Red dwarf stars are the most common type of stars in our neighborhood and probably in the Milky Way. Because of this, many of the Earth-like and potentially life-sustaining exoplanets we have discovered are in orbit around red dwarfs. The problem is that red dwarfs can display an intense flare that is much more energetic than our relatively calm sun.
What does this mean for the potential of these exoplanets to actually support life?
Most life on earth, and probably other worlds, relies on star energy to survive. The sun has been the engine of life on earth since the first cells were reproduced. But sometimes, like all stars, the sun acts and emits torches.
Sometimes it emits extremely energetic flares. The strong magnetic energy in the solar atmosphere becomes unstable and an enormous amount of energy is released. If released towards Earth, it can cause problems. This can lead to radio communication disruptions and even power outages.
But in terms of torch activity, the Sun is relatively weak compared to some other stars. Some stars, especially red dwarfs, can flare up frequently and violently. A team of researchers studied how torch activity affects the atmosphere and life potential of Earth-like planets orbiting low-mass stars, including M-type, K-type, and G-type stars.
In this image, the Solar Dynamics Observatory (SDO) captured a class X1.2 solar flare that peaked on May 15, 2013. Photo credit: NASA / SDO
The new study is called “Persistence of flare-driven atmospheric chemistry on rocky habitable zone worlds”. The lead author is Howard Chen, Ph.D. Student at Northwestern University. The paper is published in the journal Nature Astronomy.
“Our sun is more of a gentle giant,” said Allison Youngblood, an astronomer at the University of Colorado at Boulder and co-author of the study. “It’s older and not as active as younger and smaller stars. The earth also has a strong magnetic field that deflects the sun’s harmful winds. “
This explains why the earth “ripples” positively from life, as Carl Sagan described our planet. For planets orbiting low-mass stars like red dwarfs (M-dwarfs), however, the situation is very different.
“In some cases, the flaring does not erode all of the atmospheric ozone. Life on the surface might still have a chance. “
Daniel Horton, senior writer, Northwestern University
We know that solar flares and related ejections of coronal masses can severely damage the prospects for life on unprotected exoplanets. The authors write in their introduction that “stellar activity – which includes star flares, coronal mass ejections (CMEs), and star proton events (SPEs) – has a profound impact on a planet’s habitability, primarily through its effect on atmospheric ozone.”
A single flare here and there over time doesn’t have much effect. But many red dwarfs show more frequent and longer flaring.
Artist’s impression of a blazing red dwarf star orbited by an exoplanet. Photo credit: NASA, ESA and G. Bacon (STScI)
“We compared the atmospheric chemistry of planets with frequent flares to planets without flares. The long-term chemistry of the atmosphere varies widely, ”said Northwestern’s Howard Chen, first author of the study, in a press release. “Continuous torches are actually driving the atmospheric composition of a planet into a new chemical equilibrium.”
One of the things the team looked at was ozone, and the effects of flares did. Here on earth, our ozone layer protects us from UV radiation from the sun. Extreme flare activity on red dwarfs, however, can destroy ozone in the atmosphere of planets that orbit nearby. When the ozone level drops, a planet is less protected from UV radiation from its star. Strong UV radiation can reduce the risk to life.
In their study, the team used models to understand flaring and its effects on the atmosphere of exoplanets. They used flaming data from NASA’s TESS (Transiting Exoplanet Survey Satellite) and long-term data of the exoplanet climate from other studies. They found a few cases where ozone persisted despite being flared.
“We found that star flares may not preclude the existence of life,” added Daniel Horton, lead author of the study. “In some cases, the flaring does not erode all of the atmospheric ozone. Life on the surface might still have a chance. “
This figure from the study shows global mean vertical profiles of atmospheric species on a simulated planet around a sun-like star of type G. From left to right the mixing ratios for ozone, nitrous oxide, nitric acid and water vapor are given. Photo credit: Chen et al., 2020.
Planets that can at least potentially support life can find themselves in a difficult position. You need to be close enough to their stars to prevent their water from freezing, but not too close or they are too hot. But this dance with closeness can expose them to the mighty torches.
