The Sun frequently ejects high-energy bursts of plasma that then travel through interplanetary space. These so-called coronal mass ejections (CMEs) are accompanied by strong magnetic fields, which, when they interact with the Earth’s atmosphere, can trigger solar storms that can severely damage satellite systems and power grids.

In the early days of the solar system, the Sun was far more active than it is today and ejected much bigger CMEs. These might have been energetic enough to affect our planet’s atmosphere and therefore influence how life emerged and evolved on Earth, according to some researchers.

Since it is impossible to study the early Sun, astronomers use proxies – that is, stars that resemble it. These “exo-suns” are young G-, K- and M-type stars and are far more active than our Sun is today. They frequently produce CMEs with energies far larger than the most energetic solar flares recorded in recent times, which might not only affect their planets’ atmospheres, but may also affect the chemistry on these planets.

Until now, direct observational evidence for eruptive CME-like phenomena on young solar analogues has been limited. This is because clear signatures of stellar eruptions are often masked by the brightness of their host stars and flares on these. Measurements of Doppler shifts in optical lines have allowed astronomers to detect a few possible stellar eruptions associated with giant superflares on a young solar analogue, but these detections have been limited to single-wavelength data at “low temperatures” of around 10⁴ K. Studies at higher temperatures have been few and far between. And although scientists have tried out promising techniques, such as X-ray and UV dimming, to advance their understanding of these “cool” stars, few simultaneous multi-wavelength observations have been made.

A large Carrington-class flare from EK Draconis

On 29 March 2024, astronomers at Kyoto University in Japan detected a large Carrington-class flare – or superflare – in the far-ultraviolet from EK Draconis, a G-type star located approximately 112 light-years away from the Sun. Thanks to simultaneous observations in the ultraviolet and optical ranges of the electromagnetic spectrum, they say they have now been able to obtain the first direct evidence for a multi-temperature CME from this young solar analogue (which is around 50 to 125 million years old and has a radius similar to the Sun).

The researchers’ campaign spanned four consecutive nights from 29 March to 1 April 2024. They made their ultraviolet observations with the Hubble Space Telescope and the Transiting Exoplanet Survey Satellite (TESS) and performed optical monitoring using three ground-based telescopes in Japan, Korea and the US.

They found that the far-ultraviolet and optical lines were Doppler shifted during and just before the superflare, with the ultraviolet observations showing blueshifted emission indicative of hot plasma. About 10 minutes later, the optical telescopes observed blueshifted absorption in the hydrogen Hα line, which indicates cooler gases. According to the team’s calculations, the hot plasma had a temperature of 100 000 K and was ejected at speeds of 300–550 km/s, while the “cooler” gas (with a temperature of 10 000 K) was ejected at 70 km/s.

“These findings imply that it is the hot plasma rather than the cool plasma that carries kinetic energy into planetary space,” explains study leader Kosuke Namekata. “The existence of this plasma suggests that such CMEs from our Sun in the past, if frequent and strong, could have driven shocks and energetic particles capable of eroding or chemically altering the atmosphere of the early Earth and the other planets in our solar system.”

“The discovery,” he tells Physics World, “provides the first observational link between solar and stellar eruptions, bridging stellar astrophysics, solar physics and planetary science.”

Looking forward, the researchers, who report their work in Nature Astronomy, now plan to conduct similar, multiwavelength campaigns on other young solar analogues to determine how frequently such eruptions occur and how they vary from star to star.

“In the near future, next-generation ultraviolet space telescopes such as JAXA’s LAPYUTA and NASA’s ESCAPADE, coordinated with ground-based facilities, will allow us to trace these events more systematically and understand their cumulative impact on planetary atmospheres,” says Namekata.

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