One clear radio signal has at last proved that a star other than our Sun has the ability to launch a genuine coronal mass ejection into space. This is important because it takes a hypothesis that has long been assumed to be true in stellar astrophysics and turns it into a measurable risk: some of the most common types of planet-hosting stars have the ability to launch a blast of energy that could overwhelm the defenses of nearby worlds.
The star, StKM 1-1262, is a cool M dwarf at a distance of 130 light-years, and the observation leveraged a combination that has grown ever more powerful in the modern era of astronomy: a radio array on the ground and an X-ray observatory in space. The Low Frequency Array, LOFAR, recorded a brief, bright type II radio outburst, a type of event that occurs when a shock wave from an eruption pushes through the outer atmosphere of a star and into interplanetary space. The X-ray follow-up observation with XMM-Newton confirmed the nature of the star and its level of activity, revealing an object whose rotation and magnetism are far removed from the Sun’s more tranquil state.
“This type of radio signal simply would not be present unless material had completely escaped the star’s ‘bubble’ of strong magnetic fields,” said Joe Callingham of the Netherlands Institute for Radio Astronomy. The key to this observation is that it involves a type II burst, which is directly associated with an outward-moving shock, rather than simply a localized flare. In this instance, the estimated ejection velocity was 2,400 kilometers per second, which is among the most extreme when compared to solar flares.
The material consequences are a direct result of this speed. When traveling at thousands of kilometers per second, a strong CME can accelerate particle radiation and dynamic pressure to sufficiently high levels to compress a planet’s magnetosphere strongly, shrinking the region in which an atmosphere can be hidden from the solar wind by magnetic protection. “Sometimes you only need one of these big guys to come your way,” said Julián Alvarado-Gómez of the Leibniz Institute for Astrophysics Potsdam.
In the case of M dwarfs, the geometry of habitability adds to the problem.
Because these stars are cool, the temperate “Goldilocks” zone is close in, where planets are more likely to be affected by eruptions, winds, and high-energy radiation. StKM 1-1262 is known to have a magnetic field 300 times stronger than the Sun’s and to be rotating much faster, which is conducive to strong magnetic activity. While it is easier to spot planets that are close in, they are also in the direct path of destructive space weather.
The detection has been the challenge. Solar CMEs are regularly observed using coronagraphs, but this method is not feasible for other stars. However, researchers have looked for signs that scale with radio shock waves, X-ray context, and other wavelength-dependent signatures that indicate the mass being released from the corona. The new finding illustrates how better signal processing and wide-field surveys can turn a weak and brief radio flash into a physically meaningful event; this study utilized data from the LOFAR Two-metre Sky Survey, which has been monitoring tens of thousands of nearby stars.
The next step is statistics. The sensitivity of LOFAR is near the limit for this purpose, which is why there has been interest in the Square Kilometre Array, which aims to make “firsts” into catalogs. There would need to be enough detections to allow the comparison of CMEs for different types and ages of stars and to relate these to what telescopes like the James Webb Space Telescope observe when they determine if small planets orbiting M dwarfs have any atmosphere at all.
