Categories: AstronomyHabitabilityStars

It Turns out, We Have a Very Well-Behaved Star

Our Sun is a Population II star about 5 billion years old. It contains elements heavier than hydrogen and helium, including oxygen, carbon, neon, and iron, though only in tiny percentags. Image: NASA/Solar Dynamics Observatory.

Should we thank our well-behaved Sun for our comfy home on Earth?

Some stars behave poorly. They’re unruly and emit powerful stellar flares that can devastate life on any planets within range of those flares. New research into stellar flares on other stars makes our Sun seem downright quiescent.

NASA’s TESS (Transiting Exoplanet Survey Satellite) is a planet hunter. Its primary task is to watch stars for any regular dips in light. Those dips can signal the presence of a planet as it passes between us and the star. TESS is very successful at finding planets.

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But TESS does more than identify exoplanet candidates. TESS’s keen-eyed cameras reveal a lot about the stars the planets are orbiting. One of the mission’s objectives is to study about 1,000 M-dwarf (red dwarf) stars closest to us. Red dwarfs are the most plentiful stars in our galaxy, so most exoplanets are probably orbiting red dwarfs. About 75% of the stars in the Milky Way are M-dwarfs and many of them host planets in their habitable zones.

But red dwarfs are complicated stars. On the one hand, they’re the longest-lived stars, so planets orbiting them can count on stable conditions for a long time. That long-lived stability is good for the development of life.

On the other hand, red dwarfs can emit powerful flares. Stellar flares can be hard on planets and can severely limit the possibility of life on planets around red dwarfs.

“Many of these red dwarf stars can emit flares 1,000 times larger than those from the Sun…”
Ward Howard, lead author, U of C Boulder.

This is an artist’s conception of a violent stellar flare erupting on the red dwarf Proxima Centauri, our nearest neighbour. Our Sun seems relatively calm compared to red dwarfs. Credit: NRAO/S. Dagnello.

This puts planet-hunting in a new light. Planet hunting’s over-arching goal is finding planets in a star’s habitable zone where liquid water could exist on the surface. But our growing knowledge of flaring may make our understanding of habitable zones outdated.

A new study presents a statistical analysis of stellar flaring on hundreds of stars. The study is “No Such Thing as a Simple Flare: Substructure and QPPs Observed in a Statistical Sample of 20 Second Cadence TESS Flares.” The authors are Ward Howard, a post-doc researcher at the University of Colorado, Boulder, and Meredith MacGregor, assistant professor of astrophysical and planetary sciences at CU Boulder. The Astrophysical Journal will publish the study.

The study is the first large-scale analysis of stellar flaring. It’s based on data collected at 20-second intervals, a rapid cadence for observations. The faster cadence gathers more granular data.

Our Sun emits flares, which can disrupt electronic systems on Earth and in satellites. The Sun is nothing compared to the stars in this study, even though red dwarfs are smaller than the Sun.

“The sun is very well behaved,” lead author Howard said in a press release. “Many of these red dwarf stars can emit flares 1,000 times larger than those from the Sun, and you can only imagine what that might do to a planet or to life on the surface.”

TESS is in its extended mission now. The 20-second observation intervals are more rapid than the two-minute intervals used in TESS’s primary mission. The rapid intervals give astrophysicists a better window into flares. They can watch as flares develop and measure the radiation more accurately. Howard and MacGregor discovered that flares are more complicated than thought, and some can burst multiple times.

“They have all sorts of weird structure in the light curves, which indicates that some of them are bursting multiple times,” co-author MacGregor said.

This figure from the study shows the difference between 20-second intervals and 2-minute intervals. The left panel shows both intervals binned to 2-minute intervals. The right panel shows how the 20-second cadence reveals more detail in the flares. Image Credit: Ward and MacGregor 2022.

“The new 20-second cadence mode reveals significant substructure in large flares that would have been missed
at 2 min cadence,” the authors write in their paper. “Higher-cadence observations also remove degeneracy present at 2 min cadence between significantly different flare morphologies,” they say when discussing the above figure.

“We have historically had a very simple picture of stellar activity, where one loop breaks and we have one outburst of energy, and then it slowly dies away, and then we think about the frequency of that,” MacGregor continued. “That’s the model that’s been fed into everything we think about stars and their impact on planets, and it’s clearly just flat-out wrong.”

“It allows us to kind of have a statistical understanding of how often do certain things occur,” Howard said, adding that scientists have never before been able to determine how much radiation reaches planets during the peak of the superflares and how much complexity the flares have.

