Astronomers use the James Webb Space Telescope (JWST) to hunt for planets that form around young stars. The powerful space telescope delivered the goods quickly, albeit in an unexpected way.
These young planets take shape in swirling clumps of gas and dust called protoplanetary disks, gathering more and more mass as they go. Humanity has imaged many of these protoplanetary disks, but astronomers have so far only glimpsed the forming planets within them a few times.
Now a team led by scientists from the University of Michigan, the University of Arizona and the University of Victoria has added the power of JWST’s sensitive infrared instruments to this search. The team used the large telescope to observe the protoplanetary disks HL Tau, SAO 206462 and MWC 758, adding observations to data collected by the Hubble Space Telescope and the Atacama Large Millimeter Array (ALMA) in the hope of detecting planet formation.
The research also revealed previously unseen interactions between protoplanetary disks and the envelopes of gas closer to the stars at the heart of these disks.
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“In principle, in every disk we have observed with sufficiently high resolution and sensitivity, we have seen large structures such as holes, rings and, in the case of SAO 206462, spirals,” said team member and University of Michigan astronomer Gabriele Cugno in a statement. . ‘Most, if not all, of the structures can be explained by the formation of planets interacting with the disk material, but other explanations exist that do not involve the presence of giant planets.
‘If we manage to finally see these planets, we can connect some structures to formative companions and relate formation processes at much later stages to the properties of other systems. We can finally connect the dots and understand how planets and planetary systems evolve as a whole.”
Finding an unexpected planet
Cugno led a JWST study of the protoplanetary disk around the protostar SAO 206462. A protostar is a stellar body that has not yet accumulated enough mass to initiate the fusion of hydrogen to helium in its core, the process that creates a full-fledged main sequence defines. star like the sun.
In the protoplanetary disk around SAO 206462, the team saw the signals of a forming planet, but with a twist: It wasn’t the planet they expected to see.
‘Several simulations suggest that the planet should be within the disk, massive, large, hot and bright. But we didn’t find him. This means that the planet is either much colder than we think, or it is obscured by material. that prevents us from seeing it,” Cugno continued. ‘What we have found is another candidate planet, but we cannot say with 100% certainty whether it is a planet or a faint background star or galaxy contaminating our view.
“Future observations will help us understand exactly what we are looking at.”
This isn’t the first time the disk of SAO 206462 has been imaged. Hubble, Alma and the Very Large Telescope (VLT) have all studied this protoplanetary disk, with these observations revealing that it consists of two strong spirals.
These spirals are likely created by a forming planet. However, before searching for this planet with JWST, the team had expected to see a gas giant planet composed mainly of helium, like Saturn or Jupiter.
“The problem is that whatever we’re trying to detect is hundreds of thousands, if not millions, of times fainter than the star,” Cugno says. “That’s like trying to detect a small light bulb next to a lighthouse.”
JWST’s Near Infrared Camera (NIRCam) allowed Cugno and colleagues to dive deeper into SAO 206462’s disk and detect the planet’s thermal energy, some of which is released when material falls onto it at high speed.
“When material falls on the planet, it jolts at the surface and gives off an emission line at specific wavelengths,” Cugno said. “We use a series of narrowband filters to detect this accretion. This has been done before from the ground at optical wavelengths, but this is the first time this has been done with JWST in the infrared.”
This indicated a planet about 300 times the Earth-Sun distance from the central protostar. Gas giants are usually much closer to their stars, and some then migrate outward after the protoplanetary disk disappears.
The NIRCam results ruled out an object in the disk with a mass greater than 2.2 times the mass of Jupiter, with Cugno and colleagues concluding that, if there is a gas giant carving out the neat spirals of SAO 206462’s protoplanetary disk, these are very cold.
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The youngest star has the right stuff for planet formation
While Cugno and colleagues looked at the disk around SAO 206462, University of Victoria researcher Camryn Mullin used the JWST to study the star HL Tauri (HL Tau). This is a baby located about 450 light-years from Earth and has also been examined by a large number of telescopes.
With an estimated age of no more than 1 million years (compared to our 4.6 billion-year-old middle-aged Sun), HL Tau is the youngest star in the JWST protoplanetary disk survey.
“HL Tau is the youngest system in our study and is still surrounded by a dense influx of dust and gas falling onto the disk,” Mullin said. “We were amazed at the level of detail with which we could see this surrounding material with JWST, but unfortunately it obscures any signals from potential planets.”
HL Tau’s disk is known to contain a number of holes and rings the size of our Solar System that could host planets. But due to the amount of dust the disk contains and the youth of the system, even the JWST is unlikely to see planets around HL Tau directly.
Using the JWST, the team was able to distinguish a feature called a protostellar envelope. This represents the dense influx of dust and gas that is beginning to coalesce around HL Tau. This raw material flows to the star and its disk from the interstellar medium, gas and dust located between the stars, and will ultimately serve as raw material for natal planets.
The hunt for planet formation continues!
Kevin Wagner, a NASA Hubble/Sagan Fellow at the University of Arizona Steward Observatory, examined MWC 758’s protoplanetary disk with the JWST. This is another protoplanetary disk with spiral arms that could indicate the presence of a huge planet.
This possible planet and all others could not manifest in the team’s research, but the JWST’s sensitivity and power allowed them to place constraints on possible planets within this protoplanetary disk. This included ruling out the possibility that there are planets on the edge of the disk, far from the star MWC 758.
“The lack of planets detected in all three systems tells us that the planets causing the gaps and spiral arms are either too close to their host stars or too faint to be seen with JWST,” Wagner says. “If the latter is true, it tells us that they have a relatively low mass, a low temperature, are shrouded in dust, or some combination of the three – as is likely the case in MWC 758.”
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Studies like these into the formation of planets around young stars are critical to understanding how materials are distributed across young systems and how mature assemblies like the Solar System came to be, researchers said.
“Only about 15% of stars like the Sun have planets like Jupiter. It is very important to understand how they form and evolve and to refine our theories,” said Michael Meyer, team member and astronomer from the University of Michigan . “Some astronomers think that these gas giant planets regulate the supply of water to rocky planets that form in the inner parts of the disks.”
So this research may ultimately be critical to understanding how Earth formed and how it is able to support life.
The team’s research is discussed in three papers published last week in The Astronomical Journal.