Solar flares happen when magnetic-field loops threading through sunspots get twisted and break, producing massive amounts of radiation that accelerate charged particles into space. The largest one that Sol ever produced in recorded history was on September 1st, 1859, and it was observed by the British astronomer Richard Carrington. Hours later, auroras were seen in tropical latitudes and telegraph lines threw off sparks, even when disconnected from their batteries.
Flares up to 10 million times as energetic as the Carrington event have been occasionally observed in other stars, but these observations have been spotty. NASA’s Kepler spacecraft has been observing the same patch of sky continuously, and has been looking for changes in brightness in stars, which could possible signal orbiting planets.
Hiroyuki Maehara, an astronomer at the Kyoto University in Japan, and his colleagues have completed an analysis of 120 days of Kepler observations in 2009. Out 83,000 G2V-type stars like Sol, 148 (0.2%) had superflares with energies 10 to 10,000 times greater than the Carrington event. They published their findings in the journal Nature.
Most of the superflares occurred in stars that rotated in less than 10 days, which was expected since stars that rotate quickly have more magnetic energy. About a quarter of the superflares occurred on slower-spinning stars like Sol, which takes about a month to complete a rotation.
It was previously thought that magnetic interaction with Jupiter-like planets could be to blame, but none of the 148 superflare stars observed in this Kepler data had a Jupiter-like planet. It now seemed unlikely that a planet could case these flares. Slower-rotating stars are able to store up their magnetic energy and release them in the form of superflares. It’s a mystery as to how and why this happens.
While it’s improbably that Sol could have superflares, billions of years ago the situation could have been different. However, if extremely large sunspots appeared on Sol, superflares could occur. The radiation could shred the ozone layer and cause a pulse of mass extinction.
Reference: “Superflares on solar-type stars” by Hiroyuki Maehara, Takuya Shibayama, Shota Notsu, Yuta Notsu, Takashi Nagao, Satoshi Kusaba, Satoshi Honda, Daisaku Nogami and Kazunari Shibata, 16 May 2012, Nature.