Superflares from Sun-like Stars

NASA's Kepler mission is finding solar-type stars that emit jaw-dropping explosions of high-energy particles and radiation. Now astronomers are looking into why some solar-type stars emit superflares — and why the Sun never will.

Superflare illustration
A normal star emits a superflare thousands of times as bright as typical solar flares in this artist's illustration.
Casey Reed / NASA
In 1859 the Sun emitted the most powerful flare in recorded history, the so-called Carrington event. Energetic particles danced off telegraph lines, creating sparks that shocked operators and ignited fires in telegraph paper. Brilliant aurora were seen as far south as the Caribbean. Estimates are rough, but this flare probably produced some 1025 Joules of energy — a million times more than is stored in the world’s entire nuclear arsenal.

But in the grand scheme of things, that’s nothing — so-called “superflares” can be 10,000 times more powerful. And, it turns out, regular, ol’ solar-type stars are perfectly capable of releasing these massive belches of energy. New results from NASA’s Kepler mission have found 365 superflares coming from solar-type stars, suggesting that superflares might not be so uncommon in “normal” stars.

Dozens of superflares from solar-type stars have been seen before, but observations have been sporadic at best. Kepler, famous for its exoplanet search, makes the study of these rare events feasible in a systematic way.

Superflare starspots
An artist illustrates the giant starspots (dark regions) that produce superflares (white region). These starspots dwarf those seen on the visible surface of the Sun.
Hiroyuki Maehara (Kyoto University)
Hiroyuki Maehara (Kyoto University) and his colleagues searched eight months of data from 2009, when Kepler monitored 83,000 stars of the same type as the Sun. Only 148 solar-type stars (0.2%) emitted a superflare under Kepler’s watchful eye, so the events must be exceedingly rare. But every superflaring star had one thing in common: gigantic starspots. As the starspots formed and dissolved on the surface of the star, the star’s brightness varied in a quasi-periodic way. Starspots harbor magnetic energy that can be released when magnetic fields tangle and reconnect, flinging plasma off the surface of the star.

Superflaring stars also tend to rotate quickly. Maehara and his colleagues found that stars with shorter rotation periods had more (though not necessarily stronger) superflares, presumably because magnetic activity arises from the interaction between a star’s rotation and the boiling motion of its ionized gas.

But slowly rotating stars are still capable of producing flares, just not as often. Of the 365 superflares Maehara and his colleagues found, 14 came from solar-type stars with rotation periods similar to the Sun’s. Extrapolating from the superflares seen, the number of stars observed, and the length of time that Kepler watched them, the scientists found that a superflare of 1027 Joules should occur every 800 years, and one of 1028 Joules every 5,000 years.

Typical solar flare
NASA's Solar Dynamic Observatory images a typical solar flare as it spits ionized gas off the surface of the Sun
Yet, in 2,000 years of geophysical records, the Sun has never issued such a violent event. Spots on the Sun never get so big that they’re in danger of emitting a superflare. And that’s a good thing too. The strongest superflare found by Maehara would send high-energy particles “knocking around Earth's atmosphere, disassociating the nitrogen and oxygen, getting rid of the entire ozone layer,” says Bradley Schaefer (Louisiana State University). In addition to superquick sunburns and a collapsing food chain, a superflare could fry the electric grid and down the whole satellite system, he adds.

So we can count ourselves lucky that our Sun is one of the 99.8% of solar-type stars that will never emit a superflare. (This is one of those times when it’s better to be among the 99%.)

What Sets Superflaring Stars Apart?

One long-standing theory holds that “hot Jupiters,” Jupiter-sized planets that circle their host star in dizzyingly close orbits, affect a star’s magnetic activity. The planet acts as an anchor for the star’s magnetic field, Schaefer explains. As the planet orbits the star, the magnetic field lines connecting the two twist and stretch like rubber bands until sooner or later they snap.

“When a rubberband breaks,” Schaefer says, “it will snap back, and we'll feel the snap and hear the pop.” In the case of magnetic fields, a field line snapping back to the star will use some of its energy to accelerate particles, and some to emit light in every wavelength from radio to X-ray.

The hot-Jupiter theory makes good physical sense, and there are no really good alternatives at the moment. So imagine the researcher’s surprise when, of all the superflaring stars observed by Kepler, none had a hot Jupiter associated with it. If these were really behind unusual activity in solar-type stars, then about 15 hot-Jupiter transits should have been detected around superflaring stars.

But that doesn’t mean the hot-planet theory is kaput, only that it’s in need of revision. “Smaller planets could easily hide and be rare in the Kepler data,” Schaefer suggests. “The magnetic field can be anchored in any planet with a magnetic field, say a superearth or an earth, and the physics would all be the same.”

