Discovery of radio emission from the exoplanet $β$ Pictoris b

MeerKAT observations provide the first unambiguous detection of radio emission from an exoplanet: the directly imaged giant planet β Pictoris b. Emission spanning 0.85 to 3.5 GHz appears as rapidly recurring bursts, persistent weaker radiation, and strong circular polarization of 40–70%. To separate the planet from its host star, the analysis tied the radio image to the Gaia celestial reference frame using an affine astrometric correction; the source coincides with β Pic b, while the star is excluded at 4.4σ. The bursts’ variability, broadband spectra, and polarization identify electron cyclotron maser instability, or ECMI, rather than plasma emission or ordinary gyrosynchrotron radiation. Because the cyclotron frequency scales as 2.8 GHz per kilogauss, emission detected up to 3.5 GHz requires a local planetary magnetic field of at least 1.25 kG, constituting the first direct magnetic-field measurement for an exoplanet. The roughly 8-hour spacing between bursts is comparable to β Pic b’s measured 9.00 ± 0.13-hour rotation period, supporting a rotationally driven magnetosphere–ionosphere coupling mechanism analogous to auroral radio emission from Jupiter and ultracool dwarfs. Stellar-wind and planet–moon interaction models underpredict the signal by three orders of magnitude and more than an order of magnitude, respectively, favoring internally powered planetary aurorae. Continued monitoring could use rotational modulation to constrain the planet’s magnetic geometry.