Astrophysicists describe stellar flares in two phases: the rise phase between the beginning of the flare and peak brightness and the decay phase. “Many large flares exhibit complex substructure during the rise phase,” the authors write, and the 20-second cadence helps reveal the complexity, while the slower cadence doesn’t. “We find 46% of the large flares in our sample exhibit complex structure in the rise phase (201 out of 440 flares), making this a common phenomenon at the 20-second cadence.”

This figure shows the rise phases of ten of the flares in the study—nearly half of the flares show complex substructures during the rise phase. Although exceptions exist, a greater degree of complexity generally correlates with longer rise times. Resolving the complex substructure in the rise phases of large M-dwarf flares is more difficult in lower-cadence observations. Image Credit: Ward and MacGregor 2022.

The study also found other flare morphologies that the authors describe as unusual yet frequently-occurring. One is the peak-bump flare. This type of flare has an initial highly-impulsive peak followed by a less-impulsive second peak. About 17% of the flares exhibit this morphology.

Another unusual type is the flat-top flare. Most flares have a very powerful impulsive peak, but flat-top flares have more constant emission levels at their peak. Previous studies show that these flat-top flares can peak for almost one hour, though the longest-lasting peak in this study was 26 minutes. 24 of the flares in this study—about 5%—are flat-top flares.

This figure from the study shows eight flat-top flares. The 20-second cadence observations helped identify these types of flares. Image Credit: Ward and MacGregor 2022.

Red dwarfs flare differently than our Sun. But the basics are the same. All stars have powerful magnetic fields, and sometimes those fields become entangled. The entanglement spawns powerful bursts of radiation and charged particles. The result is beautiful, looping, solar prominences. Prominences remain anchored to the Sun but extend thousands of kilometres into space.

“Our sun does this, and we can get beautiful images where you see these loops of emission protruding out of the surface of the sun, and then they break and stream out into space,” MacGregor said.

This is a solar eruptive prominence seen in extreme UV light on March 30, 2010, with Earth superimposed for a sense of scale. Credit: NASA/SDO

When a solar prominence breaks free from the Sun, it’s a flare. Most flares are accompanied by coronal mass ejections (CME), masses of solar plasma and magnetic fields. When the Sun emits a CME toward Earth and strikes our planet’s magnetosphere, we get beautiful light shows: the aurorae. We also get geomagnetic storms, and if they’re powerful enough, they can damage electrical grids and satellites. But that’s rare.

“So we see beautiful lovely green lights,” MacGregor said. “What we’re actually observing is the effect of our sun splitting apart molecules in our atmosphere and then the release of energy from that splitting of things like ozone and water.”

Things play out differently on red dwarfs.

Red dwarfs are smaller than stars like our Sun. But they can rotate more rapidly than larger stars, so they can have more powerful magnetic fields. This creates more powerful flares, and sometimes what astrophysicists call superflares. Superflares can be up to 30 times more powerful than our Sun’s flares—maybe even more potent than that.

That much energy can shred a planet’s atmosphere. Most planets orbiting in a red dwarf’s habitable zone are likely tidally locked. This paints an ugly picture for life. One side of a world would be regularly blasted by powerful flares, while the other remained dark. Could life survive there?

Maybe it could. Some evidence shows that red dwarfs emit their flares from higher latitudes and poles. But planets orbit their stars in the ecliptic, which might spare them from the worst effects.

This figure is from a 2021 study showing that red dwarfs emit flares from their polar regions. The black star marks the star’s pole. The red circle shows the flare latitude, and the red dot marks the active flaring. The yellow dashed line marks the maximum typical solar flare latitude. Planets orbiting in these stars’ ecliptics likely escape the worst effects of powerful flares. Image Credit: Ilin et al. 2021.

There are no firm measurements of how much radiation from red dwarf flares would reach any planets around the stars. The authors discuss this in their paper but can’t reach solid conclusions. Scientists work with a survival concept called D90—the UV dose needed to kill 90% of a hardy bacterium called D. Radiodurans. The authors find that “… 1/3 of our 10³⁴ erg flares reach this limit during the 20-second peak epoch.” They also found that none of the flares were powerful enough to kill 100% of D. Radiodurans.

These numbers are preliminary, and there are assumptions behind them. The hypothetical planets subjected to the flares are unmagnetized and have no significant atmospheres. Magnetospheres of differing strengths and different types of atmospheres could strongly affect how much UV radiation from flares would reach a planet’s surface.

Our understanding of red dwarfs and their flaring is in the early stages. This study removes some guessing and conjecture and replaces it with some of our most detailed knowledge of flaring yet.

It doesn’t paint a pretty picture, though.

These results put our own neighbourly Sun in a pretty good light. The Sun’s flares are relatively calm and gentle compared to some powerful bursts from red dwarfs.

Complex life on Earth is only possible because of many variables that turned out just right. It looks like we can add the Sun’s relatively quiescent flaring to the list.