Planetary Debris Disks Discovered with Citizen Scientists and Virtual RealityAugust 4, 2022
Astronomers have many tools for studying the cosmos: telescopes, satellites, interplanetary spacecraft, and more. The humble human eye is a critical part of this toolkit, too, as it can often spot patterns or aberrations that algorithms miss. And our vision’s scrutinizing power has been bolstered recently by virtual reality (VR) as well as by thousands of eyes working in tandem thanks to the crowdsourcing power of the Internet.
Researchers at NASA’s Goddard Space Flight Center recently announced the discovery of 10 stars surrounded by dusty debris disks—whirling masses of gas, dust and rock left over after the earliest phases of planet formation. This result, enabled by VR and the help of citizen scientists, was recently published in the Astrophysical Journal. The findings could help astronomers piece together a time line of how planetary systems are built.
Debris disks encompass various stages of planet formation, including the youthful eras in which worlds are still embedded in the detritus from the messy, chaotic processes of their birth. Although astronomers have managed to see a few directly, most of these young planets are beyond the reach of current telescopes. Making a planetary system takes millions of years, so each debris disk observers see is just a brief snapshot of one moment in that system’s life. To uncover the whole story, astronomers search for many disk-wreathed planetary systems at different stages of evolution, gathering multiple snapshots to piece together in a time line.
To hunt for debris disks, observers usually start by looking for stars that appear especially bright in the infrared; that abnormal brightness typically comes from a surfeit of starlight-warmed dust in a disk around a star. NASA’s infrared telescope WISE (Wide-Field Infrared Survey Explorer) surveyed the entire sky, creating what in some respects is the most comprehensive catalog yet of stellar infrared measurements. With tens of thousands of data points to be analyzed and many debris disks likely hidden within the WISE catalog, what’s a scientist to do?
“It’s a great example of how so much of modern astronomy involves searching massive data sets for the proverbial needle in the haystack,” says Meredith Hughes, an astronomer at Wesleyan University, who was not involved in the study. “Even with machine-learning algorithms, it’s still hard to train computers to do this complex work of identifying noisy patterns and noticing subtle deviations from expectations, which is where the collective brainpower of citizen science comes in.”
A project called Disk Detective trained citizen scientists—regular people who want to help out on research in their spare time—to look at WISE images and compare them to those from other astronomical surveys, such as the SkyMapper Southern Sky Survey, the Pan-STARRS survey and the Two Micron All Sky Survey (2MASS), with the goal of confirming the presence of disks around each candidate star. Since the project’s start in 2014, citizen scientists have found more than 40,000 disks—that’s 40,000 snapshots of the history of how planets form.
To put these into a time line, though, astronomers need to figure out where each snapshot belongs. In other words, scientists need to know the ages of each star and its debris disk. “When we know the ages of stars and planets, we can place them in a sequence—from baby to teen to adult, if you like,” says Marc Kuchner, a NASA astrophysicist and co-author of the new study. “That allows us to understand how they form and evolve.”
Pinning down a star’s age with any substantial precision is a notoriously tricky problem in astronomy. One solution is to match up a star to its siblings, in an association known as a moving group. Stars often form in clusters from one giant cloud of gas, but many of these once-close stellar families drift apart as they age, their individual members spreading out across the Milky Way. By carefully measuring stars’ locations and velocities, researchers can determine which stars display the telltale motions that, traced backward, reveal they were collectively born at the same time and place. Once astronomers know stars in a group are related, it’s straightforward to calculate their age based on established knowledge of how stars grow and evolve.
Finding new moving group members isn’t easy. To do so, astronomers traditionally rely on analyzing preexisting lists of moving-group stars, flagging potential new members via sophisticated mathematical models. The team behind the new project wanted to try something different and more visceral: it used a VR program to zoom around the stars and get a clearer, three-dimensional perspective on how things move.
“I thought I would scare [NASA’s VR scientists] away when I said I wanted to visualize the positions and velocities of four million stars,” Kuchner says. “But they didn’t bat an eyelash!” To create this virtual stellar cornucopia, the team used data from Gaia, a European Space Agency satellite that provides the best available measurements for the positions and velocities of stars in our galaxy. The resulting VR simulation served as a sort of time machine, too—knowing how fast and in what direction a star was moving allowed Kuchner and colleagues to trace its movement backward and forward in time.
While serving as a visiting researcher at NASA, lead study author Susan Higashio strapped on a VR headset to fly around the simulation’s millions of stars. She examined where the stars with disks were in relation to known moving groups and extrapolated the stars’ motions forward and backward in time to test their potential associations. “It was so exciting when the four million stars appeared in VR, but it felt a little dizzying when they all started to swirl around me,” she recalls. “It was a really fun and interactive way to conduct science.”
Higashio traced 10 of the debris disks from Disk Detective back to their moving-group families. The team then found the estimated ages of these disks, which ranged from 18 million to 133 million years old. All of them were extremely young, compared with our home solar system, which is around 4.5 billion years old. The researchers also identified an entirely new moving group called Smethells 165, after its brightest star. “Whenever we find a new moving group, that’s a new batch of stars whose ages we know more precisely,” Kuchner explains.
The astronomers also found one strange, extreme debris disk around a star nicknamed J0925 that doesn’t quite fit into their expected time line of planet formation. It’s much brighter in the infrared—meaning it has more dust—than expected for a star of its age. As debris disks get older, some of their dust spirals into the star or is blown away by stellar winds. J0925, however, seems to have just gotten a fresh new delivery of hot dust, possibly from a recent collision between two protoplanets. Hughes highlights this star as the most interesting object uncovered in the study. “Extreme debris disks are still a bit mysterious, but they are probably similar to what our solar system would have looked like during the giant impact that formed the Earth’s moon.”
Disk Detective’s citizen-science work is still ongoing, now upgraded to use Gaia’s most recent batch of data. The team hopes to identify even more members of moving groups and new disks with their unique VR method. Lisa Stiller, one of the many citizen scientist co-authors of the study, offers encouragement for prospective volunteers. “Don’t hesitate to help out in a citizen-science project,” she says. “Your help will be needed in whatever form you choose or amount of time you choose to dedicate yourself.”
Anyone with an Internet connection can still join the Disk Detective project, no experience needed. “More than 30,000 citizen scientists have contributed,” Kuchner says. “The Disk Detectives are still working their way through hundreds of thousands of WISE images—we still need your help.”