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Even the Calmest Red Dwarfs are Wilder than the Sun

There’s something menacing about red dwarfs. Human eyes are accustomed to our benevolent yellow Sun and the warm light it shines on our glorious, life-covered planet. But red dwarfs can seem moody, ill-tempered, and even foreboding. For long periods of time, they can be calm, but then they can flare violently, flashing a warning to any life that might be gaining a foothold on a nearby planet.

Red dwarfs (M dwarfs) are the most common type of star in the Milky Way. This means that most exoplanets orbit red dwarfs, not nice, well-behaved G-type stars like our Sun. As astronomers study red dwarfs in greater detail, they’ve found that red dwarfs might not be the best stellar hosts when it comes to exoplanet habitability. Multiple studies have shown that red dwarfs can flare violently, emitting enough powerful radiation to render nearby planets uninhabitable, even when they’re firmly in the potentially habitable zone.

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But there’s still a lot astronomers don’t know about red dwarfs and their wild nature. A new study examined 177 M-dwarfs to better understand their long-term variability. The researchers found that red dwarf behaviour is more complex than thought, and even the calmest red dwarfs are wilder than the Sun. The study is titled “Characterisation of stellar activity of M dwarfs. I. Long-timescale variability in a large sample and detection of new cycles.” The paper will be published in the journal Astronomy and Astrophysics. The lead author is Lucile Mignon, a post-doctoral researcher from the University Grenoble Alpes and the French National Centre for Scientific Research (CNRS.)

All stars are variable to one degree or another. The Sun follows an 11-year cycle during which the number of sunspots on our star’s surface waxes and wanes. It’s all related to magnetic activity. But habitability hinges on longer-term cycles. Life advances in much longer timeframes than a few years. It took life on Earth billions of years to really get going. That’s one of the reasons astrophysicists are interested in red dwarfs and their long-term variability. Life appeared on Earth about 3.5 billion years ago, but complex life only really emerged about 540 million years ago during the Cambrian explosion. If life follows a similar time frame in general, could red dwarf variability prevent life from enduring?

Observing red dwarfs and reaching any conclusions is a difficult challenge. We can watch our Sun in great detail, especially in recent years. A fleet of spacecraft—including the Parker Solar Probe, the Solar Orbiter, the Solar and Heliospheric Orbiter, and others—are dedicated to monitoring it in detail. We’ve also observed the Sun and its activity over a long time period.


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