Astronomers have described the first radiation belt observed outside our solar system, using a coordinated array of 39 radio satellite dishes from Hawaii to Germany to obtain high-resolution images. Images of persistently 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-lobe structure analogous to radio images of the orbital belts. Jupiter radiation.
We are actually imaging our target’s magnetosphere by observing the radio-emitting plasma—its radiation belt—in the magnetosphere. This had never been done with a Jupiter-sized gas giant planet outside our solar system, explains Melodie Kao, a postdoctoral researcher at the University of California, Santa Cruz and first author of a paper on the new findings published in Nature. Strong magnetic fields form a “magnetic bubble” around a planet called the magnetosphere, which can trap and accelerate particles to nearly the speed of light. All of the planets in our solar system that have these types of 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.
Earth’s radiation belts, known as the Van Allen belts, are large doughnut-shaped zones of high-energy particles captured from the solar winds by the magnetic field. Most of the particles in Jupiter’s belts come from the volcanoes on its moon Io. If they could be placed side by side, the radiation belt that Kao and his team have photographed would be 10 million times brighter than Jupiter’s.
Particles deflected poleward by the magnetic field generate auroras (“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 belts. radiation outside our solar system. The ultracool dwarf observed in this study lies on the border between low-mass stars and massive brown dwarfs. Although the formation of stars and planets may be different, the physics inside them may be very similar in that soft part of the mass continuum that connects low-mass stars with brown dwarfs and gas giant planets, Kao explained.
Characterizing the strength and shape of the magnetic fields of this class of objects is largely unexplored territory. Using their theoretical knowledge of these systems and numerical models, planetary scientists can predict the strength and shape of a planet’s magnetic field, but until now they had no good way to easily check those predictions.
Auroras can be used to measure the strength of the magnetic field, but not its shape. We designed this experiment to demonstrate a method for evaluating the magnetic field shapes of brown dwarfs and eventually exoplanets, Kao explains.
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, we must take into account the role of their magnetic fields in maintaining a stable environment, in addition to aspects such as the atmosphere and climate, Kao explains. To generate a magnetic field, a planet’s interior must be hot enough to contain electrically conductive fluids, which in Earth’s case is the molten iron in its core. On Jupiter, the conductive fluid is hydrogen under so much pressure that it turns metallic. According to Kao, metallic hydrogen probably also generates magnetic fields in brown dwarfs, while inside stars the conductive fluid is ionized hydrogen.
The ultracool dwarf known as LSR J1835+3259 was the only object Kao was certain would provide the high-quality data needed to resolve its radiation belts.
Now that we’ve established that this particular type of low-level, steady-state radio emission tracks radiation belts in the large-scale magnetic fields of these objects, when we see this type of emission in brown dwarfs – and, eventually, in gas giant exoplanets – we can say with more certainty that they probably have a large magnetic field, even if our telescope is not big enough to see their shape, says Kao, adding that he is looking forward to the moment when the Next Generation Very The Large Array, currently in the planning phase by the National Radio Astronomy Observatory (NRAO), can image many more extrasolar radiation belts.
This is an essential first step towards finding many more such objects and honing our ability to search for ever smaller magnetospheres, which will allow us to study those of potentially habitable Earth-sized planets, says Evgenya Shkolnik, co-author of the study and professor at Arizona State University, who has spent many years studying magnetic fields and the habitability of planets. The team used the High Sensitivity Array, made up of 39 radio telescopes coordinated by the NRAO in the United States and the Effelsberg radio telescope, operated by the Max Planck Institute for Radio Astronomy in Germany.
Sources University of California Santa Cruz | Kao, M.M., Mioduszewski, A.J., Villadsen, J. et al. Resolved imaging confirms a radiation belt around an ultracool dwarf. Nature (2023). doi.org/10.1038/s41586-023-06138-w