Guiding light

2019-03-07 05:03:13

By Robert Adler 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 “superflares” 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 (New Scientist, 9 January, p 15). The brightening was due to “superflares” 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’s moons. 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. “The star is rotating and the planet is whipping around it every few days,” Schaefer says. “The magnetic field lines are being stretched and stretched. Sooner or later, they’re going to break and reconnect, and emit these bursts of energy.” 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’s calmer disposition. The magnetic field of the innermost planet, Mercury, is puny compared with that of a gas giant like Jupiter. “If Jupiter moved to an orbit inside Mercury, then we’d have to worry,” 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. “This is fertile ground for where extraterrestrial life might be found,” he says. Stephen Drake, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, finds the work interesting and plausible, but he’s not yet convinced that it’s the right answer. “They’ve established that some of these stars can produce big flares,” comments Drake. “But it would be nice if they’d seen flares in some of the stars that we now know have planets. So far,