Astronomers led by the University of Santa Cruz, California, have observed the first radiation belt seen outside of our solar system
High-resolution imaging of radio emissions from an ultracool dwarf shows a double-lobed structure like the radiation belts of Jupiter.
Astronomers have described the first radiation belt observed outside our solar system, using a co-ordinated array of 39 radio dishes from Hawaii to Germany to obtain high-resolution images.
The images of persistent, intense radio emissions from an ultracool dwarf 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.
Melodie Kao, a postdoctoral fellow at UC Santa Cruz and first author of the paper, said: “We are actually 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 of our solar system.”
Strong magnetic fields form a ‘magnetic bubble’ around a planet called a magnetosphere, which can trap and accelerate particles to near the speed of light.
All the planets in our solar system that have such magnetic fields, including Earth, as well as Jupiter and the other giant planets, have radiation belts consisting of these high-energy charged particles trapped by the planet’s magnetic field.
Earth’s radiation belts, known as the Van Allen belts, are large doughnut-shaped zones of high-energy particles captured from solar winds 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 that Kao and her team have imaged would be ten million times brighter than Jupiter’s.
Particles deflected by the magnetic field toward the poles generate auroras (such as the Northern Lights) when they interact with the atmosphere, and Kao’s team also obtained the first image capable of differentiating between the location of an object’s aurora and its radiation belts outside of our solar system.
The ultracool dwarf imaged in this study straddles the boundary between low-mass stars and massive brown dwarfs.
Kao explained: “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.”
Uncharted terrain
Characterising 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 easily test those predictions.
Kao said: “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.
She added: “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.”
To generate a magnetic field, 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 Jupiter, the conducting fluid is hydrogen under so much pressure it becomes metallic. Metallic hydrogen probably also generates magnetic fields in brown dwarfs, Kao said, while in the interiors of stars, the conducting fluid is ionised hydrogen.
The ultracool dwarf known as LSR J1835+3259 was the only object Kao felt confident would yield the high-quality data needed to resolve its radiation belts.
She 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.
Extrasolar radiation belts
Kao added that she is looking forward to when the Next Generation Very Large Array, currently being planned by the National Radio Astronomy Observatory (NRAO), can image many more extrasolar radiation belts.
Co-author 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.”
The team used the High Sensitivity Array, consisting of 39 radio dishes co-ordinated by the NRAO in the United States and the Effelsberg radio telescope operated by the Max Planck Institute for Radio Astronomy in Germany.
Co-author Jackie Villadsen, of Bucknell University in the US, said: “By combining radio dishes from across the world, we can make incredibly high-resolution images to see things no one has ever seen before.”
Kao emphasised that this discovery was a true team effort, relying heavily on the observational expertise of co-first author Amy Mioduszewski at NRAO in planning the study and analysing the data, as well as the multiwavelength stellar flare expertise of Villadsen and Shkolnik.
The research is published in Nature.
Image 1: Artist’s impression of an aurora and the surrounding radiation belt of the ultracool dwarf LSR J1835+3259. Credit: Chuck Carter, Melodie Kao, Heising-Simons Foundation.
Image 2: The electron radiation belt and aurora of an ultracool dwarf were imaged by combining 39 radio telescopes to form a virtual telescope spanning the globe from Hawaii to Germany. Credit: Melodie Kao, Amy Mioduszewski.
