Deadly stellar radiation blasts ‘habitable’ exoplanets every few days

Planets orbiting M-class red dwarf stars have been suggested as some of the most promising places to look for alien life, but now it seems powerful outbursts from the stars could render them uninhabitable


26 September 2022

An artist’s impression of a flaring red dwarf star and a nearby planet


Planets once considered to be the most promising candidates to host life outside the solar system may actually be uninhabitable due to powerful stellar flares, according to the largest ever survey of planet-hosting stars. However, these high-energy blasts might still be useful to kick-start chemical processes for life.

Exoplanets discovered in the so-called habitable zone of their parent star, where the conditions aren’t too hot or too cold so that liquid water could exist on their surface, often have very short orbits, on the order of weeks or days. These stars, which tend to be much cooler and smaller than our sun, can release powerful electrically charged particles in eruptions called coronal mass ejections (CMEs).

CMEs from our own sun that are powerful enough to threaten Earth are relatively rare – the most powerful recorded ejection, the Carrington event, happened in 1859, and nothing has come close to that level since. But it appears that M-class red dwarfs — the stars that host most of the exoplanets we have found so far, like those of TRAPPIST-1 or SPECULOOS-2  — could be prone to a Carrington-level event every few days, or even every few hours, says Maximillian Günther at the European Space Agency.

This would be a problem for life on close-orbiting planets. “You’re drastically closer to the star and you get bombarded with these basically billions of nuclear bombs every couple of hours,” says Günther.

To map out these stellar flares, Günther and his team used a machine learning model to comb through data taken from more than 200,000 stars using the Transiting Exoplanet Survey Satellite (TESS), which watches the sky for the nearest and brightest stars. By looking for stars that changed in brightness over time, the team could separate out flaring and non-flaring stars.

The researchers found that 30 to 40 per cent of young or middle age M-dwarf stars are flaring, on fairly short timescales and at high energies, compared with 5 to 10 per cent of the hotter F, G or K classes of stars, which include our own sun. “Our sun is orders of magnitude less active, in terms of flaring,” Günther told the Europlanet Science Congress in Granada, Spain, on 22 September.

While these frequent and powerful stellar eruptions could strip planetary atmospheres or sterilise their surfaces, they might contain enough ultraviolet energy to kick-start prebiotic chemistry. Though TESS measures in visible and infrared light, Günther says that predicted energy levels in ultraviolet light, based on how our own sun works, could match those used successfully in prebiotic experiments done on Earth. Whether any life could then arise is unclear.

Alexandra Thompson at University College London says that the stellar flare catalogue provides an excellent overview of how energetic stellar flares may affect rocky, terrestrial exoplanets, and is useful for understanding their habitability. “Understanding the host star well is extremely important for both current and future missions,” she says.

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