Neon sign identified by JWST gives clue to planet formation

Planetary systems like our Solar System seem to contain more rocky objects than gas-rich ones. Around our sun, these include the inner planets, the asteroid belt and the Kuiper belt. But scientists have known for a long time that planet-forming discs start with 100 times more mass in gas than in solids, which leads to a pressing question; when and how does most of the gas leave the disc/system?

JWST is helping scientists uncover how planets form, by advancing understanding of their birthplaces, the circumstellar discs surrounding young stars. In a new study published in the Astronomical Journal, a team of scientists including those from the University of Leicester, the University of Cambridge and led by the University of Arizona, image for the first time an old planet-forming disc (still very young relative to the Sun) which is actively dispersing its gas content.

Knowing when the gas disperses is important as it constrains the time that is left for nascent planets to consume the gas from their surroundings.

During the very early stages of planetary system formation, planets coalesce in a spinning disc of gas and tiny dust around the young star. These particles clump together, building up into bigger and bigger chunks called planetesimals. Over time, these planetesimals collide and stick together, eventually forming planets. The type, size, and location of planets that form depend on the amount of material available and how long it remains in the disc. So, the outcome of planet formation depends on the evolution and dispersal of the disc.

At the heart of this discovery is the observation of T Cha, a young star (relative to the Sun) enveloped by an eroding disc notable for its vast dust gap, approximately 30 astronomical units in radius. For the first time, astronomers have imaged the dispersing gas (aka winds) using the four lines of the noble gases neon (Ne) and argon (Ar), one of which is the first detection in a planet-forming disc. The images of [Ne II] show that the wind is coming from an extended region of the disc. The team is also interested in knowing how this process takes place, so they can better understand the history and impact on our solar system.

Scientists have been trying to understand the mechanisms behind the winds in protoplanetary discs for over a decade. The observations by JWST represent a huge step-change in the data they have to work with, compared to previous data from ground-based telescopes.

“We first used neon to study planet-forming discs more than a decade ago, testing our computational simulations against data from Spitzer, and new observations we obtained with the ESO VLT,” said co-author Professor Richard Alexander from the University of Leicester. “We learned a lot, but those observations didn’t allow us to measure how much mass the discs were losing.

“The new JWST data are spectacular, and being able to resolve disc winds in images is something I never thought would be possible.  With more observations like this still to come, JWST will enable us to understand young planetary systems as never before.”

“These winds could be driven either by high-energy stellar photons (the star's light) or by the magnetic field that weaves the planet-forming disc,” said Naman Bajaj from the University of Arizona, the study’s lead author.

To differentiate between the two, the same group, this time led by Dr Andrew Sellek of Leiden Observatory and previously of the Institute of Astronomy at the University of Cambridge, performed simulations of the dispersal driven by stellar photons. They compare these simulations to the actual observations and find dispersal by high-energy stellar photons can explain the observations, and hence cannot be excluded as a possibility.

“The simultaneous measurement of all four lines by JWST proved crucial to pinning down the properties of the wind and helped us to demonstrate that significant amounts of gas are being dispersed,” said Sellek.

To put it into context, the researchers calculate that the mass dispersing every year is equivalent to that of the moon! These results will be published in a companion paper, currently under review at the Astronomical Journal.

The [Ne II] line was discovered towards several planet-forming discs in 2007 with the Spitzer Space Telescope and soon identified as a tracer of winds by team member Professor Ilaria Pascucci at the University of Arizona; this transformed research efforts focused on understanding disc gas dispersal. Now the discovery of spatially resolved [Ne II] - as well as the first detection of [Ar III] - using the James Webb Space Telescope, could become the next step towards transforming our understanding of this process. 

The implications of these findings offer new insights into the complex interactions that lead to the dispersal of the gas and dust critical for planet formation. By understanding the mechanisms behind disc dispersal, scientists can better predict the timelines and environments conducive to the birth of planets. The team's work demonstrates the power of JWST and sets a new path for exploring planet formation dynamics and the evolution of circumstellar discs.

Reference:
Naman S Bajaj et al. ‘JWST MIRI MRS Observations of T Cha: Discovery of a Spatially Resolved Disk Wind.’ The Astronomical Journal (2024). DOI: 10.3849/1538-3881/ad22e1

Adapted from a University of Leicester press release.