You may not have noticed it, but for several weeks last year, the Earth was surrounded by an extra ring of radiation.

There are two doughnut-shaped rings of highly charged particles encircling the planet. Discovered in 1958, they're known as the Van Allen radiation belts.

So it came as a surprise when NASA's recently launched twin satellites to the treacherous region discovered a third, temporary ring. It appeared for a month before a shock wave from the sun destroyed it. Scientists are still trying to figure out how often this happens.

The twin spacecraft launched last year on a mission to explore the Van Allen belts. Solar storms can cause the rings to expand, posing a potential threat to satellites.

The discovery was reported online Thursday in the journal Science.