Planetary Formation

Webb Joins the Hunt for Protoplanets

We can’t understand what we can’t clearly see. That fact plagues scientists who study how planets form. Planet formation happens inside a thick, obscuring disk of gas and dust. But when it comes to seeing through that dust to where nascent planets begin to take shape, astronomers have a powerful new tool: the James Webb Space Telescope.

In the past few years, we’ve been getting tantalizing looks at the protoplanetary disks around young stars. ALMA, the Atacama Large Millimetre/submillimetre Array, is responsible for that. It’s imaged many of these disks around young stars, including the telltale gaps where planets are likely forming.

ALMA’s high-resolution images of nearby protoplanetary disks are the results of the Disk Substructures at High Angular Resolution Project (DSHARP). Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

Imaging the disks is now becoming a regular occurrence, but astronomers have only spotted two forming planets.

But now researchers have brought the JWST to bear on the problem. Three new studies in The Astronomical Journal present the results of that effort. They are:

The research combines new JWST observations with previous observations by the Hubble and ALMA. The astronomers behind each of the studies used the JWST to uncover new, early clues about the planet formation process, including how the process shapes the disk they’re born from. If they can identify features unique to planet formation, they can then look for these features around other disks.

HL Tau, SAO 206462 and MWC 758 are all protoplanetary disks that have been observed by other telescopes. The JWST’s powerful infrared capabilities should provide new insights into these disks and their planets. That’s because as planets gather more material to them, they release infrared radiation.

“When material falls onto the planet, it shocks at the surface and gives off an emission line at specific wavelengths,” said astronomer Gabriel Cugno, who was involved with all three papers. “We use a set of narrow-band filters to try to detect this accretion. This has been done before from the ground at optical wavelengths, but this is the first time it’s been done in the infrared with JWST.”

MWC 758 is a young star that hosts a spiral protoplanetary disk.

This JWST/NIRCam image of MWC 758 shows the star’s unusual spiral disk. Wagner et al. 2024.

Using mathematical simulations, the researchers showed that a giant planet called MWS 758c outside the spirals can produce the spirals. They also showed that the symmetry of the arms can constrain the planet’s mass. In this case, they can determine a lower range for the planet’s mass: between about 4 to 8 Jupiter masses. But they didn’t find it. There may also be an even more massive companion further out, according to the simulations, but none was detected.

SAO 206462 is another young star surrounded by a disk. It also has clearly defined spiral arms, signifying the presence of a massive planet. The astronomers studying this star and disk did find a planet, but not the one they expected.

This is a JWST image of the star SAO 296462 and its spiral disk. Image Credit: Cugno et al. 2024.

“Several simulations suggest that the planet should be within the disk, massive, large, hot, and bright. But we didn’t find it. This means that either the planet is much colder than we think, or it may be obscured by some material that prevents us from seeing it,” said lead author Gabriele Cugno, also a co-author on the other paper papers. “What we have found is a different planet candidate, but we cannot tell with 100% certainty whether it’s a planet or a faint background star or galaxy contaminating our image. Future observations will help us understand exactly what we are looking at.”

Massive gas giants are expected to be responsible for the spiral shapes. But even the JWST struggles to find them. “The problem is, whatever we’re trying to detect is hundreds of thousands, if not millions of times fainter than the star,” Cugno said. “That’s like trying to detect a little light bulb next to a lighthouse.”

HL Tau is the third star and disk that the JWST examined and the youngest, at less than 100,000 years old. HL Tau is well-known in astronomy for the telltale gaps and rings in its disk, as well as some other features. For example, astronomers found water vapour in its disk right in the location where a suspected planet is forming.

In this image of HL Tau, observations from the Atacama Large Millimeter/submillimeter Array (ALMA) show water vapour in shades of blue in the same location where astronomers thought a planet may be forming. Image Credit: ALMA (ESO/NAOJ/NRAO)/S. Facchini et al.

