MeerKAT pins repeating radio bursts to the giant planet Beta Pictoris b

Localizing the source has been the hurdle for this kind of work. Earlier radio detections from exoplanetary systems could not be traced to the planet rather than the star ^[5]. The group handled it with astrometry. They compared their MeerKAT images against the precise positions of extremely distant background quasars, which act as fixed markers, and used them to plot where the star and the planet should sit. The radio signal lined up with Beta Pictoris b ^[7]. "We can rule out the star, and ... we can say that it's from this one particular planet," Yvette Cendes of the University of Oregon, a member of the team, said ^[8].

The second line of evidence is polarization. Auroral radio emission is circularly polarized, meaning the orientation of the waves rotates in a circle, and that is what the researchers saw from Beta Pictoris b ^[9]. The emission comes from charged particles circling in the planet's magnetic field and exciting molecules in its atmosphere ^[10].

From those measurements the team derived a field of at least 1,250 gauss at the region where the radio waves are produced ^[13]. Earth's field is about 0.5 gauss and Jupiter's about 4.3 ^[14], which puts the planet's value at roughly 290 times Jupiter's ^[15] and about 2,500 times Earth's ^[16]. "This constitutes the first direct measurement of magnetic field strength for an exoplanet," the authors wrote ^[12].

The figure is a lower bound at one location, not a global surface field ^[13]. It is also close to what theory expected: the authors say it matches predictions for the fields generated by young, massive giant planets ^[17], and at ten Jupiter masses this object sits near the brown dwarf regime, where observed auroras already imply fields of thousands of gauss ^[18]. "It's an incredibly, incredibly strong magnetic field, much stronger than anything in our solar system," Cendes said ^[19].

Joe Callingham of the University of Amsterdam, who was not involved, told Science News that clinching the aurora interpretation would take seeing the radio waves pulsate in time, since the aurora should rotate in and out of view as the planet spins, roughly once every eight hours ^[22]. "So TBD, but, compelling," he said. "It's probably auroral" ^[23].

The SETI question is the one readers reach for first, and the team knows it. "When people see 'radio signal from an exoplanet,' they think aliens," Cendes said ^[31]. Live Science notes that technosignatures are a key focus of the search for extraterrestrial intelligence, and that radio signals can be emitted by natural sources too ^[26]. Suzanne Aigrain, an astrophysics professor at Oxford who was not involved in the study ^[34], named two: "There are two natural processes that can produce radio emission in exoplanets," she said, listing magnetic reconnection between a close-orbiting planet's field and its star's, and aurorae driven by stellar particles hitting the upper atmosphere ^[24]. She said the second is what the authors believe they detected ^[25], and added: "this is definitely not aliens!" ^[27].

What the measurement is actually for is interior physics. Aigrain said magnetic field measurements matter because how magnetospheres are generated, and what sets their strength, is not fully understood ^[35]. A field also shields an atmosphere: "On Earth, for example, the magnetic field has played a key role in retaining the atmosphere and shielding life on the planet from harmful high-energy radiation," she said ^[28]. "The planet in this paper is quite massive and has a thick atmosphere, so even without a strong magnetic field it could probably hold on to its atmosphere, and it is not expected to host life," she said ^[29].