Researchers pioneer new technique that could help determine habitability of planets

Our sun is surrounded by multi-million-degree plasma called the corona, beautifully visible during a total solar eclipse.

About once a day, a magnetic explosion on the sun will send a chunk of the corona hurtling into interplanetary space. This is called a coronal mass ejection (CME) and scientists believe that this almost certainly happens on other stars.

Average CMEs will produce aurorae - beautiful ribbons of colorful light that dance across the sky near the Earth's poles. The biggest CMEs are awesome, once-in-a-century events that can disrupt the electronics of satellites and even our electrical grid.

During a massive CME that hit Earth in the mid 1800s, electric currents literally shocked telegraph operators working at their stations and shut down the entire system. That CME also purportedly produced such a bright display of light that it woke people up in the middle of the night.

When CMEs, or showers of fast particles that spray out ahead of them, hit Earth, they also affect the chemistry of the atmosphere in ways we can't see, sometimes even destroying bits of the atmosphere's protective ozone.

"Even though CMEs appear obvious and dramatic when we see them through our sun-observing telescopes, CMEs from stars have proven very hard to detect," said R.O. Parke Loyd, a scientist at Eureka Scientific and previously an ASU postdoctoral research scholar in the School of Earth and Space Exploration.

Loyd, along with the help of a team of current and former researchers at ASU, including Evgenya Shkolnik, Tahina Ramiaramanantsoa, Tyler Richey-Yowell and Adam Schneider, and collaborators from several other institutions, have pioneered a new technique to measure the intensity of CMEs that will help to determine the habitability of other planets in our galaxy. Their findings were recently published in The Astrophysical Journal.

"Much like the topic of exoplanets 40 years ago, we are all but certain stellar CMEs are out there, waiting to be detected. And, like exoplanets, there have been a smattering of one-off candidate detections of stellar CMEs. The scientific community is still in search of definitive proof that stars other than the sun produce CMEs. We need methods to search for stellar CMEs that can more clearly indicate if one occurred and, if so, how big it was - how massive and how energetic," Loyd said.

As scientists, members of the team are also interested in the opposing question: If stars aren't producing CMEs, how is that proven? And what does either result say about the planets orbiting other stars?

"Our innovation is the development of a method to do both of these. The data in our pilot study enabled us to say that the sun-like star Epsilon Eri is, at the very least, not producing CMEs at a rate greater than about 10 times that of the sun. Applying this tool to new, broader and more extensive data will help us to understand how prevalent CMEs are across stars of varying size and age," Loyd said.

"One of the reasons this is so exciting and important is that most of the planets in our galaxy orbit red dwarf stars, but we don't yet know if these planets could end up as habitable as the Earth. Certainly there are plenty of red dwarf planets that could have the right surface temperature for liquid water, the basis for life," Loy said. "However, we suspect the CMEs from these stars are more intense. If they are, they could strip these planets of their atmosphere, and without an atmosphere, these planets could not have liquid surface water.

"Additionally, if they are directly exposed to the radiation from the CME, their surfaces would be harsh environments for life. Our tool is a step toward being able to finally measure the intensity of red dwarf CMEs, and all stars' CMEs, so we know whether their planets are in danger of losing their atmospheres or not."

As a proof of concept, Loyd and the team of scientists analyzed archival observations taken by the Hubble Space Telescope, including two sets of observations originally intended just for calibration of the young star, Epsilon Eridani, about 75% the size of the sun.

"The observations captured three clear flares, spikes in UV light that indicate a magnetic explosion occurred on the stellar surface, and our novel analysis enabled us to place first-of-their-kind limits on the amount of million-degree plasma that could have been ejected by CMEs accompanying those flares," Loyd said.

ASU was the setting where this work began in earnest through receipt of a grant from NASA and the Space Telescope Science Institute in 2019, but Loyd emphasizes it was a group effort.

"The project represents a broad collaboration of institutions. It was conceived at the University of Colorado, Boulder, spun up at ASU and completed at Eureka Scientific, Inc. In addition, it includes significant contributions from other researchers at ASU, CU's Laboratory for Atmospheric and Space Physics, NASA's Goddard Space Flight Center, Lockheed Martin's Solar and Astrophysics Laboratory, and the Search for ExtraTerrestrial Intelligence (SETI) Institute."

What lies ahead? With the successful demonstration of this new method, scientists can start exploring its broader application to other data from the Hubble Telescope, X-ray observatories and even future space missions.

Research Report:Constraining the Physical Properties of Stellar Coronal Mass Ejections with Coronal Dimming: Application to Far-ultraviolet Data of epsilon Eridani

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