SEARING flares that are thousands of times more powerful than those we see on
the Sun are produced when a star and one of its planets get their magnetic
fields in a tangle, a pair of astronomers suggest. They believe that these
鈥渟uperflares鈥� might help us spot distant stars that have planetary systems.
Astronomers used to think that the brightness of stars like the Sun remained
pretty stable for billions of years. But when a team led by astrophysicist
Bradley Schaefer of Yale University studied records of lone stars with the same
brightness, size and composition as the Sun, they found that some had suddenly
brightened for minutes or even days during the past century
(快猫短视频, 9 January, p 15).
The brightening was due to 鈥渟uperflares鈥� far more powerful than any that had
ever been seen on the Sun, the team concluded. If the Sun did produce such
superflares they would be powerful enough to create auroras visible all over the
world, obliterate our satellites, and melt the ice on Jupiter鈥檚 moons.
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So why do other stars have superflares? Schaefer and his Yale colleague Eric
Rubenstein think they are caused by explosive rearrangements of the magnetic
field lines of the star and a giant planet orbiting close-in.
Gas giants like Jupiter would have intense magnetic fields. 鈥淭he star is
rotating and the planet is whipping around it every few days,鈥� Schaefer says.
鈥淭he magnetic field lines are being stretched and stretched. Sooner or later,
they鈥檙e going to break and reconnect, and emit these bursts of energy.鈥�
Reconnecting solar field lines are already thought to cause many of the smaller
outbursts seen on the Sun.
The researchers, whose work will appear in The Astrophysical
Journal, think their theory also explains our Sun鈥檚 calmer disposition. The
magnetic field of the innermost planet, Mercury, is puny compared with that of a
gas giant like Jupiter. 鈥淚f Jupiter moved to an orbit inside Mercury, then we鈥檇
have to worry,鈥� says Schaefer.
If the Yale astronomers are right, most Sun-like stars with giant planets
orbiting close-in should create superflares, and this could help us pick them
out. Rubenstein speculates that superflares could provide the energy to spur the
development of life on any rocky planets in these systems. 鈥淭his is fertile
ground for where extraterrestrial life might be found,鈥� he says.
Stephen Drake, an astrophysicist at NASA鈥檚 Goddard Space Flight Center in
Greenbelt, Maryland, finds the work interesting and plausible, but he鈥檚 not yet
convinced that it鈥檚 the right answer.
鈥淭hey鈥檝e established that some of these stars can produce big flares,鈥�
comments Drake. 鈥淏ut it would be nice if they鈥檇 seen flares in some of the stars
that we now know have planets. So far, there鈥檚 no overlap.鈥