Coherent radio emission from a quiescent red dwarf indicative of star-planet interaction

Low-frequency (nu less than or similar to 150 MHz) stellar radio emission is expected to originate in the outer corona at heights comparable to and larger than the stellar radius. Such emission from the Sun has been used to study coronal structure, mass ejections and space-weather conditions around the planets(1). Searches for low-frequency emission from other stars have detected only a single active flare star(2) that is not representative of the wider stellar population. Here we report the detection of low-frequency radio emission from a quiescent star, GJ 1151-a member of the most common stellar type (red dwarf or spectral class M) in the Galaxy. The characteristics of the emission are similar to those of planetary auroral emissions(3) (for example, Jupiter's decametric emission), suggesting a coronal structure dominated by a global magnetosphere with low plasma density. Our results show that large-scale currents that power radio aurorae operate over a vast range of mass and atmospheric composition, ranging from terrestrial planets to main-sequence stars. The Poynting flux required to produce the observed radio emission cannot be generated by GJ 1151's slow rotation, but can originate in a sub-Alfvenic interaction of its magnetospheric plasma with a short-period exoplanet. The emission properties are consistent with theoretical expectations(4-7) for interaction with an Earth-size planet in an approximately one- to five-day-long orbit. Low-frequency radio emission from a normally quiescent M dwarf star suggests a radio aurora generated by the interaction between the stellar corona and an undetected Earth-sized planet.