Astronomers have described the first radiation belt observed outside our solar system, using a coordinated array of 39 radio dishes from Hawaii to Germany to obtain high-resolution images. Images of continuous, intense radio emissions from a supercooled dwarf reveal a cloud of high-energy electrons trapped in the object’s strong magnetic field, forming a double-lobe structure similar to radio images of Jupiter’s radiation belts.
“We’re actually imaging our target’s magnetosphere by observing the radio-emitting plasma — the radiation belt — in the magnetosphere. This has never been done before for something the size of a gas giant planet outside our solar system,” said Melody Kao, a postdoctoral fellow at the University of California, Santa Fe. Cruz and first author of a paper on the new findings published May 15. nature.
Strong magnetic fields form a “magnetic bubble” around a planet called the magnetosphere, which can trap particles and accelerate them to near the speed of light. All planets in our solar system that have such magnetic fields, including Earth, as well as Jupiter and other giant planets, have radiation belts made up of these charged, high-energy particles trapped by the planet’s magnetic field.
Earth’s radiation belts, known as the Van Allen belts, are large donut-shaped regions of high-energy particles captured from the solar wind by the magnetic field. Most of the particles in Jupiter’s belts are from volcanoes on its moon Io. If you could put them side by side, the radiation belt imaged by Kao and her team would be 10 million times brighter than Jupiter’s belt.
Particles deflected by the magnetic field toward the poles generate auroras (“northern lights”) when they interact with the atmosphere, and Kao’s team also obtained the first image able to distinguish the location of an object’s aurora from radiation belts outside our solar system.
The supercool dwarf imaged in this study straddles the boundary between low-mass stars and massive brown dwarfs. “While the composition of stars and planets can be different, the physics inside them can be very similar in that mushy part of the collective chain that connects low-mass stars to brown dwarfs and gas giant planets,” Kao explained.
Characterizing the strength and shape of magnetic fields for this class of objects, she said, is largely uncharted terrain. Using their theoretical understanding of these systems and numerical models, planetary scientists could predict the strength and shape of a planet’s magnetic field, but they had no good way to easily test these predictions.
“Aurora borealis can be used to measure magnetic field strength, but not shape. We designed this experiment to demonstrate a method for assessing the shapes of magnetic fields on brown dwarfs and, eventually, exoplanets,” Kao said.
The strength and shape of the magnetic field can be an important factor in determining a planet’s habitability. “When we think about the habitability of exoplanets, the role of magnetic fields in maintaining a stable environment is something to consider in addition to things like the atmosphere and climate,” Cao said.
To generate a magnetic field, the interior of the planet must be hot enough to have electrically conductive fluids, which in Earth’s case is the molten iron in its core. On Jupiter, the conducting liquid is hydrogen under so much pressure that it becomes metallic. Metallic hydrogen may also generate magnetic fields in brown dwarfs, Kao said, while in the interiors of stars the conductive fluid is ionized hydrogen.
Known as LSR J1835+3259, the supercooled dwarf was the only object Kao felt confident would produce the high-quality data needed to resolve its radiation belts.
“Now that we have established that this particular type of steady-state, low-level radio emission traces radiation belts into the large-scale magnetic fields of these objects, when we see this type of emission from brown dwarfs – and eventually from gas giant exoplanets – we can say with more confidence that They have a large magnetic field, even if our telescope isn’t big enough to see what it looks like,” Kao said, adding that she’s looking forward to the very next generation. The large array, which is currently being planned by the National Radio Astronomy Observatory (NRAO), could image many radiation belts outside the solar system.
“This is a critical first step in finding many more of these objects and honing our skills to search for smaller and smaller magnetospheres, eventually enabling us to study those potentially habitable, Earth-sized planets,” said co-author Evgenya Shkolnik of ASU. who has been studying the magnetic fields and habitability of planets for many years.
The team used the High Sensitivity Array, which consists of 39 radio dishes coordinated by NRAO in the US and the Eiffelsberg radio telescope operated by the Max Planck Institute for Radio Astronomy in Germany.
“By combining radio dishes from around the world, we can create incredibly high-resolution images to see things no one has ever seen before. Our image is comparable to reading the top row of an eye chart in California while standing in Washington, D.C.,” said co-author Jackie Feldsen of Bucknell University.
The discovery was a true team effort, Kao emphasized, drawing heavily on the observational expertise of co-first author Amy Miodoshevsky at NRAO in planning the study and analyzing the data, as well as on the multi-wavelength stellar flare expertise of Veladsen and Shkolnik. This work was supported by NASA and the Heising-Simons Foundation.