Sun-like stars experience superflares roughly once per century

Solar flares are bright flashes of light from the Sun that release large amounts of electromagnetic radiation. And while normal flares can release as much as 10²⁵ joules of energy, so-called superflares, observed on other stars, are up to 10,000 times more powerful. Even normal solar flares and the charged particles that often accompany them can impact Earth’s upper atmosphere, impeding communications signals and disrupting operations of equipment, aircraft, energy infrastructure, and more. To prepare and protect those systems, it’s vital to understand how, when, and why our Sun produces these violent burps of energy.

In a paper released today in the journal Science, astronomers tackled that challenge by analyzing data from NASA’s Kepler Space Telescope from over 50,000 Sun-like stars, discovering nearly 3,000 superflares in the process. This suggests that any given star like our Sun experiences a superflare roughly once per century. 

Beyond appearing similar to the Sun in terms of temperature and variability, these stars produce normal flares in the same general pattern as the Sun, too, so there’s no reason to think the Sun doesn’t suffer superflares at the same rate. And since we haven’t seen one that big in recent history, we could be due.

The hunt for data

While our Sun emits small flares nearly constantly, larger flares can swamp Earth’s upper atmosphere, drastically altering how radio and other signals travel through it. Flares are also often accompanied by coronal mass ejections (CMEs), outbursts of charged particles that can more directly damage satellites and aircraft, as well as cause spectacular auroral displays.

Astronomers have never directly observed a large superflare on the Sun. Several have skirted the definition, riding just at or above the dividing line of 10²⁵ joules, including (by estimates) the famous Carrington Event of 1859, as well as the 2003 Halloween solar storms. (For comparison, 10²⁵ joules is around 16,000 times the total amount of energy produced globally in a year.) And there is indirect evidence in rock records hinting that we may have received more powerful surges of energy in the distant past. 

That’s because, fortunately, superflares are much rarer than their smaller siblings. But this also means that scientists have few examples to study in order to learn more. Astronomers have only been observing flares for a few centuries, leaving them with just a handful of truly powerful recorded events. To gather more data, they can look for indirect evidence (the small chemical imprints that strong CMEs leave on rocks) of solar activity from before this period. Or, they can look at the behavior of stars similar to our Sun. 

Astronomers know that, with a few exceptions, stars of the same size and temperature are essentially similar and will follow the same evolutionary life cycle. Since stars operate on celestial timescales of billions of years, astronomers can therefore learn about stars not by watching one star for eons, but by watching many stars at different stages in the same cycle of life.

The Kepler Space Telescope is reknowned for its exoplanet-finding abilities. It accomplished this by staring at a patch of sky with some half-million visible stars, waiting for the stars to blink. Its main mission was looking for small dips of light caused by an exoplanet crossing in front of its star, blocking some light. This meant that it collected data on all of the stars it viewed, planet-bearing or not, and how their brightness varied with time. This created a rich dataset that informed astronomers about stellar variability of all kinds.

Twinsies!

That dataset is the same one mined in the new work, led by Valeriy Vasilyev of the Max Planck Institute for Solar System Research in Göttingen, Germany. The team identified 56,450 stars that are similar to our Sun in terms of their temperatures and intrinsic brightnesses. (This sample excluded stars that were known to be especially young or in binary systems.) Over four years of Kepler observations, the researchers uncovered 2,889 superflares on 2,527 stars. Their energies ranged from 10²⁶ to 10²⁹ joules, or 10 to 10,000 times the maximum energy of a normal flare. 

The researchers point out that four years of data on their 50,000-plus stars is the equivalent of being able to observe the Sun for 220,000 years — quite the hat trick. 

In general, the team found that these stars exhibited flare behavior similar to the Sun: They produced many small flares, fewer big flares, and very few enormous flares. So while the Sun has never exhibited a true superflare in recorded history, it appears that stars just like our Sun do, and on cosmically short frequencies of roughly once per century. 

This is in line with prior research that studied far fewer stars, and yet came up with a relatively similar span of roughly 350 years between superflares. 

So will our Sun emit a superflare in our lifetimes? Only time will tell.