Astronomers have observed the first radiation belt outside our solar system using a coordinated array of 39 radio dishes from Hawaii to Germany. They even have obtained high-resolution images- showing persistent, intense radio emissions from an ultracool dwarf.
Their observations reveal the presence of a cloud of high-energy electrons trapped in the object’s powerful magnetic field, forming a double-lobed structure analogous to radio images of Jupiter’s radiation belts.
The Van Allen belts, also called Earth’s radiation belts, are donut-shaped zones of very energetic particles that the magnetic field has trapped from solar winds. The majority of the debris in Jupiter’s belts originates from Io’s volcanoes. The radiation belt astronomers have imaged in this study would be 10 million times brighter than Jupiter’s if you could put them side by side.
The “northern lights” are produced when the magnetic field deflects charged particles towards the poles contact with the atmosphere.
The team also created the first image that could distinguish between the location of an object’s aurora and its radiation belts outside of the solar system.
Melodie Kao, a postdoctoral fellow at UC Santa Cruz and first author of a paper, said, “We are imaging the magnetosphere of our target by observing the radio-emitting plasma—its radiation belt—in the magnetosphere. That has never been done before for something the size of a gas giant planet outside our solar system.”
“The ultracool dwarf imaged in this study straddles the boundary between low-mass stars and massive brown dwarfs. While the formation of stars and planets can be different, the physics inside of them can be very similar in that mushy part of the mass continuum connecting low-mass stars to brown dwarfs and giant gas planets.”
“Characterizing the strength and shape of the magnetic fields of this class of objects is largely uncharted terrain. Using their theoretical understanding of these systems and numerical models, planetary scientists can predict the strength and shape of a planet’s magnetic field, but they haven’t had a good way to test those predictions easily.”
“Auroras can be used to measure the strength of the magnetic field, but not the shape. We designed this experiment to showcase a method for assessing the shapes of magnetic fields on brown dwarfs and eventually exoplanets.”
“The strength and shape of the magnetic field can be an important factor in determining a planet’s habitability. When we’re thinking about the habitability of exoplanets, the role of their magnetic fields in maintaining a stable environment is something to consider in addition to things like the atmosphere and climate.”
Generating a magnetic field requires a planet’s interior to be hot enough to have electrically conducting fluids. Metallic hydrogen probably also generates magnetic fields in brown dwarfs, while the conducting fluid is ionized hydrogen in the interiors of stars.
The only target that Kao felt confident would provide the high-quality data required to resolve its radiation belts was the ultracool dwarf LSR J1835+3259.
Kao said, “Now that we’ve established that this particular kind of steady-state, low-level radio emission traces radiation belts in the large-scale magnetic fields of these objects when we see that kind of emission from brown dwarfs—and eventually from gas giant exoplanets—we can more confidently say they probably have a big magnetic field, even if our telescope isn’t big enough to see the shape of it.”
Coauthor Evgenya Shkolnik at Arizona State University, who has been studying the magnetic fields and habitability of planets for many years, said, “This is a critical first step in finding many more such objects and honing our skills to search for smaller and smaller magnetospheres, eventually enabling us to study those of potentially habitable, Earth-size planets.”
Coauthor Jackie Villadsen at Bucknell University said, “By combining radio dishes from across the world, we can make incredibly high-resolution images to see things no one has ever seen. Our image is comparable to reading the top row of an eye chart in California while standing in Washington, D.C.”
- Melodie Kao, Resolved imaging of an extrasolar radiation belt around an ultracool dwarf, Nature (2023). DOI: 10.1038/s41586-023-06138-w