The transit of a large cloud of protoplanetary rubble across the face of a young star gave the Spitzer Space Telescope our best view yet of planetary evolution in action.

Protoplanetary debris cloud (artist's concept)
This illustration shows what a debris cloud around the young star called HD 166191 might look like up close.
NASA / JPL-Caltech

Even though planetary evolution happened in our solar system’s distant past, we can still see protoplanets forming around young stars today. But understanding what's happening is difficult because the action happens too far away for us to see clearly. Every now and then, though, planet-forming debris passes in front of the star it orbits, making a silhouette that reveals its size and location.

Now, astronomers have found a large cloud of collisional debris that orbited a young star called HD 166191, casting its silhouette and testing our ideas of planet formation.

Luck on Their Side

Starting in 2014, a group led by Kate Su (University of Arizona) spent years repeatedly observing infrared emission around young, 10 million-year-old stars, looking for the warm signature of planet formation. The team used the 3- to 5-micron band still visible to NASA’s Spitzer Space Telescope during its “warm” mission, after its cryogenic coolant ran out. The researchers expected to find dust, debris and planetesimals in orbit, crashing into each other and gradually accreting to build planets and depleting interplanetary dust.

In 2019, Su’s team reported in the Astronomical Journal five years’ worth of observations of two young Sun-like stars, named ID8 and P1121, both of which showed long-term changes in dust levels over timescales of weeks to years. They concluded that large impact events had occurred in both systems before observations had begun, but they didn’t see any debris transit either star.

But serendipity struck in their latest study of a star designated HD 166191, 329 light-years away in Sagittarius.

“We are lucky — some would say extremely lucky — to detect the impact-produced debris cloud that went in front of the star,” Su says.

As with exoplanets detected as they transit across the face of their stars, viewing a transit of debris can likewise give away its size and location relative to the star.

“It requires a perfect geometry — an almost edge-on system in order to see the transit,” Su explains. “This is why exoplanet transit missions like Kepler and TESS need to monitor millions of stars.” No wonder she considers herself lucky.

Protoplanetary Rubble

After five years of observations, Su and colleagues reported in the March 10th Astrophysical Journal that they had used Spitzer as well as ground-based telescopes to observe the system over multiple wavelengths as its infrared emission brightened starting in early 2018; it reached a peak shortly before Spitzer shut down in January 2020.

In the midst of that, the team watched a huge clump of dust pass twice in front of the star. The clump expanded between the two transits and didn't transit a third time.

The group concluded that two bodies the size of the asteroid Vesta had collided, producing a giant debris clump — perhaps covering an area hundreds of times that of the star — that orbited every 142 days (the separation between the two transits). That corresponds to a distance from the star of 0.62 astronomical unit, or slightly smaller than Venus’s orbit around the Sun.

Because longer wavelengths pass more easily through dust but are still blocked if there’s enough of it, the multi-wavelength observations also revealed the dust cloud’s structure: thick in the center and more diffuse toward the edge.

Infrared lightcurve
By tracing the changes in infrared brightness over time, the astronomers noted two dips in infrared light even as the overall brightness was increasing. These plots show a close-up of one of the dips. At longer wavelengths, the dip is both shorter and shallower, as expected for a dust cloud.
K. Su et al. / Astrophysical Journal 2022

“[The researchers] have strong evidence for ongoing collisions of the kind thought to occur during the giant impact epoch, when planets like Earth accumulate the last bit of material from a protostellar disk,” says Scott Kenyon (Harvard), who was not involved in the study.

Kenyon adds that by identifying the location and amount of debris in this system, the team now knows better what to look for in other star systems, such as with the James Webb Space Telescope.                                      

With Spitzer no longer available, Su's group is now watching activity around HD 166191 and other young star systems with other NASA infrared instruments, including the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Infrared Telescope Facility. She says continuing these observations can “provide a sanity check” on the many computer simulations of planet formation.

Kenyon notes, for example, that current simulations do not generally predict clumps as large as the one Su’s group found orbiting HD166191. Those models are — as science generally is — still a work in progress.


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