The discovery of a radiation belt outside our solar system would be a significant and game-changing discovery in the field of astronomy. Radiation belts are areas of charged particles trapped by a planet’s magnetic field, primarily electrons and protons. The Van Allen radiation belts are the most well-known radiation belts that exist around the Earth.
Astronomers used a coordinated array of 39 radio dishes from Hawaii to Germany to obtain high-resolution images of the first radiation belt observed outside our solar system. Images of an ultracool dwarf’s persistent, intense radio emissions reveal the presence of a cloud of high-energy electrons trapped in the object’s powerful magnetic field, forming a double-lobed structure similar to radio images of Jupiter’s radiation belts.
“We are imaging our target’s magnetosphere 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 of our solar system,” said Melodie Kao, a postdoctoral fellow at UC Santa Cruz and the first author of a paper on the new discoveries published in Nature.
A magnetosphere is a “magnetic bubble” formed by strong magnetic fields around a planet that can trap and accelerate particles to near the speed of light. All of our solar system’s planets with magnetic fields, including Earth, Jupiter, and the other giant planets, have radiation belts made up of these high-energy charged particles trapped by the planet’s magnetic field.
We are imaging our target’s magnetosphere 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 of our solar system.
Melodie Kao
The Van Allen belts are large donut-shaped zones of high-energy particles captured from solar winds by the Earth’s magnetic field. The majority of the particles in Jupiter’s belts come from its moon Io’s volcanoes. If you could compare them, the radiation belt imaged by Kao and her colleagues would be 10 million times brighter than Jupiter’s.
When particles deflected by the magnetic field towards the poles interact with the atmosphere, they produce auroras (“northern lights”), and Kao’s team also obtained the first image capable of distinguishing between the location of an object’s aurora and its radiation belts 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 gas giant planets,” Kao explained.
She stated that characterizing the strength and shape of the magnetic fields of this class of objects is largely uncharted territory. Planetary scientists can predict the strength and shape of a planet’s magnetic field using their theoretical understanding of these systems and numerical models, but they haven’t had an easy way to test those predictions.
“Auroras can be used to measure magnetic field strength but not shape.” “We designed this experiment to demonstrate a method for determining the shapes of magnetic fields on brown dwarfs and, eventually, exoplanets,” Kao explained.
The strength and shape of a planet’s magnetic field can play an important role in determining its 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,” Kao said.
A planet’s interior must be hot enough to have electrically conducting fluids, which in the case of Earth is the molten iron in its core, in order to generate a magnetic field. The conducting fluid in Jupiter is hydrogen under such high pressure that it becomes metallic. Metallic hydrogen, according to Kao, generates magnetic fields in brown dwarfs, whereas ionised hydrogen conducts in star interiors.
The only object Kao was confident would yield the high-quality data needed to resolve its radiation belts was the ultracool dwarf LSR J1835+3259.
“Now that we’ve established that this particular kind of steady-state, low-level radio emission traces radiation belts in these objects’ large-scale magnetic fields, when we see that kind of emission from brown dwarfs – and eventually from gas giant exoplanets – we can say with more confidence that they probably have a big magnetic field, even if our telescope isn’t big enough to see the shape of it,” Kao said, adding that she is looking forward to when the Next Generation Telescope
“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,” said coauthor Evgenya Shkolnik at Arizona State University, who has been studying the magnetic fields and habitability of planets for many years.
The High Sensitivity Array, which consists of 39 radio dishes coordinated by the NRAO in the United States, and the Effelsberg radio telescope operated by the Max Planck Institute for Radio Astronomy in Germany, were used by the team.
“By combining radio dishes from around the world, we can create incredibly high-resolution images that reveal previously unseen phenomena. Our image is comparable to reading the top row of an eye chart in California while standing in Washington, D.C.,” said Bucknell University coauthor Jackie Villadsen.
