Bouma noted that stars affect their planets through both light and streams of particles such as stellar winds and magnetic storms, but the particle component remains difficult to characterize for distant systems. "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."

Because astronomers cannot place instruments around far-off stars, Bouma and collaborator Moira Jardine of the University of St Andrews focused on a class of young, rapidly rotating M dwarfs called complex periodic variables that show recurring dips in brightness. Observers had seen these regular dimmings for some time but did not know whether they arose from starspots or from orbiting material close to the stellar surface. "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."

To resolve the origin of the dimming, Bouma and Jardine created what Bouma calls spectroscopic movies of one such complex periodic variable to track how material near the star absorbs light over time. The data indicated that the dips are produced by large clumps of relatively cool plasma trapped in the star's magnetosphere and carried around with the star by its magnetic field, forming a doughnut-shaped torus with dense clumps on opposite sides of the star. "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 in their early stages could host similar plasma tori. These natural probes can help astronomers assess how stellar particle environments are structured around young stars and how this particle flow may influence the conditions and atmospheres of any nearby planets.

A key next step is to determine whether the plasma torus material originates from the star itself, for example through outflows or flares, or from an external source associated with planetary material. "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."

Research Report:A Plasma Torus around a Young Low-mass Star

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