Stars that resemble our Sun experience massive bursts of radiation approximately every century. These spectacular events, known as superflares, unleash energy surpassing that of a trillion hydrogen bombs, dwarfing all previously noted solar flares. This finding is grounded in a study of 56,450 sun-like stars conducted by a team from the Max Planck Institute for Solar System Research (MPS) in Germany, which will be published on December 13, 2024, in the journal Science. The research indicates that earlier investigations have significantly misjudged the explosive potential of these stars. Data collected from NASA’s Kepler space telescope reveals that sun-like stars are likely to exhibit superflaring occurrences ten to one hundred times more often than earlier estimates suggested. The Sun itself is possibly capable of similar explosive events.
Undoubtedly, the Sun exhibits unpredictable behavior, a fact emphasized by this year’s exceptionally strong solar storms. Some of these storms even produced stunning auroras at low latitudes. But could our star become even more volatile? Evidence of the most intense solar “outbursts” can be traced back to ancient tree rings and samples from glacial ice that date back thousands of years. However, these indirect methods do not allow for a reliable determination of superflare frequency. Direct measurements of radiation received from the Sun have only been available since the dawn of the space age.
Another approach to understanding our Sun’s long-term activity involves studying other stars, which is the focus of the new research. Advanced space telescopes monitor the brightness changes of thousands of stars in visible light. During superflares, which release over one octillion joules of energy in a brief time, there is a noticeable surge in brightness. “We cannot observe the Sun over thousands of years,” explained Prof. Dr. Sami Solanki, Director at the MPS and one of the study’s authors. “However, we can observe the behaviors of numerous stars similar to the Sun over shorter periods, which helps us estimate the frequency of superflares.”
Searching for the Sun’s close kin
In this study, researchers from multiple institutions, including the University of Graz (Austria) and the University of Oulu (Finland), examined data from 56,450 sun-like stars collected by NASA’s Kepler telescope between 2009 and 2013. “Altogether, the Kepler data provide us with evidence spanning 220,000 years of stellar activity,” noted Prof. Dr. Alexander Shapiro from the University of Graz.
Key to the research was the careful selection of stars to study. Only those with surface temperatures and brightness similar to the Sun were included. Researchers also eliminated potential sources of error, such as cosmic radiation and nearby non-sun-like stars that could falsely appear to flare near a sun-like star in the Kepler images. Each potential superflare, typically just a few pixels large, was meticulously analyzed, with only events that could be confidently attributed to the selected stars counted.
Through this process, the team discovered 2,889 superflares occurring on 2,527 of the observed stars, indicating that a sun-like star experiences a superflare roughly once every century.
“Advanced dynamo simulations of these solar-type stars effectively elucidate the magnetic processes that lead to the significant energy release during superflares,” stated coauthor Dr. Allan Sacha Brun from the Commissariat of Atomic and Alternative Energies of Paris-Saclay and the University of Paris-Cité.
Surprisingly common
“We were astonished to find that sun-like stars are so frequently prone to superflares,” said lead author Dr. Valeriy Vasilyev from the MPS. Previous studies had typically estimated that such events occurred every thousand to ten thousand years. Earlier research, unable to pinpoint the exact sources of flares, focused on stars without too many close neighbors in their images. This current study is the most detailed and sensitive to date.
In contrast, previous studies seeking evidence of intense solar storms affecting Earth suggested longer intervals between extreme solar events. When a significant surge of energetic particles from the Sun reaches Earth’s atmosphere, they generate detectable levels of radioactive isotopes such as carbon-14 (14C). These isotopes are stored in natural records like tree rings and glacial ice. Through modern technological methods, researchers can measure 14C levels to identify high-energy solar particle events that occurred thousands of years ago.
Utilizing this method, researchers found five major solar particle events and three additional possible occurrences over the last twelve thousand years of the Holocene, leading to an average rate of once every 1,500 years. The most significant of these is thought to have taken place in 775 AD. However, there may have been more of such violent events in the past, and it remains unclear whether these colossal flares always come with coronal mass ejections or how they correlate with extreme solar particle events, necessitating further investigation, as noted by co-author Prof. Dr. Ilya Usoskin from the University of Oulu in Finland. Thus, research into terrestrial evidence for past extreme solar events might underestimate the occurrence of superflares.
Anticipating hazardous space weather
Although this new study does not predict when the Sun will have its next outburst, it emphasizes the need for caution. “The new findings starkly remind us that even extreme solar events are part of the Sun’s regular behavior,” remarked coauthor Dr. Natalie Krivova from the MPS. During the Carrington event of 1859, one of the most severe solar storms in recent history resulted in widespread disruptions to the telegraph system across northern Europe and North America, and it is estimated that the energy released in that event was merely one-hundredth that of a superflare. Today, satellites are especially vulnerable alongside terrestrial infrastructure.
Thus, the best preparation for intense solar storms remains effective and timely forecasting. As a precautionary measure, satellites could potentially be deactivated. Starting in 2031, ESA’s space probe Vigil will contribute to this effort. Positioned in space, it will monitor the Sun from a lateral viewpoint, enabling earlier detection of any processes that could lead to hazardous space weather. The MPS is in the process of creating a Polarimetric and Magnetic Imager for this mission.
