Even the quietest purple dwarfs are wilder than the solar
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 be moody, grumpy, and even gloomy.
They can be quiet for a long time, but then they can flash violently, sending out a warning to any life that might take hold on a nearby planet.
Red dwarfs (M dwarfs) are the most common type of star in the Milky Way. That means most exoplanets orbit red dwarfs, not nice, well-behaved G-type stars like our sun. As astronomers study red dwarfs more closely, they have found that red dwarfs may not be the best stellar hosts when it comes to exoplanet habitability. Several studies have shown that red dwarfs can flare up violently, emitting enough radiation to render nearby planets uninhabitable even when firmly within 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 looked at 177 M dwarfs to better understand their long-term variability. The researchers found that red dwarf behavior is more complex than previously thought, and even the quietest red dwarfs are wilder than the Sun.
The study is titled “Characterization of M-dwarf stellar activity. I. Long-term variability in a large sample and detection of new cycles.” The work is published in the journal Astronomy and Astrophysics. The lead author is Lucile Mignon, postdoctoral researcher at the University of Grenoble Alpes and the French National Center for Scientific Research (CNRS).
All stars are variable to some degree. The Sun follows an 11-year cycle in which the number of sunspots on our star’s surface waxes and wanes. It’s all related to magnetic activity. But habitability depends on longer-term cycles. Life progresses in much longer periods of time than a few years. Life on Earth took billions of years to really get going.
This is 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 arose about 540 million years ago during the Cambrian Explosion. If life generally follows a similar time frame, could red dwarf variability prevent life from persisting?
Observing red dwarfs and drawing conclusions is a difficult challenge. Especially in the last few years we have been able to observe our sun in great detail. A fleet of spacecraft — including the Parker Solar Probe, Solar Orbiter, Solar and Heliospheric Orbiter, and others — is dedicated to detailed surveillance. We have also observed the sun and its activity over a long period of time.
An artist’s illustration of the Parker Solar Probe approaching the sun. Spacecraft like this mean we understand our Sun and its activity in much more detail than red dwarfs. Photo credit: NASA
Unfortunately, we could not observe individual red dwarfs over extremely long periods of time. Instead, the researchers have to be content with datasets that span several decades. In this new research, Mignon and her co-authors studied 177 M dwarfs observed by HARPS (High Accuracy Radial Velocity Planet Searcher) from 2003 to 2020. Activity on this timescale provides clues as to how these stars behave over longer periods of time.
HARPS is essentially a spectrograph, and the authors of this study collected chromospheric emissions for the red dwarfs. Chromospheric emissions come from a star’s magnetic field activity rather than its merger. Flaring is an artifact of magnetic activity, so studying flaring means studying a star’s chromosphere. In addition to the chromospheric emissions, the team also analyzed the photometric properties of the red dwarfs.
This artist’s illustration shows a red dwarf emitting extremely powerful X-rays. Astronomers want to know more about red dwarfs and their flares to see how it affects the potential habitability of exoplanets around red dwarfs. In this study, researchers examined the red dwarfs’ chromospheric activity for signs of variability. Photo credit: NASA’s Goddard Space Flight Center
The difficulty in studying red dwarf variability arises from our limited long-term data. “Unequivocally identifying a cycle requires measurements that show its repetition over multiple periods. This requires data that has been collected over a long period of time,” they explain.
In the absence of this, the researchers worked with the idea of what they call “seasons”. By identifying seasons for individual stars, they were able to better analyze the data. “We have these seasons as bins of 150 days (to average the rotational modulation as best as possible) with at least five observations (150 days is the typical maximum limit for the rotational period of M dwarfs) and gaps between observations of less than 40 days defined in a 150-day bin,” they explain.
This identified a subsample of 57 stars.
This number from the study shows the number of nights each star was observed and the time span of the observations. The blue stars are the 57-star subsample, and the red stars are the remaining stars. Photo credit: Mignon et al. 2023
The results show that variability is a defining trait among M-dwarfs. “We find that most stars are significantly variable, even the faintest stars,” the researchers wrote. “Most of the stars in our sample (75%) exhibit long-term variability, mainly manifested by linear or quadratic variability, although the true behavior can be more complex.” (Linear variability is simpler, while quadratic variability suggests a cycle. )
The researchers found cycles ranging from several years to more than 20 years in their sample. But they are quick to point out that their results have limitations and that their study is only a first step towards a better understanding of red dwarfs. For many of the stars, there is strong evidence that long-term variability exists. “…Better sampled stars might exhibit more complex behavior if sampled better,” they write. Nonetheless, their results “…indicate the strong presence of long-term variability and suggest that these stars exhibit strong long-term variability, which is important in searches for exoplanets.”
This number from the study is an example of some of the team’s findings. It shows the variability for a red dwarf named GJ 273, better known as Luyten’s star. One of its planets is in the star’s circumstellar habitable zone. Note the exponential time scale showing variability over longer periods of time. Photo credit: Mignon et al. 2023
There could be multiple layers of cycles and variability interacting with one another, making stellar behavior very difficult to decipher. Their puzzling behavior “… may be due to a complex underlying variability on different time scales simultaneously,” the authors write.
The researchers say they’ve made progress despite their limited data. “Even if time coverage is insufficient for some stars, our data can be used to estimate a minimum cycle length, if one exists.” But some conclusions are elusive for now. Their analysis “…is insufficient to guarantee that the signal is periodic or even quasi-periodic.”
A slam-dunk answer for red dwarf habitability is out of reach for now. It may be that, as this study suggests, there are so many differences between red dwarfs that they are forever unpredictable. But don’t bet against the science revealing more details.
Red dwarf flaring is well documented. The strongest stellar flare ever discovered comes from a red dwarf. In 2019, Proxima Centauri, a red dwarf and our nearest stellar neighbor, emitted a flare 14,000 times brighter than its pre-flare luminosity, and it took only a few seconds to flare that brightly. The exoplanet Proxima Centauri b is in the star’s potentially habitable zone, and such a bright flare could rule out the possibility of life or even liquid water on the planet. Even if Proxima Centauri flared up that brightly every million years or even longer, it could rule out the possibility of life.
The quest for life or habitability on other worlds inevitably involves a focus on red dwarfs. Their abundance means they need to be studied in more depth. It could result in many of the planets we think are habitable, like the well-known TRAPPIST-1 planets, simply getting too much radiation from their red dwarf hosts. The more variable they are, the less likely life is to persist or even thrive on exoplanets around red dwarfs.
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