July 17, 2024
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Solar System

Webb Telescope Sees Evidence of Asteroid Collision Around Nearby Star – Sky & Telescope

Beta Pictoris Debris Disc
This artist's concept shows the young star Beta Pictoris from the outer edge of a disk of planet-forming debris. (In reality, there are probably several debris disks around the star, as well as two companions that were not known when this concept was created.)
NASA / FUSE / Lynette Cook

The James Webb Space Telescope (JWST) has found evidence that asteroids collided about 20 years ago around Beta Pictoris, a star 60 light years from Earth. But that evidence is not based on what JWST saw, but on what he did not see.

“About 20 years ago I was studying Beta Pictoris with the Spitzer space telescope,” says Christine Chen (Space Telescope Science Institute), who presented the results at the 244th meeting of the American Astronomical Society on June 10. In the Spitzer observations, Chen's team saw indicators of dust, including grains containing crystalline silicates. These crystals are common on Earth and other bodies in the solar system; They are also commonly seen around young, planet-forming stars.

But when Chen led a team to observe the system again in 2023, the data that came back brought some surprises. The chemical signatures of those crystalline silicates, which should have been seen in near- and mid-infrared spectroscopy, had disappeared.

Flavien Kiefer (Sorbonne University, France), who studied Beta Pictoris but was not involved in the current study, adds that Similar features were also observed in 2010. in longer wavelength data collected by the Herschel Space Observatory.

“The disappearance of spectral features in the JWST observation is surprising!” Kiefer says. “This must have happened between 2010 and now!”

“This was really an unexpected observation that changed our understanding of the planetary system,” Chen says, “and leads us to believe that there has recently been a giant collision in the system.” That collision, which could have occurred deep in the “terrestrial zone” just before Spitzer observed the system in 2004, pulverized what would have been a “really big asteroid in the solar system.” Now, decades later, the star's radiation has swept away the evidence.

The tumultuous years

The star Beta Pictoris and the orbits of its giant companions
This diagram shows the orbits of the giant companion planets, Beta Pictoris b and c, whose orbits are shown in white. The square and circle symbols show individual observations of the positions of the planets. For reference, Jupiter and Saturn orbit our Sun at 5 astronomical units (au) and 10 au, respectively.
Axel Quetz / MPIA Graphics Department

Beta Pictoris is a “teenage” system, which has lost the gaseous disk of its childhood but is still in the midst of the tumult that accompanies the formation of terrestrial planets. Multiple debris discs, packed with asteroids, comets and dust, still surround the 20-million-year-old star. And two giant companions, each more than 10 times the mass of Jupiter, help shape the system's outer belts and rings.

Observations made since the late 1980s have appeared evaporated comet pieces that was too close to Beta Pic. More recently, the transiting exoplanet survey satellite even detected three of these comets as they passed across the face of the star, leaving a trail of cometary material behind them.

Beta Pictoris: JWST observations off Spitzer
JWST observations did not see the same dust signatures that Spitzer detected in 2004 and 2005. That huge amount of dust, likely formed during a recent collision, has already disappeared from the system.
Plot: Roberto Molar Candanosa / Johns Hopkins University, with modifications; Beta Pictoris concept art: Lynette Cook/NASA

But what JWST showed was not evidence of such bodies or the dust that carried them, but the absence of them. The two brightest emission regions Chen's team saw in the Spitzer data, centered at 18 and 23 microns, likely came from very small dust grains released when asteroids collided near the star, where planets could be forming. terrestrial. Since then, radiation from the young star, exerting a force of its own, has carried away the tiny grains.

The spectra also show that at somewhat shorter wavelengths, between 5 and 15 microns, the overall light level has decreased between the 2005 Spitzer observation and the 2023 JWST data. That change comes from hot dust from the collision which has also disappeared since then, Chen explains, also due to the star's radiation pressure.

Kiefer agrees that Chen's assessment is reasonable, although he believes it is also possible that the colliding bodies were comets. “The fact that the features disappear, and therefore that the dust is not replenished over time, strongly suggests that, in fact, some rare phenomenon that led to the release of a large amount of dust from cometary bodies must have happened in the past,” he says. “Collisions are one way to do it.”

If the collision occurred between comets, then they could have been thrown into the inner system from a region like the Kuiper Belt in the solar system, Kiefer adds. Therefore, Chen's discovery could indicate the existence of Kuiper-like belts around Beta Pictoris.

Planet formation in action

Astronomers used to think that collisions between planetesimals occur continuously, constantly knocking down small bodies and replenishing dust from debris disks around young stars. The JWST data now adds to the evidence that collisions can occur as individual events, and we can observe how they happen.

Previous observations have shown that comets are likely circling and dipping into Beta Pictoris on daily, even hourly, timescales. But we don't look at the system that often. “I think this really highlights how little we have studied these systems in the time domain,” Chen says.

The window that JWST opens on Beta Pictoris (and similar systems) is closed from scratch. Water vapor in our atmosphere absorbs the near- and mid-infrared wavelengths that Chen and others want to observe. And since the JWST mission receives nine times more requests for observing time than it can accept, it will be difficult to make follow-up observations from space. But Chen remains hopeful. “It's definitely worth following up.”

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