Red dwarfs are smaller and cooler than our sun, which means that the habitable zone for any planets orbiting them is smaller and much closer to the star than Earth is to the sun. Not only does this expose them to torches, it also causes planets to be tidally bound to their stars. The combination of flaring and tidal locking can be bad for life prospects. The Earth’s rotation creates its protective magnetosphere, but tidal-locked planets cannot create one and are largely unprotected from stellar UV radiation.
“We studied planets orbiting in the habitable zones of M and K dwarf stars – the most common stars in the universe,” Horton said. “Habitable zones around these stars are narrower because the stars are smaller and less powerful than stars like our sun. M and K dwarf stars, on the other hand, are believed to flare up more frequently than our Sun, and their tidal-locked planets are unlikely to have magnetic fields that help deflect their stellar winds. “
This illustration from the study shows how repeated star flaring can alter the atmospheric gases in a simulated Earth-like planet around a Sun-like star. Photo credit: Chen et al., 2020.
This study also has a more positive side. The team found that torchlight activities can actually help in the search for life. The flares can make it easier to see some gases that are biomarkers. In this case, they found that energy from flaring can highlight the presence of gases such as nitric acid, laughing gas, and laughing gas, all of which can be indicators of life processes.
This illustration from the study shows how repeated star flaring can affect atmospheric chemistry on a modeled Earth-like planet around a type K star. Note the increased levels of detectable NO, a potential bio-marker. Photo credit: Chen et al., 2020.
“Space weather events are usually viewed as a disadvantage to habitability,” said Chen. “However, our study has shown quantitatively that some space weather can actually help us identify signatures of important gases that could indicate biological processes.”
But only a few. In other cases, their work showed that flaring can destroy potential biosignatures from anoxic life. “Although we report on the 3D effects of star flares on oxidizing atmospheres, strong flares can have other unexpected effects on atmospheres with reducing conditions. For example, hydrogen oxide species originating from star flares could destroy important anoxic biosignatures such as methane, dimethyl sulfide and carbonyl sulfide61 and thereby suppress their spectroscopic properties, ”the authors report.
Another interesting finding from this study concerns exoplanet magnetospheres. They find that hyper-flares can help reveal the nature and extent of magnetospheres. “More speculatively, proton events during hyperflares can reveal the existence of magnetic fields on a planetary level by highlighting certain regions of the planet. By identifying fingerprints of nitrogen or hydrogen oxide emitting flows during magnetic storms and / or auroral precipitation events, it may be possible to determine the geometric extent of exoplanetary magnetospheres. “
Hyperflares could help us understand the extent of exoplanet magnetospheres by identifying the extent of nitric oxide flow fingerprints. Photo credit: Chen et al., 2020.
Other recent research has shown that exoplanets exposed to flaring, especially around red dwarf stars, are not great places to find life. The torch activity is too detrimental. However, this study shows that the situation is more complex.
Overall, it has been shown that flaring can in some cases contribute to the detection of biosignatures. It also shows that while flaring can disrupt the atmosphere of exoplanets, it returns to normal in many cases. It is also a fact that low-mass stars live much longer than stars like our sun, which means there is more time on their planets for life to evolve.
This new work shows how complicated the search for life is and how many variables are involved. And it contains at least one surprise. While flaring has largely been viewed as detrimental to exoplanet habitability, the fact that it can help detect biosignatures means that there is more going on than expected.
Artist’s impression of rocky exoplanets orbiting Gliese 832, a red dwarf star that is only 16 light years from Earth. Many of the Earth-like exoplanets we have discovered orbit red dwarf stars. Photo credit: ESO / M. Kornmesser / N. Risinger (skysurvey.org).
This research required the collaboration of scientists from many disciplines. It relied on climatologists, astronomers, observers and theorists and of course on exoplanet scientists.
“This project was the result of fantastic teamwork, said Eric T. Wolf, a planetary scientist at Boulder University and co-author of the study. Our work shows the benefits of interdisciplinary efforts in studying the conditions on extrasolar planets
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