Contrary to the human need for organising objects in well-defined and distinct categories, our Solar System seems to be replete with celestial objects that defy our tidy definitions of comets, asteroids, and planets. Now, the discovery of a new sub-stellar object within our Sun’s galactic neighborhood, made with NASA’s Wide-field Infrared Survey Explorer, or WISE, and Spitzer Space Telescope, comes to further blur the line between planets and brown dwarfs.
Launched in December 2009, WISE conducted two all-sky surveys in infrared wavelengths in 2010, before running out of its hydrogen coolant in October of that year. Having a sensitivity 1,000 times greater than that of previous space-based infrared observatories, it scanned the sky continuously for 10 months at four different infrared wavelengths—3.3, 4.7, 12, and 23 micrometers—conducting observations that were of unprecedented detail. By analysing this wealth of data, astronomers were able to discover millions of black hole candidates in distant galaxies, thousands of new asteroids in the Solar System, dozens of new comets, and a new ultra-cool class of brown dwarfs.
Although theorised to exist as early as the 1960s, the objects that would be later known as “brown dwarfs” were first discovered during the mid-1990s, when technical advances in the field of infrared imaging technology made their detection possible. Often being described as “failed stars,” brown dwarfs are thought to have masses ranging from several to 75 times that of Jupiter. Like stars, they are formed from the gravitational collapse of interstellar clouds of gas and dust. But what defines the final outcome of this formation process is the mass of the created protostar inside the collapsing cloud. If its mass is lower than 0.08 solar masses, then the protostar will fail to undergo thermonuclear fusion of hydrogen in its core and to ignite to become a main-sequence star. The resulting object will instead spend its life slowly radiating away the internal leftover heat from its formation, in the form of thermal radiation.
Like all main-sequence stars, brown dwarfs are classified into different spectral types according to their chemical characteristics and surface temperatures. All-sky infrared surveys conducted with ground- and space-based instruments during the past two decades have revealed the presence of more than 2,000 brown dwarfs to date, ordered from highest temperature to lowest into spectral types M, L, T, and Y. The latter, in particular, were a hypothesized class of brown dwarfs, whose surfaces were thought to have temperatures lower than 600 K (326 degrees Celsius), leading to heated debates among astronomers regarding where to draw the line that separated brown dwarfs from planets. “The distinction between hydrogen-fusing stars and brown dwarfs is well-defined,” wrote Dr. Adam Burgasser for the Physics Today magazine in 2008, now an associate professor of Physics at the University of California, San Diego. “But what distinguishes brown dwarfs from planets, given their similar sizes and atmospheric properties? Pluto’s recent demotion has focused attention on the ambiguity of the term ‘planet’ in the Solar System. Brown dwarfs are forcing us to re-examine a related ambiguity in a galactic context.”
Making the distinction between gas giant planets and brown dwarfs even more ambiguous, scientists studying data from WISE finally confirmed the existence of these sub-stellar/planetary-mass hybrids in August 2011, by announcing the first-ever discovery of Y-class brown dwarfs at distances ranging from 9 to 50 light-years from the Sun. Most remarkably, one of the newly found objects, named WISE 1828+2650, was estimated to have a surface temperature no greater than 298 K (25 degrees Celsius). As detailed in a study published earlier this month in The Astrophysical Journal Letters, written by Dr. Kevin Luhman, an associate professor from The Penn State University’s Department of Astronomy and Astrophysics, astronomers can now add a new member to this family of ambiguous objects: an ultra-cold, Y-class brown dwarf, named WISE J085510.83-071442.5, or WISE 0855–0714 for short.
Luhman is no stranger to brown dwarf research. In March 2013, he entered the spotlight by finding the third closest star system to the Sun, named Luhman 16, consisting of a pair of brown dwarfs that were located just 6.5 light-years away. WISE 0855–0714 comes next in line, as the fourth closest brown dwarf.
Luhman first spotted the newly found object, in a series of images taken with WISE in May and November 2010. As a follow-up, he also analysed the image archives of the Spitzer Space Telescope and the Gemini Observatory in Chile, to trace the object’s path in the sky. Since the images were taken six months apart, by using the parallax method he was able to determine that WISE J0855-0714 was lying at a distance of approximately 7.2 light-years away. “This object appeared to move really fast in the WISE data,” says Luhman. “That told us it was something special.” .
The data from Spitzer were also used to pin down the brown dwarf candidate’s surface temperature. The space telescope observed WISE 0855–0714’s absolute magnitude and color at infrared wavelengths between 3.6 and 4.5 micrometers. Based on these observations and certain theoretical models of stellar evolution, Luhman reached at a surprising estimate: The newly found brown dwarf had a temperature ranging from 225 to 260 K (-48 to -13 °C). In addition, its mass was found to be between three and 10 Jupiter masses. “It’s very exciting to discover a new neighbor of our solar system that is so close,” says Luhman. “And given its extreme temperature, it should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures.”
By taking these results at face value, one could be forgiven for arguing that WISE 0855–0714 constitutes just another rogue planet that was probably ejected from its parent star and left drifting through interstellar space, like several other similar objects discovered in recent years. One of them, Cha 110913-773444, also discovered by Luhman, is a planetary-mass sub-dwarf having an estimated mass eight times that of Jupiter. Yet Spitzer observations have indicated the presence of a protoplanetary disk surrounding Cha 110913-773444, not unlike those found around main-sequence stars during the early phases of planetary formation. Since, according to theory, protoplanetary disks can only form around protostars, does that mean that objects like Cha 110913-773444 and the newly found WISE 0855–0714 are indeed “failed stars” instead of planets? “Astronomers vigorously debating that semantic question, fall mainly in two camps,” writes Burgasser. “One advocates a definition based on formation—a brown dwarf condenses out of giant molecular clouds, whereas a planet forms via core accretion in a circumstellar debris disk. The other focuses on interior physics: A brown dwarf must be heavier than the mass threshold for core fusion of any element, roughly 13 Jupiter masses.”
The 13 Jupiter-mass limit has also been adopted by the International Astronomical Union as the dividing line between brown dwarfs and smaller planetary bodies. The rationale for this distinction is that any objects above this limit can, unlike planets, undergo thermonuclear fusion of deuterium in their core, thus falling under the brown dwarf category. On the other hand, many astronomers have challenged this definition, arguing that according to current theoretical models of stellar evolution the minimum mass a protostar can have to form a brown dwarf is that of Jupiter’s.
Despite the ongoing debates regarding their nature, the discovery of objects like WISE 0855–0714 is invaluable to astronomers in their efforts to better understand the processes of stellar and planetary formation. “These objects are important, as they can either help us understand more about how planets may be ejected from planetary systems, or how very light objects can arise from the star formation process,” says Philippe Delorme, an astronomer at the Institute of Planetology and Astrophysics of Grenoble in France, who was the lead discoverer of CFBDSIR 2149-0403, another planetary-mass sub-dwarf candidate found in 2012.
As reported in a previous AmericaSpace article, results from WISE have indicated that large Jupiter-mass planetary bodies probably do not exist within our Solar System beyond the orbit of Neptune. Yet the presence of objects like Luhman 16 and WISE 0855–0714 underscores our lack of knowledge about the uncharted territory in our own star’s vicinity. “It is remarkable that even after many decades of studying the sky, we still do not have a complete inventory of the Sun’s nearest neighbors,” says Michael Werner, Project Scientist for NASA’s Spitzer Space Telescope at the Jet Propulsion Laboratory in Pasadena, Calif.
Indeed, what else is out there?