This artist’s concept illustrates a close-up view of a cool brown dwarf. Objects like this, drifting just beyond our solar system, have been imaged by NASA’s Wide-field Infrared Survey Explorer and could be discovered by Backyard Worlds: Planet 9.
Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body. NASA’s Wide-field Infrared Survey Explorer uncovered these elusive objects for the first time, using its heat-sensing, infrared vision. The telescope found six Y dwarfs, ranging in atmospheric temperatures from 350 degrees Fahrenheit (175 degrees Celsius) to less than about 80 degrees Fahrenheit (25 degrees Celsius).
Y dwarfs belong to a larger family of objects called brown dwarfs. Brown dwarfs begin their lives like stars but they never accumulate enough mass to fuse atoms steadily at their cores and shine with starlight — as our sun does so well. Instead, they fade and cool with time, giving off most of their light in infrared wavelengths.
WISE was able to pick up this faint glow for six Y dwarfs, which are the coldest class of brown dwarfs and the latest letter in the stellar classification scheme. This scheme describes stars of all temperatures, beginning with the hottest “O” stars and now ending with the coldest Y dwarfs. The entire scheme includes the classes: O, B, A, F, G, K, M, L, T, Y. Our yellow sun belongs to the G class of stars. M stars are colder than our sun, and reddish in color.
While the O through K classes are all considered stars, M and L objects are a mixture of stars and brown dwarfs, and T and Y objects are all brown dwarfs. The term “brown dwarfs” was chosen because at that time, astronomers didn’t know what colors these objects would actually have at the visible wavelengths our eyes see, and brown is not a true color of light (there are no “brown photons”). Astronomers now know that T dwarfs would appear reddish, or magenta, to the eye. But they are not certain what color Y dwarfs are, since these objects have not been detected at visible wavelengths. The purple color shown here was chosen mainly for artistic reasons. In addition, the Y dwarf is illustrated as reflecting a faint amount of visible starlight from interstellar space.
NASA-funded Website Lets the Public Search for New Nearby Worlds
NASA is inviting the public to help search for possible undiscovered worlds in the outer reaches of our solar system and in neighboring interstellar space. A new website, called Backyard Worlds: Planet 9, lets everyone participate in the search by viewing brief movies made from images captured by NASA’s Wide-field Infrared Survey Explorer (WISE) mission. The movies highlight objects that have gradually moved across the sky.
“There are just over four light-years between Neptune and Proxima Centauri, the nearest star, and much of this vast territory is unexplored,” said lead researcher Marc Kuchner, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Because there’s so little sunlight, even large objects in that region barely shine in visible light. But by looking in the infrared, WISE may have imaged objects we otherwise would have missed.”
WISE scanned the entire sky between 2010 and 2011, producing the most comprehensive survey at mid-infrared wavelengths currently available. With the completion of its primary mission, WISE was shut down in 2011. It was then reactivated in 2013 and given a new mission assisting NASA’s efforts to identify potentially hazardous near-Earth objects (NEOs), which are asteroids and comets on orbits that bring them into the vicinity of Earth’s orbit. The mission was renamed the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE).
The new website uses the data to search for unknown objects in and beyond our own solar system. In 2016, astronomers at Caltech, in Pasadena, California, showed that several distant solar system objects possessed orbital features indicating they were affected by the gravity of an as-yet-undetected planet, which the researchers nicknamed “Planet Nine.” If Planet Nine — also known as Planet X — exists and is as bright as some predictions, it could show up in WISE data.
The search also may discover more-distant objects like brown dwarfs, sometimes called failed stars, in nearby interstellar space.
“Brown dwarfs form like stars but evolve like planets, and the coldest ones are much like Jupiter,” said team member Jackie Faherty, an astronomer at the American Museum of Natural History in New York. “By using Backyard Worlds: Planet 9, the public can help us discover more of these strange rogue worlds.”
Unlike more distant objects, those in or closer to the solar system appear to move across the sky at different rates. The best way to discover them is through a systematic search of moving objects in WISE images. While parts of this search can be done by computers, machines are often overwhelmed by image artifacts, especially in crowded parts of the sky. These include brightness spikes associated with star images and blurry blobs caused by light scattered inside WISE’s instruments.
Backyard Worlds: Planet 9 relies on human eyes because we easily recognize the important moving objects while ignoring the artifacts. It’s a 21st-century version of the technique astronomer Clyde Tombaugh used to find Pluto in 1930, a discovery made 87 years ago this week.
On the website, people around the world can work their way through millions of “flipbooks,” which are brief animations showing how small patches of the sky changed over several years. Moving objects flagged by participants will be prioritized by the science team for follow-up observations by professional astronomers. Participants will share credit for their discoveries in any scientific publications that result from the project.
“Backyard Worlds: Planet 9 has the potential to unlock once-in-a-century discoveries, and it’s exciting to think they could be spotted first by a citizen scientist,” said team member Aaron Meisner, a postdoctoral researcher at the University of California, Berkeley, who specializes in analyzing WISE images.
Backyard Worlds: Planet 9 is a collaboration among NASA, UC Berkeley, the American Museum of Natural History in New York, Arizona State University in Tempe, the Space Telescope Science Institute in Baltimore, and Zooniverse, a collaboration of scientists, software developers and educators who collectively develop and manage citizen science projects on the internet.
source: NASA – Jet Propulsion Laboratory – California Institute of Technology