Artist rendition of DG CVn, a binary star system made up of two red dwarf stars. Illustration shows it unleashing powerful flares that could impact planet habitability. (Credit: NASA)
In a nutshell
- Stellar flares emit 3-12 times more far-ultraviolet radiation than previously thought, based on analysis of 182 flares from 158 nearby stars
- Higher UV levels could have dual effects: potentially kickstarting chemical reactions needed for life while also possibly destroying protective atmospheric layers on planets
- Red dwarf stars, which make up 75% of our galaxy’s stars, are particularly prone to these intense flares, complicating the search for habitable planets around them
HONOLULU — When stars have tantrums, they don’t just get a little brighter, they blast out intense ultraviolet radiation that could make or break the chances for life on nearby planets. A study analyzing data from NASA’s Galaxy Evolution Explorer (GALEX) telescope has found that these stellar outbursts, called flares, emit far more high-energy ultraviolet light than scientists previously thought, a finding that could reshape our understanding of where life might exist in the universe.
Scientists typically estimated flare radiation using a simplified model that assumed flares behave like extremely hot objects heated to 9,000 Kelvin (about 15,740°F). Under this assumption, flares should produce more low-energy ultraviolet light (near-UV) than high-energy ultraviolet light (far-UV). However, when researchers from Hawaii examined real stellar flares, they discovered the opposite pattern. Results, published in Monthly Notices of the Royal Astronomical Society, reveal that, on average, flares produce three times more far-UV radiation than expected, with peak values reaching up to 12 times the predicted levels.
A research team led by astronomer Vera Berger analyzed 182 flares from 158 stars within 100 parsecs (about 326 light-years) of Earth. Using innovative computational techniques, they mined through GALEX’s decade of observations from 2003 to 2013, examining data from nearly 300,000 nearby stars.
“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,” says study co-author Benjamin J. Shappee, in a statement.
NASA/Ames Research Center/Daniel Rutter)
The study focused particularly on red dwarf stars, cosmic lightweights that are smaller and cooler than our Sun. These stars make up about 75% of all stars in our galaxy, making them the most common type of stellar neighbor. Despite their small size, red dwarfs can remain active for billions of years longer than Sun-like stars, with younger red dwarfs frequently subjecting their planets to powerful flares.
“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,” says co-author Jason Hinkle, a Ph.D. candidate at the University of Hawaii Institute for Astronomy.
Red dwarfs interest astronomers because their “habitable zones,” regions where liquid water could exist on a planet’s surface, are much closer to the star than in our solar system. This proximity makes any planets in these zones more vulnerable to the effects of stellar flares. A planet orbiting in a red dwarf’s habitable zone might receive hundreds or thousands of times more UV radiation during a flare than Earth receives from the Sun.
This abundance of far-UV radiation plays a complex role in the potential for life beyond Earth. Like a cosmic spark plug, far-UV light can help drive the chemical reactions needed to create RNA building blocks, essential components for life. However, too much can also destroy protective ozone layers in planetary atmospheres, potentially exposing any surface life to harmful radiation.
“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,” says Berger, who conducted the research while in the Research Experiences for Undergraduates program at the University of Hawaii.
The exact cause of this stronger far-UV emission remains unclear. The researchers suggest it might result from flare radiation being concentrated at specific wavelengths, indicating the presence of atoms like carbon and nitrogen in the stellar atmosphere. This concentration effect could explain why the far-UV radiation is so much more intense than previously predicted models.
Space telescopes currently searching for potentially habitable worlds, such as NASA’s Transiting Exoplanet Survey Satellite (TESS), might need to factor in these higher UV radiation levels when assessing planetary habitability. The study suggests that planets we once thought might be suitable for life could face more challenging conditions than previously believed.
The research also highlights the importance of continued UV observations of stellar flares. Current space telescopes primarily observe stars in visible light, missing crucial information about their UV output. Future missions specifically designed to study stellar UV radiation could help scientists better understand these powerful events and their impact on planetary habitability.
By revealing the true intensity of ultraviolet radiation from stellar flares, it fundamentally changes our understanding of the conditions that distant planets face. Astronomers may need to rethink which worlds might harbor the right conditions for life.
Paper Summary
Methodology
The researchers used GALEX’s unique capability to simultaneously observe both FUV (1350-1750 Å) and NUV (1750-2750 Å) light with high time resolution. They selected stars from the Gaia Catalogue of Nearby Stars, focusing on those within 100 parsecs (about 326 light-years) of Earth. To identify flares, they looked for sudden brightness increases that stood out from the background stellar activity. The team carefully filtered out contaminated data and required that flares be visible in both UV bands to be included in their analysis.
Results
The study found that 98% of observed flares showed higher FUV/NUV ratios than predicted by the standard 9,000 K blackbody model. The median ratio of FUV to NUV energy was 0.50, about three times higher than expected. At peak brightness, some flares showed FUV/NUV ratios up to 12.6 times higher than the model predicted. The team also found tentative correlations between FUV output and both stellar type and flare energy.
Limitations
The research was limited to stars bright enough for GALEX to detect and could only observe flares that produced detectable signals in both UV bands. The team also notes that their method of identifying flare peaks using NUV data might slightly bias against NUV-faint events. Additionally, the study couldn’t definitively determine whether the excess FUV radiation came from thermal processes or line emission.
Discussion and Takeaways
These findings suggest that the standard approach of modeling flares as simple blackbodies needs revision. The excess FUV radiation could significantly affect calculations of “abiogenesis zones” where flares might help create conditions favorable for life, as well as “ozone depletion zones” where flares might make planets uninhabitable. The results also highlight the importance of obtaining detailed UV spectra of flares to better understand their emission mechanisms.
Funding and Disclosures
The research was supported by multiple institutions and grants, including the Research Experience for Undergraduates program at the Institute for Astronomy, University of Hawaii-Manoa (NSF grant #2050710), NASA grant 80NSSC21K0136, and various other NSF grants. The lead author was supported by a Churchill Scholarship.
Publication Information
This study was published in Monthly Notices of the Royal Astronomical Society, Volume 532, Pages 4436-4445 (2024), titled “Stellar flares are far-ultraviolet luminous” by Vera L. Berger et al. The paper became available through advance access on August 5, 2024, and was originally submitted on December 16, 2023.