As Kepler continues monitoring solar-type stars (the mission is currently slated to carry on for another four years), the ever-climbing mountain of data will help astronomers understand what makes a superflare. Plus, astronomers could soon get some help. Schaefer suggests that looking for more flares could make a perfect citizen science project. In the not-too-distant future, the answers might lie in the hands of the public.

17 thoughts on “Superflares from Sun-like Stars

  1. Bruce

    This is a hugely important finding! What must this mean for the famous Drake equation? Monica Young’s statement, "So we can count ourselves lucky that our Sun is one of the 99.8% of solar-type stars that will never emit a superflare" begs for more explaination. How do you define "solar-type stars" here, Dr. Young? Also, I’m very happy to hear that "our Sun … will never emit a superflare", but it’s rare to hear scientists use an absolute term such as "never" in a science report. Why are you so sure, Dr. Young?

  2. Monica YoungMonica Young

    Hi Bruce,
    By solar-type stars, I mean G-type, which is a temperature classification. Normal/main-sequence stars range in surface temperature from 2,000 to 40,000 Kelvin and are labeled by a pretty random sequence of letters: OBAFGKM (commonly remembered as the pneumonic, Oh be a fine guy(or girl) kiss me). So by solar-type stars, I mean stars that have the same surface temperature as the Sun. Note that a solar-type star can rotate much more quickly than the Sun, so many (but not all) superflaring, solar-type stars are actually rotating very quickly, generating the magnetic energy needed for the superflares.

  3. Monica YoungMonica Young

    @Bruce: I never use the term "never" lightly! 🙂 We can say our Sun has probably never emitted a superflare because 1) in 2,000 years of recorded history, no such event has been recorded, and it is highly unlikely that this would be the case if the Sun were capable of emitting superflares, and 2) if a superflare event had happened in the Earth’s recent history, it would have had severe effects on the Earth’s atmosphere, which have not been observed. And then of course there’s the statistical argument: only 0.2% of solar-type stars emit superflares, and the vast majority of these are quickly rotating stars, so it is very, very unlikely that the Sun has the capability to emit a superflare. I think in this case, given the statistics in our favor, the term "never" can be used.

  4. Edward Schaefer

    Let’s see: For other slowly rotating stars like the Sun, it was found that " a superflare of 10^27 Joules should occur every 800 years, and one of 10^28 Joules every 5,000 years." So of those, how often will one occur with the Earth in the parh of the resulting CME? And of those, how often will the magnetic fields of the CME and the Earth be well enough aligned to let a lot of the energy down towards the Earth? If the odds of a major flare’s CME both heading our way and not being deflected are as little as 1 in 10, then even if the Sun flares like other similar stars, the average time between 10^27 Joules superflares that affect the Earth goes to 1 in 8,000 years, and for 10^28 Joules superflares that average time goes to 50,000 years. Now having 2,000 years of reliable records (if those are even reliable for the whole 2,000 years) does not seem so significant.

  5. Bruce

    Edward Schaefer’s point is why I questioned Dr. Young’s use of the word “never”. (Actually, I think she’s “probably” right:) Her excellent article shows that the only common factor of all these super-flaring G stars is the formation of extremely bad cases of startspotitis. (Sorry, but what do we call this?) The article implies that the rate of this stellar condition goes to zero as the stellar rotation rate drops, but our Sun’s rotation is still fast enough (as Edward’s comment points out) for it to possibly be susceptible. As having a star hugging magneto-planet seems to be a contributing factor to this stellar “disease” we can be thankful that Mercury isn’t much closer to the Sun. These spin-off Kepler stellar physics findings may prove to be even more important that the planet hunt in the long run. We might “never” travel to other stars, but we have to live with the Sun.

  6. Peter WilsonPeter

    Stars spin quickly when young, but slow with age. So by the time life around a sun-like star like ours evolves to the point of such complexity that super-flares could kill it off, super-flares will "never" occur anymore. Design or lucky accident? The debate goes on.

  7. Rod

    Peter, yes indeed the debate goes on. This report indicates luck decides how the Earth-Sun relationship became established in origins science and why we find ourselves orbiting such a friendly star and habitable planet. Secular science is very much rooted in a philosophy of origins that excludes there being a Creator. Something I suspect Galileo would never agree with.

  8. Bruce

    To Rod, certainly Isaac Newton would never have accepted an evolutionary origin ether. To Peter, in view of conservation of angular momentum the slowdown of stellar spin rate must be extremely slow, unless there’s a close planet causing a large tidal drag. Is there a known deceleration rate for the Sun’s rotation? In the commonly taught evolutionary timeline the Earth was without even complex aquatic life for almost 4 billion years. Then, according to what’s taught about the fossil record, the so-called Cambrian explosion took place at about 550 million years ago, when sizeable and diverse animal life proliferated in the seas. I see this sudden emergence of complex life in the fossil record as evidence of Creation. Perhaps prior to this time the sun may not have been such a friendly star.

