Nearly 100 'cool' worlds called brown dwarfs found in our cosmic backyard, will offer insights on exoplanets

The brown dwarfs are among the coolest known, with a few approaching the temperature of Earth, cool enough to harbor water clouds


                            Nearly 100 'cool' worlds called brown dwarfs found in our cosmic backyard, will offer insights on exoplanets
(NOIRLab/NSF/AURA/P Marenfeld/Acknowledgement: William Pendrill)

Discovering and characterizing astronomical objects near the Sun is fundamental to the understanding of our place in the universe as well as its history. Yet astronomers are still unearthing new residents in our solar neighborhood. A breakthrough has now been achieved by astronomers and citizen scientists who have discovered 95 cool worlds near our Sun's vicinity — objects more massive than planets but lighter than stars, known as brown dwarfs — outside the solar system.

Several of these newly-discovered worlds are among the very coldest brown dwarfs known, with a few approaching the temperature of Earth — cool enough to harbor water clouds. Many of these brown dwarfs are within a few dozen light-years of the Sun. They are well outside the solar system, so do not experience heat from the Sun, but still inhabit a region astronomers consider our "cosmic neighborhood".

According to scientists, the cool worlds offer the opportunity for new insights into the formation and atmospheres of planets beyond the solar system. "These cool worlds are crucial laboratories for studying exoplanet-like atmospheres, free from the glare of a much brighter host star. These discoveries will also allow us to make the best estimates of the number of free-floating planets roaming interstellar space nearby the Sun," lead author of the study Aaron Meisner from the National Science Foundation’s NOIRLab tells MEA WorldWide (MEAWW).

Meisner explains that these newly discovered worlds will be "excellent targets for detailed atmospheric characterization" using NASA's soon-to-launch James Webb Space Telescope. "These discoveries also represent a critical step toward making the best ever three-dimensional map of the Sun’s cosmic neighborhood," he says. 

Why this discovery is important

Brown dwarfs are not massive enough to power themselves like stars but are still many times heavier than planets. Lacking the mass needed to sustain nuclear reactions in their core, brown dwarfs resemble cooling embers. Their low mass, low temperature and lack of internal nuclear reactions make them extremely faint — and therefore, extremely difficult to detect. Because of this, when searching for the very coolest brown dwarfs, astronomers can only hope to detect such objects relatively close to the Sun.

The discoveries will allow scientists to make the best estimates of the number of free-floating planets roaming interstellar space nearby the Sun, according to lead author Aaron Meisner (Getty Images)

Despite their name, brown dwarfs would appear magenta or orange-red to the human eye if seen close up. While brown dwarfs can be extremely hot, even thousands of degrees Fahrenheit, many of the newly-discovered ones are colder than the boiling point of water. Some even approach the temperature of the Earth and are cool enough to harbor water clouds.

"Brown dwarfs with low temperatures are also small in diameter and therefore, faint in visible light. Still, they give off heat in the form of infrared light, which is invisible to the human eye yet detectable by telescopes such as the Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) and Spitzer. For cold brown dwarfs like those in this study, the infrared signal is also faint, so they are easier to find the closer they are to our solar system," explains NASA.

The astronomers say that with their relatively cold temperatures, the current discoveries bridge a previously empty gap in the range of low-temperature brown dwarfs, identifying a long-sought missing link within the brown dwarf population. Since the same physical processes may form both planets and brown dwarfs, the study offer prospects for research into worlds beyond the solar system. "It’s awesome to know that our discoveries are now counted among the Sun's neighbors and will be targets of further research," writes co-author Jim Walla, a citizen scientist and Astro Data Lab user.

In 2014, scientists discovered the coldest-known brown dwarf, called WISE 0855, using data from NASA’s WISE mission in infrared light. WISE 0855 is about minus 10 degrees Fahrenheit or minus 23 degrees Celsius. No other brown dwarf came close to this object’s low temperature. This new batch of brown dwarfs, together with others recently discovered using NEOWISE and Spitzer, puts 0855 in context. “Our new discoveries help connect the dots between 0855 and the other known brown dwarfs," says astrophysicist Marc Kuchner, the principal investigator of Backyard Worlds and citizen science officer for NASA's Science Mission Directorate. 

It was a gigantic effort

Members of the public helped make these discoveries through 'Backyard Worlds: Planet 9', a NASA-funded citizen science project. It is a collaboration between volunteers and professional scientists. Backyard Worlds incorporates data from NEOWISE satellite along with all-sky observations collected between 2010 and 2011. Data from NASA’s retired Spitzer Space Telescope and the facilities of NOIRLab were also instrumental in the analysis.

The research team used archival data from the Nicholas U Mayall 4-meter Telescope at Kitt Peak National Observatory (KPNO) and the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), which were made available through the Community Science and Data Center (CSDC), all programs of NSF's NOIRLab. Large survey data sets were then made available to the Backyard Worlds volunteers using NOIRLab’s Astro Data Lab science platform. The results, to be published in The Astrophysical Journal, demonstrate the rapidly growing role of survey and archival data research in astronomy today, say scientists. 

One of the new Backyard Worlds brown dwarf discoveries, as seen in sky maps from WISE (right) and Legacy Surveys DR8 (left). The orange and red colors are indicative of a very low-temperature brown dwarf (A Meisner/NOIRLab/NSF/AURA)

According to the analysis, to help find the Sun's coldest and nearest neighbors, the astronomers of the Backyard Worlds project turned to a worldwide network of more than 100,000 citizen scientists. These volunteers inspected trillions of pixels of telescope images to identify the subtle movements of brown dwarfs and planets. Despite the abilities of machine learning and supercomputers, there is no substitute for the human eye when it comes to scouring telescope images for moving objects

Backyard Worlds volunteers have already discovered more than 1,500 stars and brown dwarfs near the Sun. The latest discovery of 95 brown dwarfs is the largest published sample of these objects ever discovered through a citizen science project. According to Meisner, this is a record for any citizen science program by a factor of about 20, and 20 citizen scientists across 10 different countries are listed as co-authors of the study. "Vast modern data sets can unlock landmark discoveries, and it’s exciting these could be spotted first by a citizen scientist. The Backyard Worlds' discoveries show that members of the public can play an important role in reshaping our scientific understanding of our solar neighborhood," says Meisner.

NASA’s Spitzer Space Telescope provided the brown dwarf temperature estimates. Brown dwarfs are expected to cool as they age, passing from "near-stellar temperatures down to planetary temperatures and below, fading all the while and eventually winking out," say experts. The new discoveries "attest to this picture by uncovering elusive examples" of brown dwarfs approaching Earth-temperatures, they add.

"This paper is evidence that the solar neighborhood is still uncharted territory and citizen scientists are excellent astronomical cartographers. Mapping the coldest brown dwarfs down to the lowest masses gives us key insights into the low-mass star-formation process while providing a target list for detailed studies of the atmospheres of Jupiter analogs," says co-author Jackie Faherty of the American Museum of Natural History in New York.

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