James Webb Space Telescope takes its first images of forming planetary systems

Image of HL Tau taken by the James Webb Space Telescope, overlaid with data from ALMA.
Image of HL Tau taken by the James Webb Space Telescope, overlaid with data from ALMA. Credit: Mullin; ALMA (ESO/NAOJ/NRAO)

March 27, 2024

How planets form is one of astronomy's most active fields of research. The process of moving from a vortex of gas and dust swirling around a young star to system of planets  remains shrouded in mystery – quite literally, as telescopes have historically struggled to peer through the dense clouds of dust surrounding planetary systems when they're in their infancy.

Taking advantage of the dust-penetrating capabilities of the James Webb Telescope’s (JWST) infrared instruments, an international team consisting of University of Victoria  (UVic) astronomy professor Ruobing Dong, NRC-Hertzberg researcher and adjunct professor Doug Johnstone and astrophysics graduate student Camryn Mullin, alongside researchers from the University of Arizona and the University of Michigan, has obtained the first direct observations of gas and dust feeding a nascent planetary system with raw material for planet formation.

The researchers focused on observing three different nascent planetary systems: HL Tau, SAO 206462 and MWC 758, with the hope of detecting any planets that might be forming.

By combining JWST's images with prior observations by the Hubble Space Telescope and the Atacama Large Millimeter Array (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.

"Protoplanetary disks are important targets for astronomical observations. They allow us a glimpse of how our own solar system formed 4.6 billion years ago. Catching planets in the act of forming can help us better understand the formation process and how chemical elements get distributed throughout a planetary system.”

- Camryn Mullin, UVic graduate student and lead author on the UVic-led paper

The team is presenting its findings in three separate papers accepted in The Astronomical Journal. Two other papers are in preparation.

The birth of a planetary system

Nascent planetary systems, also known as protoplanetary disks, form when a vast cloud of interstellar gas and dust condenses around a young star under the effect of gravity. While the dust and gas will initially be in a relatively spherical distribution, over time it will collapse into a flatter disk shape, like a pancake or a DVD, with the star at the centre.

On astronomical time scales, protoplanetary disks are very short-lived, lasting up to 10 million years at most. By that point, most of the material in the disk has dissipated. Astronomers don’t yet fully understand how that happens, but it is likely that much of the material gets accreted onto the star, some is blown away by stellar radiation and the rest coalesces to form planets, asteroids and comets.   

Although protoplanetary disks have previously been observed in various levels of detail, it is still extremely difficult to make out any planets that may be forming within. Researchers have relied on features such as gaps and rings to infer the presence of planets as they plow through the disk. 

Observing HL Tau reveals details of proto-stellar envelope

The UVic team of Mullin, Dong and Johnstone focused on observing the 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. ALMA images had previously revealed 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. 

While they weren’t able to detect a planet using the JWST (no definitive planets were detected in any of the three systems observed), JWST images did reveal unexpected details of a structure surrounding the disk: the proto-stellar envelope – essentially a dense inflow of dust and gas surrounding the young star that is just beginning to coalesce.

“The lack of a planet detection for HL Tau isn’t unexpected. The HL Tau disk is the youngest in our program, and therefore is surrounded by the most material remaining from star formation. The envelope makes it even more difficult to detect planets. If searching for the light of a planet around its star is like trying to spot a firefly next to a lighthouse, then the envelope is like added fog. The planets are either too close to the star or too faint to be imaged with JWST.”

- Mullin

Despite the lack of planet detection, the team is still thrilled with the detailed envelope images. The envelope contains several active components with material flowing in and out. The researchers could clearly detect streamers feeding material from the envelope to the inner disk, increasing the mass available for planet formation and providing any potential planets with different material than may be available in the disk alone.

Combining instruments results in groundbreaking sensitivity

The sensitivity achieved by using JWST is groundbreaking, and has allowed researchers to place the most stringent constraints yet on the suspected planets.

Each of the telescopes involved in the study – Hubble, ALMA and JWST – makes observations in a different wavelength band. By overlaying the images, researchers are able to get a more complete understanding of the system.

“Combining different instruments helps us to really see the full picture,” says Mullin. “Because JWST looks at the infrared, aka heat, it’s ideal for trying to spot young, hot planets. But we needed ALMA (which uses radio waves) to first see through the envelope to the disk to give us an idea of where the planets are most likely to be.”

With the ongoing refinement of observational techniques and more advanced instruments coming online, future observations of forming planetary systems are promising.

“I’m excited for what we’ll be able to see in the future and how future observations will help us refine theories about the processes involved in creating these planetary systems,” says Mullin. “This study is just the beginning.”

This research was supported by the National Sciences and Engineering Research Council of Canada, the Alfred P. Sloan Foundation, the Government of Canada’s New Frontiers Research Fund and NASA’s JWST/NIRCam project.

Read the papers: