Image caption A superflare would be associated with very large starspots - much larger than those on the Sun

Nasa's Kepler space telescope has provided fresh insight on the colossal explosions that can afflict some stars.

These enormous releases of magnetic energy - known as superflares - could damage the atmosphere of a nearby orbiting planet, putting at risk any lifeforms that might reside there.

Fortunately, Kepler shows superflares to be much less frequent on slow-rotating stars like our Sun.

The new observations are reported in the journal Nature.

The biggest recorded flare on the Sun was probably the "Carrington event" of 1 September 1859.

Described by the English astronomer Richard Carrington, this outburst sent a surge of electromagnetic radiation and charged particles towards the Earth.

The magnetic fields embedded in the bubble of matter buffeted the Earth's own magnetic field, producing spectacular auroral lights. Electric fields generated in telegraph wires disrupted communications.

Remarkably, a Carrington flare is puny compared to some of the events witnessed by Kepler. These superflares can be 10,000 times more energetic.

The US space agency telescope is currently staring at 100,000 stars in a patch of sky about 600 to 3,000 light-years from Earth.

Image caption The flares seen on our Sun are timid compared with the events witnessed by Kepler

It is looking for tell-tale dips in light that might indicate orbiting planets have just moved across the face of those stars. But in making those observations, Kepler is also gathering information on the sudden brightening that might be associated with flares.

Hiroyuki Maehara from Kyoto University, Japan, and colleagues have now reviewed the data to compile the best statistics yet on the frequency and size of superflares.

Kepler saw a total of 365 superflares during an observation period lasting 120 days.

The numbers confirm that very few (only 0.2%) Sun-like stars experience monster flares.

That may be explained by current models which indicate superflares could be caused by magnetic interactions with closely orbiting giant planets - not something we see in our Solar System, where the largest worlds, Jupiter and Saturn, circle far away from the Sun.

But, this is a situation seen in some distant planetary systems where worlds even bigger than our gas giants orbit extremely close to their host stars.

One other interesting observation Kepler makes is that the stars which have superflares display extremely large starspots.

Image caption Carrington sketched what he saw on the Sun's surface in 1859

These are regions of the star's surface that are relatively cooler than the typical surface temperatures around them.

Carrington famously sketched the spot group on the Sun associated with his big flare, but according to Kepler this group would be dwarfed by the sort of spots on a superflare star.

Scientists have long speculated about the impact of a superflare on our Sun might have on Earth. The expectation is that it would strip away the ozone layer, leading to increased radiation at ground level. Widespread extinctions could result.

There is a flip side to this, however. In some distant planetary systems, superflares might actually be life-enabling by providing sufficient energy in the atmospheres of these worlds to initiate the chemistry necessary for biology to get going.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter