All planets in our solar system with global magnetic fields have radiation belts, doughnut-shaped regions confined by magnetic fields where particles are trapped and accelerated, glowing in radio light. All of this suggests that there should also be radiation belts wherever there is a stable global magnetic field.
However, detecting the faint glow of an extrasolar radiation belt is difficult, because this faint glow of a radiation belt is difficult to resolve. But difficult does not mean impossible: for the first time, astronomers have photographed a radiation belt wrapped around an extrasolar object.
This object is a very low-mass red dwarf star named LSR J1835+3259 that is slightly larger in diameter than Jupiter, has about 77 times the mass of Jupiter, and is about 20 light-years away.
“We actually imagine our target’s magnetosphere by observing the radio-emitting plasma – its radiation belt – in the magnetosphere,” says astronomer Melodie Kao from the University of California, Santa Cruz. “This has never been done before for anything the size of a gas giant planet outside of our solar system.”
The Earth has its Van Allen belts, filled with particles from the solar wind. Uranus, Neptune, Mercuryand Saturn all have radiation belts.
JupiterThe enormous radiation belts of are mainly powered by volcanic moon Io as it spits out large drops of volcanic material. Even Ganymede, Jupiter’s moon – the only moon in the solar system with its own magnetic field – has some sort of radiation belt.
And although the radiation belts and the magnetic fields that confine them have not been detected in extrasolar objects, we have seen hints of their presence.
Low-mass stars and brown dwarfs showed activity similar to solar system auroras. Auroras – seen on several planets – are generated when accelerated charged particles are funneled along magnetic field lines to fall into a planet’s atmosphere and interact with particles there.
Having shown signs of this auroral activity (thus suggesting the presence of a global magnetic field), LSR J1835+3259 was the ideal place to observe the radiation belts up close.
Using a network of 39 radio telescopes around the world to efficiently create an Earth-sized radio telescope, Kao and his colleagues took observations of the star, looking closely at the space around it, where a belt of radiation, viewed from the side, would appear as two radio-emitting lobes.
Sure enough, the images revealed a double-lobed structure around the star, emitting faint radio waves, similar to the lobes in Jupiter’s radiation belt. However, because the star is much farther away than Jupiter, its radio lobes are inherently much, much brighter, about 10 million times brighter than Jupiter’s.
And the observed radiation is of a type that has been seen before in low-mass stars and brown dwarfs, but was attributed to flares in the stellar corona.
These findings not only confirm that objects such as stars can have radiation belts, but they also mean that we may have seen radiation belts before in other such objects and didn’t know what that we were looking at.
“Now that we’ve established that this particular type of steady-state low-level radio emission traces radiation belts in the large-scale magnetic fields of these objects, when we see this type of brown dwarf emission – and possibly of gas giant exoplanets – we can say with more confidence that they probably have a large magnetic field, even if our telescope is not big enough to see their shape”, Kao says.
It’s a finding that astronomers hope will help search for potentially habitable worlds in the future as techniques and instruments are refined. This is because the earth’s magnetic field is considered essential for the flourishing of life. It deflects harmful solar radiation from reaching the surface, protecting the atmosphere and vulnerable organisms that inhabit the surface.
Tools that allow us to find magnetic fields around other worlds will help us find similarly shielded planets.
It’s still a bit far, but this discovery puts us on the right track.
“This is an essential first step in finding many more such objects and honing our skills in searching for smaller and smaller magnetospheres,” he added. says astronomer Evgenya Shkolnik from Arizona State University, “ultimately allowing us to study those of potentially habitable Earth-sized planets”.
The research has been published in Nature.