The Sun’s Magnetic Field is Behaving Oddly. Astronomers Still Don’t Know Why.

In 2007, the German astronomer Erika Böhm-Vitense surveyed the stars. She noticed two distinct groups: Younger stars were spinning fast and bursting with magnetic activity, and older were stars spinning slowly with much tamer magnetism. But right in the center, conspicuously defying those trends, was our sun.

“Clearly the sun is not a good standard star for the discussion of stellar activity,” Böhm-Vitense wrote. But she wondered if the sun’s alienation among its brethren might hint at a bigger mystery: “Is its special position between the sequences necessary to permit life on Earth to evolve and survive?”   

Böhm-Vitense’s finding kicked off a two-decades-long investigation into the sun’s odd behavior. Since then, astronomers have come to understand that our star is not an outlier after all. Doing so has required a rethink of how stars spin—and, thus, generate magnetic fields—over the course of their lifetimes. “The sun’s doing what all the other stars are doing—it’s just that they’re all doing something different than what we expected,” says Travis Metcalfe, an astronomer at White Dwarf Research Corporation in Colorado. Now, observations and simulations are converging to suggest that when stars are approaching middle age (as our sun is) they undergo a sort of magnetic crisis during which their large-scale magnetic fields begin to wane.

What that means for our sun—and what comes next for it—is still up for debate. But, just as Böhm-Vitense suggested, the answer could be critical for predicting which stars can support life. As new telescopes start to hunt for biosignatures on distant planets, scientists are racing to rewrite the story of how the magnetic activity of stars evolves.

“We’re still in the first salvo of what’s going on here,” says Jeremy Drake, an astronomer at the Lockheed Martin Solar and Astrophysics Laboratory in California. “It’s really a central question we need to answer if we want to work out if we are alone in the universe.”

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Dancing With Stars

Like all stars, our sun is a giant molten ball of charged particles that twirls around its vertical axis as it burns through its reservoir of hydrogen fuel over the course of its lifecycle. The swirling of charge creates what scientists call a “dynamo,” which generates a global magnetic field with north and south poles—essentially turning the sun into a giant fridge magnet. Because the equator spins faster than the poles, the magnetic field winds up like a rubber band. Periodically, when the tension is especially high, the sun lashes out in energetic explosions that create space weather on Earth—radiation and particles that light up our sky with auroras and, occasionally, satellite-disrupting flares.

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In the 1970s, scientists realized that the sun’s global magnetic field also acts as a parachute, gradually slowing down its spin over time. A few decades later, Sydney Barnes, now at the Leibniz Institute for Astrophysics Potsdam in Germany, suggested astronomers leverage this relationship to determine the age of distant stars from how fast they’re spinning, a technique he called “gyrochronology.”

Gyrochronology was a breakthrough for dating stars—something that’s just as challenging in astronomy as it is in Hollywood. But it’s become clear that the age-to-spin relationship isn’t as straightforward as scientists originally thought—particularly for older stars that have already burned through a good chunk of their hydrogen fuel. In 2016, Metcalfe and colleagues showed that some stars older than the sun weren’t continuing to slow down as gyrochronology predicted. Instead, these elder stars appeared to hit cruise control and maintain a relatively constant rotational speed.

The observations were at first controversial, but as the sample size grew and the relationship was validated with different methods, even initial skeptics such as Barnes began to believe that these old stars are spinning suspiciously fast. “The question is: What is the explanation?” Barnes says.

The Dying Dynamo

In that 2016 work, Metcalfe’s team put forth a proposal for the interplay between age, spin, and magnetism: A star’s magnetic field does restrain its rotation—but only to a point. Beyond a certain stage (when a star has burned through around half of the hydrogen in its core), the star is rotating too slowly to be able to maintain its dynamo, and its global magnetic field begins to shut down.

Over the last few years, their team has hunted for more direct evidence of this shutdown sequence. They started with magnetic field measurements of the sun taken by different spacecraft. But “studying the sun in isolation is like watching one really detailed scene in the middle of a movie,” Metcalfe says. “What you really need is even just still frames from earlier and later scenes.” So, they collected measurements of the X-ray brightness, rotation, and magnetic field strength in 17 stars of different ages using the Large Binocular Telescope in Arizona, the largest optical telescope on Earth.

