Sun-like Stars and the Threat of Superflares
An international study involving the University of Graz reveals how frequently Sun-like stars are prone to powerful radiation outbursts.
The findings cast new light on potential risks to Earth — and emphasize the importance of accurate space weather forecasting.
The Sun governs the daily rhythms of all living organisms, and the solstices in summer and winter are celebrated in diverse cultural traditions around the world. From a distance of 150 million kilometers, our star appears rather peaceful. Yet upon closer inspection, the Sun reveals itself to be a highly temperamental star — as evidenced by the unusually intense solar storms of the past year, which manifested in striking auroras visible even at low latitudes.
But can our star also become a true cosmic fury?
Evidence for the most extreme solar "tantrums" can be found in prehistoric tree rings and samples of ancient glacial ice. However, the frequency of such superflares cannot be reliably determined from these indirect records. Direct measurements of the radiation flux reaching Earth from the Sun have only been available since the dawn of the space age.
Another approach to investigating the Sun’s long-term behavior — as demonstrated in the current study — is to look to the stars. Modern space telescopes observe hundreds of thousands of stars and record their variations in brightness.
Superflares, which release more than quadrillions of joules of energy within a short time, are identified in observational data by brief, extremely intense brightness peaks in visible light.
In a recent study, the team led by Sami Solanki, Director at the Max Planck Institute for Solar System Research (MPS), analyzed measurements from 56,450 Sun-like stars observed by NASA’s Kepler space telescope between 2009 and 2013.
“We cannot observe the Sun over thousands of years,” explains Solanki, co-author of the study.
“But we can monitor the behavior of thousands of Sun-like stars over short time periods. That helps us assess how frequently superflares occur,” he adds.
The research team included scientists from the University of Oulu (Finland), the National Astronomical Observatory of Japan, the University of Colorado Boulder (USA), the French Alternative Energies and Atomic Energy Commission (CEA) Paris-Saclay, the University of Paris-Cité, and the University of Graz.
Distant Stars as Proxies for the Sun
Alexander Shapiro, recently appointed Professor of Astrophysics at the University of Graz, emphasizes the study’s relevance for the institution:
“The investigation of solar activity and solar eruptions has been a focus of astrophysical research at the University of Graz for over two decades. Recently, this research has expanded to include distant stars, which can provide missing pieces of the puzzle in our quest to understand the Sun and the solar-terrestrial connection.
The Kepler telescope data used in this study cumulatively represent 220,000 years of stellar activity.”
The key to this research lies in the careful selection of target stars — those exhibiting particularly close "kinship" with the Sun.
Only stars with surface temperatures and luminosities similar to the Sun’s were included. Moreover, the researchers eliminated numerous potential sources of error, such as cosmic rays, passing asteroids or comets, and non-solar-type stars that might coincidentally appear near a Sun-like star in the telescope’s field of view.
To ensure reliability, the team meticulously examined the small-pixel images of each candidate superflare and included only those events that could be clearly attributed to one of the selected stars.
Through this approach, the researchers identified 2,889 superflares on 2,527 of the 56,450 stars analyzed. This suggests that, on average, a Sun-like star emits one superflare per century.
More Frequent Than Expected
“We were very surprised by how frequently Sun-like stars tend to produce such massive radiation outbursts,” says Valeriy Vasilyev of the MPS, lead author of the study.
Previous surveys by other research groups had estimated occurrence rates of once every thousand to ten thousand years. However, those earlier studies could not always pinpoint the exact source of the observed brightness flare and had to exclude stars that appeared too close to other objects in the telescope's view. The current study is likely the most precise and sensitive to date.
Larger average recurrence intervals of extreme solar events had also been suggested by Earth-based studies looking for signs of strong solar storms. When a particularly intense flux of energetic solar particles enters Earth’s atmosphere, it generates measurable quantities of radioactive isotopes — such as carbon-14 (¹⁴C) — which are stored in "natural archives" like tree rings and glacial ice. Even millennia later, modern measurement techniques allow scientists to detect such solar particle events through ¹⁴C anomalies.
Using this method, scientists have identified five extreme particle events and three candidates within the last 12,000 years. The most intense is believed to have occurred in 775 CE. However, it remains quite possible that more superflares and extreme particle outbursts occurred in the past.
“It is unclear whether massive radiation outbursts are always accompanied by particle events, or how the two phenomena are related. More research is needed,” cautions co-author Ilya Usoskin from the University of Oulu.
Consequently, estimates of superflare frequency based on terrestrial evidence may understate the true occurrence rate.
Forecasting Space Weather
Although the new study cannot predict when the Sun’s next major outburst might occur, its results are a clear warning.
“The new figures are a strong reminder that even the most extreme solar storms are part of the Sun’s natural behavior,” says co-author Natalie Krivova of the MPS.
During the Carrington Event of 1859 — one of the most intense solar storms in the past 200 years — telegraph systems across large parts of North America and Northern Europe failed.
Yet the associated radiation release is estimated to have delivered only one hundredth of the energy of a true superflare.
Today, such an event would not only threaten ground-based infrastructure but also pose serious risks to satellites in orbit.
Therefore, reliable and timely forecasting is the most important form of preparedness. For example, satellites can be temporarily shut down to protect them.
Beginning in 2031, the ESA’s Vigil mission aims to assist in this effort. From its unique side-on vantage point in space, Vigil will monitor the Sun and detect brewing solar activity earlier than Earth-based or Earth-orbiting observatories.
The Polarimetric and Magnetic Imager, developed by the MPS, is currently under construction for this mission.