NASA's Kepler mission is finding solar-type stars that emit jaw-dropping explosions of high-energy particles and radiation. Now astronomers are looking into why some solar-type stars emit superflares — and why the Sun never will.In 1859 the Sun emitted the most powerful flare in recorded history, the so-called Carrington event. Energetic particles danced off telegraph lines, creating sparks that shocked operators and ignited fires in telegraph paper. Brilliant aurora were seen as far south as the Caribbean. Estimates are rough, but this flare probably produced some 1025 Joules of energy — a million times more than is stored in the world’s entire nuclear arsenal.
But in the grand scheme of things, that’s nothing — so-called “superflares” can be 10,000 times more powerful. And, it turns out, regular, ol’ solar-type stars are perfectly capable of releasing these massive belches of energy. New results from NASA’s Kepler mission have found 365 superflares coming from solar-type stars, suggesting that superflares might not be so uncommon in “normal” stars.
Dozens of superflares from solar-type stars have been seen before, but observations have been sporadic at best. Kepler, famous for its exoplanet search, makes the study of these rare events feasible in a systematic way.Hiroyuki Maehara (Kyoto University) and his colleagues searched eight months of data from 2009, when Kepler monitored 83,000 stars of the same type as the Sun. Only 148 solar-type stars (0.2%) emitted a superflare under Kepler’s watchful eye, so the events must be exceedingly rare. But every superflaring star had one thing in common: gigantic starspots. As the starspots formed and dissolved on the surface of the star, the star’s brightness varied in a quasi-periodic way. Starspots harbor magnetic energy that can be released when magnetic fields tangle and reconnect, flinging plasma off the surface of the star.
Superflaring stars also tend to rotate quickly. Maehara and his colleagues found that stars with shorter rotation periods had more (though not necessarily stronger) superflares, presumably because magnetic activity arises from the interaction between a star’s rotation and the boiling motion of its ionized gas.
But slowly rotating stars are still capable of producing flares, just not as often. Of the 365 superflares Maehara and his colleagues found, 14 came from solar-type stars with rotation periods similar to the Sun’s. Extrapolating from the superflares seen, the number of stars observed, and the length of time that Kepler watched them, the scientists found that a superflare of 1027 Joules should occur every 800 years, and one of 1028 Joules every 5,000 years.Yet, in 2,000 years of geophysical records, the Sun has never issued such a violent event. Spots on the Sun never get so big that they’re in danger of emitting a superflare. And that’s a good thing too. The strongest superflare found by Maehara would send high-energy particles “knocking around Earth's atmosphere, disassociating the nitrogen and oxygen, getting rid of the entire ozone layer,” says Bradley Schaefer (Louisiana State University). In addition to superquick sunburns and a collapsing food chain, a superflare could fry the electric grid and down the whole satellite system, he adds.
So we can count ourselves lucky that our Sun is one of the 99.8% of solar-type stars that will never emit a superflare. (This is one of those times when it’s better to be among the 99%.)
What Sets Superflaring Stars Apart?
One long-standing theory holds that “hot Jupiters,” Jupiter-sized planets that circle their host star in dizzyingly close orbits, affect a star’s magnetic activity. The planet acts as an anchor for the star’s magnetic field, Schaefer explains. As the planet orbits the star, the magnetic field lines connecting the two twist and stretch like rubber bands until sooner or later they snap.
“When a rubberband breaks,” Schaefer says, “it will snap back, and we'll feel the snap and hear the pop.” In the case of magnetic fields, a field line snapping back to the star will use some of its energy to accelerate particles, and some to emit light in every wavelength from radio to X-ray.
The hot-Jupiter theory makes good physical sense, and there are no really good alternatives at the moment. So imagine the researcher’s surprise when, of all the superflaring stars observed by Kepler, none had a hot Jupiter associated with it. If these were really behind unusual activity in solar-type stars, then about 15 hot-Jupiter transits should have been detected around superflaring stars.
But that doesn’t mean the hot-planet theory is kaput, only that it’s in need of revision. “Smaller planets could easily hide and be rare in the Kepler data,” Schaefer suggests. “The magnetic field can be anchored in any planet with a magnetic field, say a superearth or an earth, and the physics would all be the same.”
As Kepler continues monitoring solar-type stars (the mission is currently slated to carry on for another four years), the ever-climbing mountain of data will help astronomers understand what makes a superflare. Plus, astronomers could soon get some help. Schaefer suggests that looking for more flares could make a perfect citizen science project. In the not-too-distant future, the answers might lie in the hands of the public.