In a paper published last September, Metcalfe and his colleagues stitched together those frames and found a clear trend. Right around halfway through their lives, the strength of the stars’ global magnetic field appeared to drop drastically—up to 100 times weaker than their cousins a few billion years younger. The sun falls right on that cusp. Their interpretation was simple: “We’re basically watching as the sun’s dynamo slowly collapses,” Metcalfe says, “but that process is going to take the entire second half of its lifetime to play out.”

Some astronomers find the explanation compelling. “This clarifies the picture a lot, I think, and it’s got a good solid physical basis to it,” says Patrick Young, an astrophysicist and astrobiologist at Arizona State University. Others aren’t yet convinced by the analysis. “There’s nothing wrong per se—it could be true … it just seems a little contrived,” Barnes says. “My feeling is that the dots can be connected in a different way.”

One alternative is that the global magnetic field isn’t dying out but rather approaching a temporary minimum. Using computer simulations, Allan Sacha Brun, an astrophysicist at the University of Paris-Saclay in France, found that if a star rotates even more slowly than the sun, its poles can begin rotating faster than the equator, and the magnetic field can strengthen again—giving birth to a new dynamo that no longer cycles in strength. “If a star spins down too much, he may actually revive his dynamo,” Brun says.

One possible explanation for that magnetic dip is if the star’s magnetic field is shape-shifting, Brun suggests. The “dipole” magnetic field—with a north pole and a south pole like a bar magnet—is the simplest way for the dynamo to organize. But it can also form fancier shapes with four poles, or eight. In 2018, Cecilia Garraffo at the Harvard-Smithsonian Center for Astrophysics in Boston called for a “revolution revolution,” arguing that astronomers had been overlooking the complexity of magnetic fields in driving stellar rotation.

At a meeting in Tokyo in June, researchers presented their different ideas for how stellar magnetism evolves. Metcalfe now believes it all fits into one neat story. “This might just be the natural evolution of dynamos,” he says. “It’s a sequence.” A star could start out with very complex magnetic fields. As it ages, those messy fields gradually order into a strong dipole, which makes the star spin slower. Around halfway through its life, it’s spinning so slowly that the magnetic field morphs back into more complicated shapes, which cuts the brakes on its rotation. Finally, as the star runs out of its hydrogen fuel, it begins to expand, slowing the rotation further until the dipole takes over once again.

In April, Metcalfe and colleagues observed a star in the constellation Aquila that appears to validate this narrative. The star is at a later stage in its evolution than the sun, and yet it seems to sport a very strong, stable magnetic field—just as Brun’s simulations predict. But “we have not completely closed the loop,” Brun cautions. His team is now trying to model whether switching between different magnetic field shapes can explain the observed changes in rotation.

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Life in the Stars

Ironing out the details of this magnetic saga is vital. The activity—and thereby rotation—of stars shapes the environments of the planets that orbit them. If Metcalfe is correct that the sun’s global magnetic field is collapsing, we should expect the next few billion years on Earth to feature progressively calmer space weather: fewer cataclysmic solar flares and diminishing auroras. If Brun is right about stellar activity reviving, the distant future would hold a rapid uptick in activity—making for more consistent geomagnetic storms on Earth.

But the lessons wouldn’t apply only to Earth. If we can manage to construct a consistent framework of magnetic evolution in which the sun fits in rather than sticks out, we could use our star as a “Rosetta Stone” for learning about other solar systems.

The clock is ticking. In the early 2040s, NASA plans to launch the Habitable Worlds Observatory (HWO) to look for chemical signatures of life on exoplanets orbiting other stars. Its primary goal is to directly image 25 Earthlike planets. To do so, scientists need to know where to point the telescope—which means knowing which stars are most likely to harbor exo-Earths. To winnow down potentially habitable targets, astronomers are beginning to look to the magnetic activity of their stars.

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Take the star TRAPPIST-1, for instance. Considered a top contender for a habitable neighborhood, the star hosts seven rocky planets in the “Goldilocks zone” where liquid water can exist. But when Garraffo and Drake simulated TRAPPIST-1’s space weather, they found those particles would blast into the planets, stripping away their atmospheres—and any chance of life as we know it.

Young, who’s helping to prioritize targets for HWO, thinks stellar activity history could make a useful filtering criteria for habitability. “If you have a star that’s not been terribly active for a while, that’s one less thing to worry about,” they say. “This is absolutely something to consider.”

But before advising on alien-hunting, astronomers first need a better handle on how stars spin. “There’s undoubtedly stuff we don’t understand,” Barnes says. “I’ve worked a lifetime on rotation, and I’m still at the beginning.”