Citizen scientists help create 3D map of local stellar neighborhood

Is our solar system located in a typical Milky Way neighborhood? Scientists have gotten closer to answering this question, thanks to the NASA-funded Backyard Worlds: Planet 9 project, a "citizen science" collaboration between professional scientists and members of the public. Scientists tapped into the worldwide network of 150,000 volunteers using Backyard Worlds: Planet 9 to find new examples of brown dwarfs. These objects are balls of gas that are not heavy enough to be stars, since they can't power themselves through nuclear fusion the way stars do. And while "brown" is in the name, they would appear magenta or orange-red if a person could see them close up. By making a complete map of these objects, scientists could find out whether different kinds of brown dwarfs are evenly distributed in our solar system's neighborhood. Telescopes can detect brown dwarfs because they emit heat, in the form of infrared light, left over from their formation. Infrared light is invisible to human eyes, but it can reveal tantalizing details about brown dwarfs and other objects throughout the universe. The result of the new citizen science effort is the most complete map to date of L, T and Y dwarfs in the vicinity of the solar system. These brown dwarf varieties can have temperatures of up to thousands of degrees Fahrenheit, but the Y dwarfs, which are the coolest, may have below-freezing temperatures and clouds made of water.

This image shows Earth surrounded by the nearest brown dwarfs, shown in red, against the backdrop of surrounding constellations.


Of course, an astronomer's idea of a neighborhood is different in space than on Earth. The map encompasses a radius of 65 light-years, or about 400 trillion miles, with "close neighbors" inhabiting space within about 35 light-years, or 200 trillion miles.

Since 2017, citizen scientists have been searching for brown dwarf candidates as part of Backyard Worlds, using data from NASA's Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) satellite along with all-sky observations collected between 2010 and 2011 under its previous moniker, WISE. The Backyard Worlds team also collaborated with Caltech's Summer Research Connection program to involve high school students in finding brown dwarfs. Both worldwide volunteers and high school students in the Pasadena, California, area are listed as co-authors of the study, which was presented at the 237th meeting of the American Astronomical Society.

While brown dwarfs are millions to billions of years old, this team of professional and citizen scientists had a much shorter deadline to find them. They knew that NASA's Spitzer Space Telescope was the only operating observatory that could confirm the distances and positions of the brown dwarfs they were interested in, and Spitzer was set to retire in January 2020. It was a frantic rush to find as many brown dwarfs as they could so Spitzer could reveal their locations more precisely.

Fortunately, citizen scientists helped save the day - they discovered dozens of new brown dwarfs.

"Without the citizen scientists, we couldn't have created such a complete sample in so short a time," said J. Davy Kirkpatrick, scientist at Caltech/IPAC in Pasadena and lead author of the study. "Having the power of thousands of inquiring eyes on the data enabled us to find brown dwarf candidates much faster."

Professional astronomers then used Spitzer to observe 361 local brown dwarfs of types L, T, and Y, and combined them with previous discoveries to make a 3D map of 525 brown dwarfs. Besides the citizen science discoveries, scientists made use of CatWise, a NASA-funded catalog of objects from WISE and NEOWISE, to complete their census.

And there's a surprise: One of our solar system's neighbors - the galaxy's coldest known Y dwarf, with temperatures likely below freezing - represents a rare resident in the cosmic neighborhood. Astronomers would have expected to find a lot more of them in the vicinity. But this may be because current telescopes aren't sensitive enough to find them, since these objects are so faint.

As previous research has found, of the seven objects nearest to our solar system, three are rare types of brown dwarfs. The rest are normal stars: red dwarfs Proxima Centauri and Barnard's Star, and Sun-like stars Alpha Centauri A and B.

"If you were to put the Sun at a random place within our 3D map and you were to ask, 'Typically, what do its neighbors look like?' We find that they would look very different from what our actual neighbors are," said Aaron Meisner, assistant scientist at the National Science Foundation's NOIRLab and co-author of the study.

So, is the Sun in an unusually diverse cosmic neighborhood, or is it just that nearby Y dwarfs are easiest to spot? Astronomers will need to investigate further to find out.

Some of these L, T, and Y dwarfs have masses and temperatures similar to exoplanets - planets beyond our solar system. Getting details about distant planets can be challenging because if they orbit other stars, starlight is a lot brighter than the planet. Since brown dwarfs in this study do not orbit stars, a telescope does not have to subtract starlight to look at them. This makes brown dwarfs a new kind of laboratory for understanding exoplanets.

Scientists will learn even more about brown dwarfs with NASA's forthcoming James Webb Space Telescope, which will examine these mysterious objects in detail in infrared light. NASA's upcoming SPHEREx mission, which will be an all-sky infrared survey, also presents new opportunities to characterize more brown dwarfs.

The Backyard Worlds: Planet 9 project is ongoing and open to anyone worldwide who wants to join the quest to find more mysterious objects in spacecraft data. In addition to a total of about 3,000 brown dwarfs, volunteers have helped find the oldest, coldest white dwarf surrounded by a disk of debris.

