Astronomers have identified massive rings of plasma around young M dwarf stars that function as built-in monitors for stellar space weather. These structures, presented at the American Astronomical Society meeting, could help assess conditions for habitable planets orbiting these common stars. The findings come from research by Carnegie's Luke Bouma and Moira Jardine of the University of St Andrews.
Young M dwarf stars, which are smaller, cooler, and dimmer than the Sun, often host rocky planets similar in size to Earth. Many of these worlds face challenges like intense flares and radiation that could strip away atmospheres. To study how these stars affect their planets through particles like solar winds, researchers turned to unusual dips in starlight observed in rapidly spinning young M dwarfs known as complex periodic variables. Previously unexplained, these brightness dips have now been linked to clouds of cool plasma trapped in the stars' magnetospheres, forming doughnut-shaped tori carried along by magnetic fields. Luke Bouma of Carnegie Institution for Science explained the significance: 'Stars influence their planets... through particles -- or space weather -- like solar winds and magnetic storms, which are more challenging to study at great distances.' By creating 'spectroscopic movies' of one such star, Bouma and Moira Jardine confirmed the plasma's role. '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 said. The team estimates that at least 10 percent of M dwarfs exhibit these structures in their early stages, offering a new tool for astronomers. Bouma noted future work will trace the plasma's origins, either from the star or external sources. The research appears in The Astrophysical Journal Letters (2025; 988 (1): L3). 'This is a great example of a serendipitous discovery... space weather is going to be an important part of answering' whether planets around M dwarfs can host life, Bouma concluded.