Radio burst from a stellar coronal mass ejection

Coronal mass ejections (CMEs) are massive expulsions of magnetized plasma from a star and are the largest contributors to space weather in the Solar System^1,2. CMEs play an important role in planetary atmospheric erosion, especially for planets that are close to their host star^3-5. However, this conclusion remains controversial as there has not been an unambiguous detection of a CME from a star outside our Sun. Previous stellar CME studies have only inferred the presence of a CME through the detection of other types of stellar eruptive event^6-9. A signature of a fast CME is a type II radio burst^10,11, which is emitted from the shock wave produced as the CME travels through the stellar corona into interplanetary space. Here we report an analogue to a type II burst from the early M dwarf StKM 1-1262. The burst exhibits identical frequency, time and polarization properties to fundamental plasma emission from a solar type II burst. We demonstrate that the rate of these events with similar radio luminosity from M dwarfs is $0.84_{-0.69}^{+1.94}\times 10^{-3}$ per day per star. Our detection implies that we are no longer restricted to extrapolating the solar CME kinematics and rates to other stars, allowing us to establish observational limits on the impact of CMEs on exoplanets.