"I enjoy this project because the objects that we send to the researchers might get observed with a big telescope," said Melina Thevenot, a citizen scientist in Germany who is listed as a co-author of the new study. "I think we volunteers can really see the fruits of our efforts with this project and the publications by the science team."

Citizen scientists invited to join quest for new worlds

The Backyard Worlds: Planet 9 project re-launches this week, with a call to volunteer citizen scientists to join the search for cold worlds near the Sun. With its newly revamped online interface and equipped with twice as much data as before, the project offers new opportunities to discover planets lurking yet unseen in the outer reaches of the Solar System (e.g., Planet 9, Planet X) as well as cold nearby "failed stars" (a.k.a. brown dwarfs). The re-launch coincides with the publication of the project's latest discovery: a record-setting white dwarf star encircled by mysterious dusty rings that challenge astronomers to rethink the long-term evolution of planetary systems. With Backyard Worlds, available through the Zooniverse web portal, citizen scientists use images from NASA's WISE mission to uncover previously overlooked objects in the Sun's local cosmic neighborhood. The WISE satellite is currently surveying the entire sky at infrared wavelengths. WISE's infrared sensitivity has the unrivaled potential to reveal extremely cold and faint new neighbors of the Sun, some of which may reside closer to us than even the nearest known star, Proxima Centauri. Such neighbors would most likely be brown dwarfs colder than room temperature, of which only a few tantalizing examples are currently known to exist.

It Takes a Village - Crowd-sourced Mining of Vast Astronomical Maps

Nearby celestial objects appear to move against the backdrop of more distant stars and galaxies, and that relative motion is used to identify members of the Sun's local neighborhood.

But no lone astronomer could single-handedly study all of the WISE images by eye to find these cold worlds; and a completely automated search is not possible due to noise and artifacts in the WISE images. Backyard Worlds' solution is an online interface that presents large cohorts of volunteers with animated "flipbooks" of WISE images in which to visually search for moving objects.

"Professional researchers routinely use supercomputers and machine learning, but there's still no substitute for the human eye when it comes to recognizing subtle motions in astronomical images," said Backyard Worlds co-founder Aaron Meisner, an astronomer at the NSF's National Optical Astronomy Observatory in Tucson, Arizona.

The crowd-sourced approach works. Since the project's inception two years ago, Backyard Worlds volunteers have discovered more than a thousand cold, nearby brown dwarfs - more than one per day!

Finding the Unexpected: an Ancient White Dwarf with Enigmatic Dust Rings

Alongside its re-launch, Backyard Worlds also announces its latest discovery, a surprising object unlike anything the team expected to find. Whereas most Backyard Worlds discoveries have been brown dwarfs, this newly published member of the solar neighborhood is a white dwarf - the faint, dense remnant of a stellar death long ago.

The white dwarf in question - LSPM J0207+3331, or J0207 for short - is unusually bright in WISE images, indicating that it's encircled by a warm, dusty circumstellar disk. Such disks are thought to result from the ongoing breakup of small rocky planetesimals in orbit around the white dwarf. But with an age of roughly 3 billion years, J0207 is colder and nearly three times older than any other white dwarf known to harbor such a disk.

The peculiar infrared properties of J0207 were first reported to Backyard Worlds researchers by citizen scientist Melina Thevenot of Germany.

"This white dwarf is so old that whatever process is feeding material into its ring must operate on billion-year timescales," said John Debes, an astronomer at the Space Telescope Science Institute in Baltimore and lead author of the J0207 study co-authored by Thevenot. "Most of the models scientists have created to explain rings around white dwarfs only work well up to around 100 million years, so this star is really challenging our assumptions of how planetary systems evolve."

Adding to this puzzle, the J0207 disk may be composed of more than one distinct ring-like component, an arrangement never before seen in circumstellar material surrounding a white dwarf.

Critical follow-up of J0207 was obtained by Backyard Worlds collaborator Adam Burgasser, a professor at UC San Diego, using the Keck II telescope on the Mauna Kea summit in Hawaii, one of the world's premier astronomical observing facilities. "That is a really motivating aspect of the search," said Thevenot, one of more than 150,000 citizen scientists who have contributed to the Backyard Worlds project. "The researchers will move their telescopes to look at worlds you have discovered. What I especially enjoy, though, is the interaction with the awesome research team. Everyone is very kind, and they are always trying to make the best out of our discoveries."

More Discoveries in Store!

The newly upgraded Backyard Worlds interface is expected to unleash a new wave of additional discoveries. The rebooted edition doubles the amount of WISE data that can be examined, provides access to additional visualization resources, and incorporates upgraded image processing algorithms that better remove artifacts.

"The synergy of careful human scrutiny combined with massive computations has fueled the remarkable discoveries made by Backyard Worlds volunteers," said Meisner. "I'm excited to see what happens next!"

Backyard Worlds: Planet 9 is a collaboration between NASA, the American Museum of Natural History in New York, Arizona State University, National Optical Astronomy Observatory, the Space Telescope Science Institute in Baltimore, the University of California San Diego, Bucknell University, the University of Oklahoma, and Zooniverse, a collaboration of scientists, software developers and educators who collectively develop and manage citizen science projects on the internet.