Astronomers uncover risks to planets that could host life

A groundbreaking study has revealed that red dwarf stars can produce stellar flares that carry far-ultraviolet (far-UV) radiation levels much higher than previously believed. This discovery suggests that the intense UV radiation from these flares could significantly impact whether planets around red dwarf stars can be habitable. Led by current and former astronomers from the University of Hawaii Institute for Astronomy (IfA), the research was recently published in the Monthly Notices of the Royal Astronomical Society.

"Few stars have been thought to generate enough UV radiation through flares to impact planet habitability. Our findings show that many more stars may have this capability," said astronomer Vera Berger, who undertook the study while in the Research Experiences for Undergraduates program at IfA, an initiative supported by the National Science Foundation.

Berger and her team used archival data from the GALEX space telescope to search for flares among 300,000 nearby stars. GALEX is a now-decommissioned NASA mission that simultaneously observed most of the sky at near-and far-UV wavelengths from 2003 to 2013. Using new computational techniques, the team mined novel insights from the data.

"Combining modern computer power with gigabytes of decades-old observations allowed us to search for flares on thousands and thousands of nearby stars," said Michael Tucker, a PhD graduate of IfA and now a postdoctoral fellow at Ohio State University.

UV's double edge

According to researchers, UV radiation from stellar flares can either erode planetary atmospheres, threatening their potential to support life, or contribute to the formation of RNA building blocks, which are essential for the creation of life.

This study challenges existing models of stellar flares and exoplanet habitability, showing that far-UV emission from flares is on average three times more energetic than typically assumed, and can reach up to twelve times the expected energy levels.

"A change of three is the same as the difference in UV in the summer from Anchorage, Alaska to Honolulu, where unprotected skin can get a sunburn in less than 10 minutes," said Benjamin J. Shappee, an Associate Astronomer at IfA who mentored Berger.

Hidden causes

The exact cause of this stronger far-UV emission remains unclear. The team believes it might be that flare radiation is concentrated at specific wavelengths, indicating the presence of atoms like carbon and nitrogen.

"This study has changed the picture of the environments around stars less massive than our Sun, which emit very little UV light outside of flares," said Jason Hinkle, a PhD candidate at IfA who co-authored the study.

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The upside of volatile space weather

 

Giant solar flare illustration.

Although violent and unpredictable, stellar flares emitted by a planet's host star do not necessarily prevent life from forming, according to a new Northwestern University study.

Emitted by stars, stellar flares are sudden flashes of magnetic imagery. On Earth, the sun's flares sometimes damage satellites and disrupt radio communications. Elsewhere in the universe, robust stellar flares also have the ability to deplete and destroy atmospheric gases, such as ozone. Without the ozone, harmful levels of ultraviolet (UV) radiation can penetrate a planet's atmosphere, thereby diminishing its chances of harboring surface life.

By combining 3D atmospheric chemistry and climate modeling with observed flare data from distant stars, a Northwestern-led team discovered that stellar flares could play an important role in the long-term evolution of a planet's atmosphere and habitability.

"We compared the atmospheric chemistry of planets experiencing frequent flares with planets experiencing no flares. The long-term atmospheric chemistry is very different," said Northwestern's Howard Chen, the study's first author. "Continuous flares actually drive a planet's atmospheric composition into a new chemical equilibrium."

"We've found that stellar flares might not preclude the existence of life," added Daniel Horton, the study's senior author. "In some cases, flaring doesn't erode all of the atmospheric ozone. Surface life might still have a fighting chance."

The study will be published on Dec. 21 in the journal Nature Astronomy. It is a joint effort among researchers at Northwestern, University of Colorado at Boulder, University of Chicago, Massachusetts Institute of Technology and NASA Nexus for Exoplanet System Science (NExSS).

Horton is an assistant professor of Earth and planetary sciences in Northwestern's Weinberg College of Arts and Sciences. Chen is a Ph.D. candidate in Horton's Climate Change Research Group and a NASA future investigator.

Importance of flares

All stars -- including our very own sun -- flare, or randomly release stored energy. Fortunately for Earthlings, the sun's flares typically have a minimal impact on the planet.

"Our sun is more of a gentle giant," said Allison Youngblood, an astronomer at the University of Colorado and co-author of the study. "It's older and not as active as younger and smaller stars. Earth also has a strong magnetic field, which deflects the sun's damaging winds."

Unfortunately, most potentially habitable exoplanets aren't as lucky. For planets to potentially harbor life, they must be close enough to a star that their water won't freeze -- but not so close that water vaporizes.

"We studied planets orbiting within 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. On the flip side, M and K dwarf stars are thought to have more frequent flaring activity than our sun, and their tidally locked planets are unlikely to have magnetic fields helping deflect their stellar winds."

Chen and Horton previously conducted a study of M dwarf stellar systems' long term climate averages. Flares, however, occur on an hours- or days-long timescales. Although these brief timescales can be difficult to simulate, incorporating the effects of flares is important to forming a more complete picture of exoplanet atmospheres. The researchers accomplished this by incorporating flare data from NASA's Transiting Exoplanet Satellite Survey, launched in 2018, into their model simulations.

Using flares to detect life

If there is life on these M and K dwarf exoplanets, previous work hypothesizes that stellar flares might make it easier to detect. For example, stellar flares can increase the abundance of life-indicating gasses (such as nitrogen dioxide, nitrous oxide and nitric acid) from imperceptible to detectable levels.

"Space weather events are typically viewed as a detriment to habitability," Chen said. "But our study quantitatively shows that some space weather can actually help us detect signatures of important gases that might signify biological processes."

This study involved researchers from a wide range of backgrounds and expertise, including climate scientists, exoplanet scientists, astronomers, theorists and observers.

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Studying the sun as a star to understand stellar flares and exoplanets

 New research shows that sunspots and other active regions can change the overall solar emissions. The sunspots cause some emissions to dim and others to brighten; the timing of the changes also varies between different types of emissions. This knowledge will help astronomers characterize the conditions of stars, which has important implications for finding exoplanets around those stars.

An international research team led by Shin Toriumi at the Japan Aerospace Exploration Agency added up the different types of emissions observed by a fleet of satellites including "Hinode" and the "Solar Dynamics Observatory" to see what the Sun would look like if observed from far away as a single dot of light like other stars.

The team investigated how features like sunspots change the overall picture. They found that when a sunspot is near the middle of the side of the Sun facing us, it causes the total amount of visible light to dim. In contrast, when the sunspots are near the edge of the Sun the total visible light brightens because at that viewing angle bright structures known as faculae surrounding the sunspots are more visible than the dark centers.

In addition, X-rays which are produced in the corona above the solar surface grow brighter when a sunspot is visible. The coronal loops extending above the sunspots are magnetically heated, so this brightening appears before the sunspot itself rotates into view and persists even after the sunspot has rotated out of view.

Because the changes in the overall solar emissions and their timings carry information about the location and structure of features on the surface of the Sun, astronomers hope to be able to deduce the surface features of other stars such as starspots and magnetic fields. This will help astronomers to better recognize dimming caused by the shadow of an exoplanet. With better knowledge about the effects of starspots, we can estimate the parameters, such as the radii and orbits, of exoplanets more accurately.

As in-depth investigations into the Sun proceed, a better understanding of the detailed mechanisms of atmospheric heating and flare eruptions will be gained. Toriumi comments, "To this end, the next-generation solar-observing satellite Solar-C(EUVST), being developed by Japan in close collaboration with US and European partners, aims to observe the Sun in emissions that probe the chromosphere, transition region, and corona as a single system."

From Science Daily