, but also how our own planet Earth was born.
The results were published on Wednesday (Nov. 8) in the journal Astrophysical Journal Letters.