Flaring Stars and Fragile Worlds: Can Planets Around Red Dwarfs Be Habitable?
This white paper, led by first author Rebecca Szabó, explores whether planets orbiting highly active, flaring stars can truly be considered habitable. The motivation comes from the rapid growth in known exoplanets that meet the basic requirement for habitability: having temperatures that allow liquid water to exist on their surfaces. Many of these planets orbit cool, low-mass stars known as M-dwarfs, which are abundant near the Sun and easier to study than Sun-like stars. However, M-dwarfs are also known for strong ultraviolet (UV) and X-ray activity, raising concerns about whether their planets can retain atmospheres and support life. The authors argue that answering this question requires studying stellar flares on a much larger scale than is currently possible.
Exoplanet Habitability and the Habitable Zone
The paper begins by introducing the concept of the habitable zone (HZ), defined as the range of distances from a star where a planet’s equilibrium temperature allows liquid water on its surface. While this definition is simple, the authors stress that many additional factors, such as planet mass, atmospheric composition, and stellar activity, affect real habitability. M-dwarfs are particularly interesting because their HZs lie close to the star, making planets easier to detect and monitor. Future missions like Gaia and PLATO are expected to greatly expand the number of known potentially habitable planets. Yet the close-in nature of these orbits also exposes planets to intense stellar flares, which may strip atmospheres or damage surface conditions.
Stellar Flares and Superflares
The authors then explain what stellar flares are: sudden releases of magnetic energy that produce bursts of radiation across the electromagnetic spectrum, including extreme ultraviolet and X-rays. While the Sun produces flares, other stars can generate much more energetic “superflares.” Observations over the past decade have shown that even single, Sun-like stars can experience such events, raising questions about how common extreme flares really are and how often planets may be exposed to them. Historical evidence from Earth, such as so-called Miyake events seen in tree rings, suggests that very strong solar events may have occurred in the past, reinforcing concerns about flare impacts on planetary environments .
Planetary Environments and the Potential for Life
Next, the paper focuses on how stellar flares affect planetary atmospheres and the potential for life. High-energy flares can deplete protective ozone layers, allowing harmful UV radiation to reach the surface. The authors cite studies showing that repeated flares with energies around 10³⁴ erg could destroy nearly all ozone on an Earth-like planet in the HZ of an M-dwarf. At the same time, flares may play a positive role by driving prebiotic chemistry. The concept of an “abiogenesis zone” is introduced: a region where UV radiation from flares provides enough energy to trigger chemical reactions important for the origin of life. Whether flares help or hinder life therefore depends on their energies, frequencies, and spectral properties.
What We Learn from Solar and Stellar Flares
The discussion then turns to what astronomers know from studying solar and stellar flares. Solar flares have been observed in great detail using spectroscopy and space-based monitoring, leading to a solid understanding of their properties. In contrast, spectroscopic observations of flares on other stars are still limited. While large surveys and amateur observations provide photometric data, detailed spectral information is rare. Existing studies show that emission in certain chromospheric lines signals flare activity, but much more data are needed to link flare behavior to stellar type and age in a statistically meaningful way.
Open Questions and a Path Forward
Finally, the authors outline key open questions and propose a path forward. They argue that understanding flare statistics and their biological impact requires monitoring thousands of stars with high time resolution. A future facility like the Wide Field Survey Telescope would allow astronomers to capture short-lived flares and measure their spectra in detail. By combining large surveys with rapid, flexible follow-up observations, researchers could determine how common dangerous flares are and whether planets around flaring stars can maintain conditions suitable for life. The paper concludes that only with such large-scale, coordinated observations can the habitability of planets around active stars be reliably assessed.
Source: Szabó