  9. Rod

    Bruce, you may want to check out the Faint Young Sun Paradox (read problem) concerning the early Sun and Precambrian earth. Getting back to class G stars, now list 770 exoplanets, 321 have class G host stars. The average exoplanet mass is 2.17 Jupiters and average e = 0.216, so very eccentric orbits for many. The average class G star in this table is 1.116 M_sun with average temp of 5666 K, thus many of the G stars with exoplanets are larger than our Sun. The mass range was 0.7 M_sun to 2.7 M_sun. I don’t have a complete list of variables that affect the Earth-Sun relationship that allow the Earth to be very habitable today orbiting our class G Sun but I will not claim the configuration we enjoy is because of luck or a series of cosmic accidents when the solar system formed.

  10. Bruce

    The first link in Monica Young’s article provides answers to some of these questions. As Peter posted, stars’ spin rates do slow down, and it is thought that the Sun was a super-flare generator in the distant past. Something to ponder as we all (safely, I hope) watch today’s eclipse.

  11. Rod

    Bruce, where in Earth’s fossil record do we see a Faint Young Sun with <=25-30% solar energy than present and rapid rotation <=3 days vs. 25 days or so today?

  12. Peter WilsonPeter

    Bruce: "Is there a known deceleration rate for the Sun’s rotation?" Curiously, it is the rate physicist thought the Hubble expansion would be decelerating…until the discovery of dark energy. Seriously, I do not know how fast the sun is slowing down, or why, but I remember reading that all stars of a given mass begin their life with the same rotation rate. The reason is that angular momentum prevents a proto-stellar cloud of gas from collapsing, except at the rate it can shed angular momentum. Therefore, for a given mass, fusion does not begin until AM has been reduced to the same quantity.

  13. Bruce

    Peter, as Spock would say, your post is, fascinating. I predict that before this blog thread drops off the web-page we will have answers to the solar spin down rate and it’s cause questions, as someone who either knows or who finds out will post a comment. For now I want to address some of the interesting comments Rod has made. Rod, as you suggested I did check out the Faint Young Sun Paradox, which for the benefit of those who hadn’t heard about this (like me before Rod’s comment) is the “problem” that the sun is supposed to have been about 30-35% cooler in it’s billions of years ago youth, and yet the Earth’s geologic record shows much more evidence for liquid water rather than ice on our planet’s surface. Here’s my first approximation attempt as a logical Bible reader to resolve this conundrum: One of the commonly used answers to this is the suggestion that the greenhouse effect was stronger in the past, but the working gas mentioned the most is CO2. May I suggest an even more common compound, H2O. Water vapor is in fact the most important greenhouse gas even today. Genesis 1:6,7 states that, “And God went on to say, ‘Let an expanse come to be in between the waters and let a dividing occur between the waters and the waters.’ Then God proceeded to make the expanse and to make a division between the waters that should be beneath the expanse and the waters that should be above the expanse.” These verses are from the second “day” of the creation account. Presently about 71% of the Earth’s surface is covered by oceans, and the average depth of the oceans are 3.79km (12,430ft.). If in the Earth’s distant past a large fraction this water had been suspended high in the atmosphere as vapor it would have provided plenty of greenhouse effect, allowing a balmy earth even with a cooler Sun. I don’t see the problem here.

  14. Bruce

    Responding to more of Rod’s comments, I think that it’s still too soon in the search for exoplanets and the stars they orbit to draw any major conclusions. I don’t think you’re alone in not having “a complete list of variables that affect the Earth-Sun relationship that allow the Earth to be very habitable today.” I doubt that anyone here on Earth has such a “complete” list yet, in view of all that’s still to be discovered. I’m with you totally however in your sentiment about the Earth-Sun not being just an accident. As to your question contained in your “Rapid rotating Precambrian Sun” post, perhaps there is evidence in the fossil record related to the news Monica is here reporting. Paleontologists tell us that the fossil record contains several mass extinction events. Could one or more of these have been caused by solar superflaring, which in turn would have been caused by a more rapidly rotating Sun? This seems logically plausible to me.

  15. Henrik

    Thankyou Dr Young for this illuminative and well-written article. It’s as gratifying to me as it must be vexing to the Moonies that our Sun is incapable of a Nibiru Event. Just be prepared for the inevitable challenge.

  16. Bruce

    Henrik, I had never seen the word "Nibiru" before your post. After googling it I find that I agree with you completely. I also agree completely with Dr. Young’s somewhat amended statement in the third comment in this thread; "our Sun has probably never emitted a superflare" speaking of historical times. I also join you in happily agreeing with her statements in the article and even in it’s title that the Sun will in the future never emit a superflare.

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