New data suggest the star Beta Pictoris has cleared away the remains of an asteroid pulverized as recently as two decades ago.

Beta Pictoris debris disk
This artist’s concept shows the young star Beta Pictoris from the outer edge of a planet-forming debris disk. (There are actually probably multiple debris disks around the star, as well as two companions that weren't known when this concept was created.)
NASA / FUSE / Lynette Cook

The James Webb Space Telescope (JWST) has found evidence that asteroids collided some 20 years ago around Beta Pictoris, a star 60 light-years from Earth. But that evidence rests not on what JWST saw, but on what it didn’t see.

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

But when Chen led a team in observing the system again in 2023, the data that came down held some surprises. The chemical fingerprints of those crystalline silicates, which should have been seen in the near- and mid-infrared spectroscopy, were gone.

Flavien Kiefer (LESIA-Observatory of Paris), who has studied Beta Pictoris but was not involved in the current study, adds that similar features were also seen in 2010 in longer-wavelength data collected by the Herschel Space Observatory.

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

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

The Tumultuous Years

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

Beta Pictoris is a “teenaged” system, having lost the gaseous disk of its infancy but still in the midst of the tumult that accompanies terrestrial planet formation. Multiple debris disks, stuffed full of asteroids, comets, and dust, still circle the 20 million-year-old star. And two giant companions, each with more than 10 times Jupiter’s mass, help shape the system’s outer belts and rings.

Observations since the late 1980s have turned up evaporated bits of comets that came too close to Beta Pic. More recently, the Transiting Exoplanet Survey Satellite even spotted three of these comets as they passed across the face of the star, trailing cometary material behind them.

Beta Pictoris: JWST vs. Spitzer observations
The JWST observations didn't see the same dust signatures that Spitzer detected in 2004 and 2005. That massive amount of dust, probably made 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 trailing them, but the absence of it. The two brighter regions of emission that Chen’s team saw in Spitzer data, centered on 18 and 23 microns, probably came from very small dust grains, released when the asteroids collided near the star, where terrestrial planets might be forming. The young star’s radiation, which exerts a force of its own, has since blown the tiny grains away.

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

Kiefer agrees that Chen’s assessment is reasonable, though he thinks it's also possible the bodies that collided might have been comets. “The fact that the features disappear, and thus that the dust is not replenished through time, strongly suggests that indeed some rare phenomenon leading to the release of large amount of dust from cometary bodies must have happened in the past,” he says. “Collisions are one way to do that.”

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

Planet Formation in Action

Astronomers used to think that collisions between planetesimals happen continually, constantly grinding down small bodies and replenishing the dust of debris disks around young stars. The JWST data now add to building evidence that collisions can occur as individual events — and we can watch them happen.

Previous observations have shown that comets are probably circling and plunging into Beta Pictoris on daily, even hourly timescales. But we don’t observe the system that frequently. “I think this just really highlights how little we have studied these systems in the time domain,” Chen says.

The window that JWST opens on Beta Pictoris (and systems like it) is closed from the ground. Water vapor in our atmosphere absorbs the near- and mid-infrared wavelengths that Chen and others want to look at. And with the JWST mission receiving nine times more requests for observing time than it can accept, follow-up observations from space will be tough to get. But Chen remains hopeful.  “It’s definitely worth following up on.”

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