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| 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 |
Astronomers used a coordinated network of 39 radio dishes from Hawaii to Germany to produce high-resolution photos of the first radiation belt discovered outside our solar system. photos of an ultracool dwarf’s persistent, intense radio emissions suggest the presence of a cloud of high-energy electrons trapped in the object’s enormous magnetic field, generating a double-lobed structure similar to radio photos 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 lead author of a report on the new findings published in Nature on May 15.
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 major planets, have radiation belts made up of these high-energy charged particles trapped by the planet’s magnetic field.
The Van Allen belts are huge donut-shaped zones of high-energy particles collected 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 seen by Kao and her colleagues would be 10 million times brighter than Jupiter’s.
When particles deflected by the magnetic field toward the poles interact with the atmosphere, they produce auroras (“northern lights”), and Kao’s team also produced the first image capable of distinguishing between the position of an object’s aurora and its radiation belts outside our solar system.
This study’s ultracool dwarf crosses the line between low-mass stars and large brown dwarfs. “While the formation of stars and planets is different, the physics inside 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 structure of the magnetic fields of this class of objects is mostly unknown territory. Planetary scientists can forecast the intensity and form of a planet’s magnetic field using their theoretical understanding of these systems and numerical models, but they haven’t had an easy means to validate 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 intensity and form of a planet’s magnetic field can play an essential 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 heated 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 on Jupiter is hydrogen under so high pressure that it becomes metallic. Metallic hydrogen, according to Kao, forms magnetic fields in brown dwarfs, but ionized hydrogen conducts in star interiors.
Kao was sure that the ultracool dwarf LSR J1835+3259 would provide the high-quality data required to resolve its radiation belts.
“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 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,” Kao said, adding that she is looking forward to when the Next Generation Very Large 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 co-author Evgenya Shkolnik of Arizona State University, who has long studied magnetic fields and planet habitability.
The High Sensitivity Array, which consists of 39 radio dishes controlled by the NRAO in the United States, and the Effelsberg radio telescope maintained by the Max Planck Institute for Radio Astronomy in Germany, were employed 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 co-author Jackie Villadsen.
Kao stressed that this was a genuine team effort, depending largely on the observational experience of co-first author Amy Mioduszewski at NRAO in designing the study and interpreting the data, as well as Villadsen and Shkolnik’s multiwavelength stellar flare knowledge.
