By looking for stars that temporarily dim, researchers have spotted 21 instances of plasma being thrown from the stars' outer layers, more than all previous observations of stars besides the sun.

The dimming technique, published April 22 in Nature Astronomy, sets the stage for understanding how coronal mass ejections can affect the habitability of exoplanets, as the superhot plasma could strip the atmospheres of otherwise life-friendly planets.

Coronal mass ejections emerge from a star's corona, its plasmatic outer layer that is many times hotter than its surface. Often following bright flashes of radiation known as solar flares, the sun spews large amounts of its corona into the rest of the solar system during rearrangements of its magnetic field. These ejections can pose threats to satellite technology and the safety of space-traveling humans. 

Other stars also release coronal mass ejections, but they are much more difficult to measure from Earth. Scientists normally block out the body of the sun to observe its corona — similar to how the corona is visible along the edges of the moon during a solar eclipse — but the approach won't work on other stars because they are too far away, said Astrid Veronig, a professor of astrophysics at the University of Graz in Austria and a lead author of the paper.

Fewer than 20 coronal mass ejections have been detected in previous research, Veronig said. One pre-existing method is detecting the Doppler shift of an ejection moving toward Earth, as its electromagnetic radiation should appear to have a higher frequency than the star from which it emerged.

To create a "more direct approach" for detecting stellar coronal mass ejections, the study's authors began by analyzing the extreme ultraviolet light emitted by the sun during its own ejections. They saw that these emissions suddenly decreased in the corona after a coronal mass ejection occurred alongside a solar flare.

This dimming is caused by the corona becoming less massive and subsequently emitting less light, according to Veronig. The corona eventually gets replenished by lower layers of the star.

"We see the increase by the flare, which is like an hour," she said, "but then later this diminishing by this coronal dimming takes much longer because the corona needs time to refill, so they have this decrease in emission, which lasts for several hours."

Using their solar observations as a benchmark, the researchers used a similar approach to analyze observations of 201 stars that are similar to the sun or cooler in temperature. By searching for flares followed by dimming in extreme ultraviolet light and X-rays, they found evidence of 21 coronal mass ejections at 13 stars, more than doubling the total of observed stellar ejections.

Following the ejections, the stars' X-ray and ultraviolet brightness decreased by between 5% to 56%, which is an order of magnitude more dramatic than the sun's dimming periods, according to the authors. 

Understanding coronal mass ejections may also be important to judging which planets outside the solar system are fit for life, according to Veronig. With more than 4,000 exoplanets discovered, judging their habitability requires taking into account whether they experience "energetic outbursts so strong that they would basically destroy the atmosphere of the planet" and prevent them from hosting liquid water, she said.

Two of the dimming events occurred at Proxima Centauri, the closest star to Earth other than the sun, including the ejection that caused a 56% decrease in brightness. The star has exoplanets and even a "starspot" cycle similar to the sun's sunspots, where coronal mass ejections often originate, so it is a good candidate for studying ejections and their interactions with planets, Veronig said.

The study is a proof-of-concept for the dimming method, Veronig said, and many more observations need to be made to identify patterns in coronal mass ejections of other stars. She and her co-authors said their method could be strengthened by more sensitive extreme ultraviolet and X-ray measurements and by using other wavelengths of light.

The study, "Indications of stellar coronal mass ejections through coronal dimmings," published April 22 in Nature Astronomy, was authored by Astrid Veronig, Petra Odert, Martin Leitzinger and Nikolaus Fleck, University of Graz; Karin Dissauer, University of Graz and NorthWest Research Associates; and Hugh Hudson, University of Glasgow and University of California, Berkeley.