Astronomers monitoring data from thousands of distant stars have come to an unnerving conclusion: every 2000 to 3000 years, ones just like the sun can produce superflares 100 or more times larger than anything ever recorded in human history.
Such an event, if it were to occur today, would produce a blast of radiation that would destroy satellites, disrupt electronics, knock out communications, and devastate power grids worldwide.
“Our study shows that superflares are rare events,” says Yuta Notsu, a visiting researcher at the University of Colorado, Boulder, US, “but there is some possibility that we could experience such an event in the next 100 years or so.”
The disturbing find comes from studying data collected by the Kepler Space Telescope, designed to continuously monitor a field of about 150,000 distant stars.
Its primary mission is to look for brightness changes caused by the silhouettes of planets passing in front of their suns. But the same data also allows scientists to collect enormous amounts of data about these stars’ flares.
Flares are sudden releases of energy thought to be caused by releases of magnetic energy stored near starspots — the extrasolar equivalent of sunspots. Superflares are simply big versions.
Conventional wisdom held that superflares are a product of young, fast-rotating stars, unlike the sun, which in middle age has seen its rotation slow to about once every 25 days.
But it turns out that as stars age and slow, they don’t quit having flares. They simply have them less often.
“Young stars have superflares every week or so,” Notsu says. “For the sun, it’s once every few thousand years, on average.”
Nobody knows when, or if, the next such superflare will hit the Earth.
The largest flare on record is the Carrington Event, a giant flare observed by English astronomer Richard Carrington in 1859, which created northern lights that spread as far south as Hawaii and southern lights that spread as far north as Santiago, Chile – slightly farther north than the Australian city of Sydney.
People in the northeastern US claimed they could read the newspaper just from the light of the aurora, and operators of the then-newfangled telegraph reported sparks leaping off their equipment, melting wires and starting fires.
But by superflare standards, the Carrington Event was just a baby, Notsu said recently at a meeting of the American Astronomical Society (AAS) in St. Louis, Missouri.
Its estimated energy was “merely” 1033 ergs — the equivalent of a 100,000,000,000-megaton thermonuclear explosion.
The mammoths observed by Notsu’s team are immensely larger, suggesting that stars like the sun are capable of producing flares of 1035 ergs every few thousand years — one hundred times larger than the Carrington Event.
“So we have no record of any flare as big as you’re describing, which means it’s yet to come,” Rick Fienberg, AAS’s press officer, said, only partially in jest, at Notsu’s recent press conference.
That doesn’t mean, however, that it’s impossible to determine if the sun has ever produced such flares. That’s because their radiation would create a spike in carbon-14 levels in the upper atmosphere.
Carbon-14 is a radioactive form of carbon which, like the ordinary form, finds its way into biological tissues. It is formed when highly energetic radiation from outer space produces neutrons in the upper atmosphere. These are captured by nitrogen-14, which then decays into carbon-14.
A 2012 study in Nature found a spike in carbon-14 levels in tree rings, suggesting that something, possible a massive solar flare, sent carbon-14-forming radiation sleeting into the Earth’s upper atmosphere as recently as AD 775.
“Such events can be used to investigate long-term [solar] activity,” Notsu says.
Meanwhile, he adds, it might be wise to prepare by protecting electronics on the ground and in orbit from massive radiation surges.
“If a superflare occurred 1000 years ago,” he says, “it was probably no big problem. People may [simply] have seen a large aurora. Now it’s a much bigger problem because of our electronics.”
In addition to being presented at the AAS meeting, Notsu’s research was published in The Astrophysical Journal.