Astronomers have always been interested in how planets form. They believe that planets begin as spinning disks of gas and dust around young stars. These protoplanetary disks eventually come together to form planets, driven by gravity. However, observing the development of worlds within the disks has been difficult.
Now three universities are using the powerful James Webb Space Telescope (JWST) in their research.
The University of Michigan (U-M), University of Arizona (UArizona), and University of Victoria recently worked together on three studies to gain new insights into protoplanetary disk dynamics. By using the Hubble Space Telescope, Atacama Large Millimeter Array data, and the imaging capabilities of the JWST, scientists have begun to understand the early stages of planet formation and interactions. They used JWST to help observe protoplanetary disks HL Tau, SAO 206462, and MWC 758 in the hope of finding any forming planets.
The papers, published in The Astronomical Journal, brought together previously unseen interactions between disks where planets form and the gas and dust envelope surrounding the young stars.
Scientists found a potential planet around SAO 206462 that could provide insight into the birth of planets. However, it was not the large, bright gas giant they had expected, which suggests the object could be cooler or hidden by gas, making discovery challenging.
“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 U-M astronomer Gabriele Cugno, a co-author on all three 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.”
These investigations reveal the possibility of planet formation and the complex internal structure in the disks. For example, the disk around HL Tau has gaps and rings similar to those in the early Solar System. However, the large clouds of dust prevent direct observation of developing planets.
“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’s disk is known to have several rings and gaps similar in size to those in our Solar System where planets could be found.
“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, an astronomer at the UArizona Steward Observatory. “We have already begun looking at other young systems with known planets to help form a more complete picture.”
The JWST revealed unexpected details of a different feature: the proto-stellar envelope; essentially a dense inflow of dust and gas surrounding a young star just starting to form. Material from the interstellar medium falls inward onto the star and disk under the influence of gravity, providing the raw material for planets and their precursors.
No new planets yet, but astronomers are making progress.
The absence of planets in the observed disks may actually provide as much information as their presence. This suggests that faint planets, proximity to their host stars, or dust coverage could be responsible for shaping disk features such as gaps and spiral arms. This helps improve understanding of disk temperature, planetary mass, and material distribution.
Kevin Wagner, a researcher at UArizona Steward Observatory, conducted an investigation of MWC 758’s protoplanetary disk. The recent research did not discover new planets, but it allowed astronomers to place the strictest limitations yet on the planets they believe might be present. The results eliminate the possibility of additional planets in the outer areas, supporting the idea of a single giant planet causing the spiral arms.
“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.”
Understanding how protoplanetary disks move and how gas giants form is crucial because it influences the delivery of water and minerals to rocky planets forming near the star. Additionally, this study aids scientists in comprehending the formation of planetary systems and determining the necessary conditions for habitable worlds to exist.
“It’s really important to understand how (these planets) form and evolve, and to refine our theories,” said Michael Meyer, U-M 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.”
The papers were published in The Astronomical Journal:
- JWST/NIRCam Imaging of Young Stellar Objects. I. Constraints on Planets Exterior to the Spiral Disk Around MWC 758
- JWST/NIRCam Imaging of Young Stellar Objects. II. Deep Constraints on Giant Planets and a Planet Candidate Outside of the Spiral Disk Around SAO 206462
- JWST/NIRCam Imaging of Young Stellar Objects III. Detailed Imaging of the Nebular Environment Around the HL Tau Disk
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