The JWST found the known stellar envelope, outflow cavity, and other features. But, unfortunately, no planet.

This image from the paper shows an ALMA image of HL Tau and a JWST image of HL Tau. The JWST is able to see details that the ALMA image doesn’t show, including a feature called the hook-shaped clump. Image Credit: Mullin et al. 2024

“HL Tau is the youngest system in our survey and still surrounded by a dense inflow of dust and gas falling onto the disk,” said Mullin, a co-author of all three studies. “We were amazed by the level of detail with which we could see this surrounding material with JWST, but unfortunately, it obscures any signals from potential planets.”

One of the difficulties with HL Tau is its youth. The younger a star is, the more gas and dust is in the disk. It eventually gets taken up by planets, and the rest is dissipated by disk wind. But HL Tau is so young that the disk is very thick.

“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,” said Jarron Leisenring, the principal investigator of the observing campaign searching for forming planets and astronomer at the University of Arizona Steward Observatory. “We have already begun looking at other young systems with known planets to help form a more complete picture.”

But astronomy is full of surprises, especially when working with a powerful tool like the JWST. Astronomers often set out to find one thing and find something else they didn’t expect. That’s what happened with HL Tau.

This image of HL Tau from 2016 shows an inner gap and an outer gap where planets may be forming. Unfortunately, the JWST wasn’t able to detect them. But it did find other features. Image Credit: Yen et al. 2016.

In this case, the JWST detected HL Tau’s stellar envelope, where in-falling material gathers around the still coalescing young star. This material eventually becomes part of the star, disk, and planets.

While the astronomers behind all three papers hoped to find planets, that proved difficult. But the JWST’s sensitivity still helped them make progress.

“The lack of planets detected 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 Wagner, a co-author of all three studies. “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.”

Planet formation could be the key to understanding how some planets end up with water and how other chemical elements are distributed in a solar system. Astronomers think that massive gas giants like Jupiter end up regulating the movement and flow of elements. But not all stars host planets so massive.

“Only about 15 percent of stars like the sun have planets like Jupiter. It’s really important to understand how they form and evolve and to refine our theories,” said U-M Michael Meyer, University of Michigan astronomer and coauthor of all three studies. “Some astronomers think that these gas giant planets regulate the delivery of water to rocky planets forming in the inner parts of the disks.”

Image of Jupiter taken by NASA’s Juno spacecraft. Massive gas giants like Jupiter might govern the movement of water in a young solar system, affecting which planets get it. That’s just one of the reasons why astronomers want to find them around young stars. (Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill)

In every disk that astronomers can get a good look at, they find gaps, rings, and sometimes spirals and other structures that can be explained by the formation of giant planets. But they also can’t rule out other explanations. And this is where the issue stands, for now.

“Basically, in every disk we have observed with high enough resolution and sensitivity, we have seen large structures like gaps, rings and, in the case of SAO 206462, spirals,” Cugno said. “Most if not all of these structures can be explained by forming planets interacting with the disk material, but other explanations that do not involve the presence of giant planets exist.”

Finding these massive planets forming around young stars is the next step. Even though the JWST didn’t find them, it still made progress on the issue. That’s how science works. Because if astronomers can eventually see some of these planets, they can then untangle the relationships between all the other features the JWST has observed with the planets themselves.

“If we manage to finally see these planets, we can connect some of the structures with forming companions and relate formation processes to the properties of other systems at much later stages,” Cugno said. “We can finally connect the dots and understand how planets and planetary systems evolve as a whole.”

Upcoming telescopes can make even more progress. The ESO’s Extremely Large Telescope will probe the earliest stages of planetary formation and will also detect water and organic chemicals in protoplanetary disks. Its first light is scheduled for 2028.

The Giant Magellan Telescope will also study the formation of planetary systems with its Near-Infrared Spectrograph. The GMT will see its first light in the 2030s.

Evan Gough

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