Brown dwarfs are often viewed as stellar underachievers. They're nicknamed "failed stars," "sub-stellar objects" or, simply, "substars."
While they may be too small to be known as stars - they cannot sustain nuclear fusion in their cores for long, except for the largest brown dwarfs that have the gravitational clout to fuse deuterium and lithium - they are also too big to be called planets.* They are in a class of their own; so rather than being the proverbial runt of the stellar litter, they are wonderfully interesting objects that form a mysterious bridge between the planets and stars.
Now, using the awesome power of the 305-meter (1000-ft) Arecibo radio telescope in Puerto Rico, astronomers have revealed yet another puzzle that only amplifies the mystery surrounding these enigmatic objects.
Penn State University researchers have detected the flaring emissions from a very cool brown dwarf 33.6 light-years away called "J1047+21." The emissions detected were of radio wavelengths, suggesting some kind of interaction between a magnetic field and charged particles. What's more, it has become a record-breaking radio-emitting brown dwarf.
"This object is the coolest brown dwarf ever detected emitting radio waves - it's half the temperature of the previous record holder, making it only about five times hotter than Jupiter," said graduate student Matthew Route, the lead author of the discovery paper. The surface temperature of J1047+21 is approximately 630 degrees Celsius (1,160 Fahrenheit).
Other brown dwarfs have been discovered emitting infrared radiation at cooler surface temperatures - down to a record-breaking 25 degrees Celsius (80 Fahrenheit) in the case of a Y-class ultra-cool brown dwarf called "WISE 1828+2650," 30 light-years from Earth. But J1047+21 is the coolest "radio-star" detected to date.
But how radio waves are generated by a lone brown dwarf is a puzzle unto itself. The emissions "must be generated by electrons spiraling along the magnetic field," Alex Wolszczan, project leader of this study, told Discovery News. "The real puzzle is how do you generate plasma in such a cool environment?"
Primarily, there needs to be a magnetic field surrounding the brown dwarf, not dissimilar to the magnetosphere that surrounds our planet. But to generate radio waves, electrons need to be supplied to the magnetic field - as they are negatively charged, the magnetic field causes them to spiral. It's the motion of electrons that generates the radio waves detected by Arecibo.
"One idea is that the necessary energy is generated by magnetic field reconnection, which leads to creating a hot plasma bubble that travels upwards through the atmosphere," Wolszczan added, pointing out that the data suggests motion in the brown dwarf.
Magnetic reconnection occurs when magnetic fields come into contact and are forced together. If the conditions are right, the magnetic fields may "snap" and "reconnect" with one another. As a consequence, plasma deep inside the brown dwarf is rapidly energized, potentially supplying the brown dwarf's magnetosphere with electrons.
Presumably, the flaring radio emissions indicate magnetic reconnection inside the brown dwarf, causing bubbles of plasma to erupt to the surface, releasing electrons into the magnetosphere. As each bubble is released, a new radio flaring event is detected.
"This is a really exciting result. We hope that in the future we'll be able to detect yet colder brown dwarfs, and possibly even giant planets around other stars," Wolszczan said in the Penn State press release. Indeed, this research goes well beyond detecting the radio emissions from brown dwarfs - it also has a fascinating application in the detection of exoplanets that may harbor alien life.
The magnetosphere around Earth deflects the most energetic particles that constantly slam into our planet, thereby protecting the otherwise vulnerable biosphere below from solar radiation. As space plasma interactions with planetary magnetospheres generate radio emissions, once our radio telescopes become sensitive enough, we may be able to detect aurorae crackling over exoplanets that possess global magnetic fields.
Once we can do this, we may reveal worlds that not only have the potential to support life, we may have the ability to identify the ones that can protect their evolving alien biospheres from the ravages of stellar radiation too.
This research was published in the March 10 edition of Astrophysical Journal Letters.
*Rather than "failed stars," perhaps they should be re-labeled "overachieving planets" or "super-planetary objects"? Just a thought.
Image credit: NASA