Can Our Sun Produce a Killer Flare?

The short answer to this is "no".

The longer answer is a little more involved. Whilst a solar flare from out Sun, aimed directly at us, could cause secondary problems such as satellite damage and injury to unprotected astronauts and blackouts, the flare itself is not powerful enough to destroy Earth, certainly not in 2012. I dare say, in the far future when the Sun begins to run out of fuel and swell into a red giant, it might be a bad era for life on Earth, but we have a few billion years to wait for that to happen. There could even be the possibility of several X-class flares being launched and by pure bad luck we may get hit by a series of CMEs and X-ray bursts, but none will be powerful to overcome our magnetosphere, ionosphere and thick atmosphere below.

Scientists using NASA's Swift satellite have spotted a stellar flare on a nearby star so powerful that, had it been from our sun, it would have triggered a mass extinction on Earth. The flare was perhaps the most energetic magnetic stellar explosion ever detected.

The flare was seen in December 2005 on a star slightly less massive than the sun, in a two-star system called II Pegasi in the constellation Pegasus. It was about a hundred million times more energetic than the sun's typical solar flare, releasing energy equivalent to about 50 million trillion atomic bombs.

Fortunately, our sun is now a stable star that doesn't produce such powerful flares. And II Pegasi is at a safe distance of about 135 light-years from Earth.