How much does a star determine what its planets are like? And could that influence whether those worlds can support life? Carnegie's Luke Bouma is investigating a new way to tackle this question by using naturally occurring "space weather stations" that appear around some young stars. His findings are being presented this week at the American Astronomical Society meeting.
M dwarf stars are smaller, cooler, and dimmer than our Sun, yet most of them host at least one rocky planet about the size of Earth. Many of these worlds are not considered friendly to life. They may be too hot, lack stable atmospheres, or be exposed to frequent flares and intense radiation. Still, they offer valuable opportunities to study how stars influence the environments around their planets.
"Stars influence their planets. That's obvious. They do so both through light, which we're great at observing, and through particles -- or space weather -- like solar winds and magnetic storms, which are more challenging to study at great distances," Bouma explained. "And that's very frustrating, because we know in our own Solar System that particles can sometimes be more important for what happens to planets."
A New Way To Study Stellar Space Weather
Directly placing instruments around distant stars to measure space weather is not possible.
Or is it?
Bouma, working with Moira Jardine of the University of St Andrews, focused on an unusual class of M dwarfs known as complex periodic variables. These young stars spin rapidly and show repeated dips in brightness. Scientists were unsure whether those dips were caused by dark spots on the star or by material orbiting nearby.
"For a long time, no one knew quite what to make of these oddball little blips of dimming," Bouma said. "But we were able to demonstrate that they can tell us something about the environment right above the star's surface."
Plasma Rings Act Like Natural Space Weather Stations
To investigate further, the team created "spectroscopic movies" of one of these stars. Their analysis revealed that the dimming comes from large clouds of relatively cool plasma trapped within the star's magnetosphere. These plasma clumps are carried along by the star's magnetic field, forming a doughnut shaped structure called a torus.
"Once we understood this, the blips in dimming stopped being weird little mysteries and became a space weather station," Bouma exclaimed. "The plasma torus gives us a way to know what's happening to the material near these stars, including where it's concentrated, how it's moving, and how strongly it is influenced by the star's magnetic field."
Bouma and Jardine estimate that at least 10 percent of M dwarfs may have these plasma structures during their early stages. That means astronomers could use them to better understand how stellar particles influence planetary environments.
What This Means for Alien Worlds
Bouma's next goal is to determine where the material in the torus originates, whether it comes from the star itself or from an external source.
"This is a great example of a serendipitous discovery, something we didn't expect to find but that will give us a new window into understanding planet-star relationships," Bouma concluded. "We don't know yet if any planets orbiting M dwarfs are hospitable to life, but I feel confident that space weather is going to be an important part of answering that question."
