Radiation belts spotted outside our solar system for the first time

The high-resolution images obtained with a large array of radio dishes revealed "persistent, intense radio emissions" from this stellar object. 
Mrigakshi Dixit
Artist's impression of an aurora and the surrounding radiation belt of the ultracool dwarf LSR J1835+3259.
Artist's impression of an aurora and the surrounding radiation belt of the ultracool dwarf LSR J1835+3259.

Chuck Carter, Melodie Kao, Heising-Simons Foundation 

Astronomers have discovered evidence of a radiation belt outside our solar system for the first time. 

The radiation belts have been spotted around LSR J1835+3259, an ultracool dwarf (low-mass star) located 18 light-years from Earth.

The high-resolution images obtained with a large array of radio dishes revealed "persistent, intense radio emissions" from this stellar object. 

What the images show

The images revealed a cloud of high-energy particles trapped in the magnetic field of the object.

"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," said Melodie Kao, a postdoctoral fellow at the University of California in Santa Cruz and first author of this study, in an official statement. 

A magnetosphere is the magnetic field-dominated region surrounding a celestial object, where charged particles are trapped.

Our planet also has giant donut-shaped clouds of radiation belts called the Van Allen belts, which trap high-energy particles from the Sun. Other large planets in our solar system, like the gas giant Jupiter also have radiation belts that capture the energetic electrons emitted by the volcanic moon Io.

The newly identified radiation belts are similar to Jupiter's radiation belts. When compared side by side, the belts of this object are "10 million times brighter" than Jupiter's.

As per the study, the ultracool dwarf straddles the boundary between low-mass stars and massive brown dwarfs.

Understanding magnetic fields

Understanding radiation belts can often provide insight into the magnetic field shape and interior structure of a cosmic object.

For example, the Earth's interior is hot enough to have electrically conducting fluids, which aids the generation of its strong magnetic field, thereby supporting life on the planet.

In the case of Jupiter, the liquid metallic hydrogen generates a magnetic field. According to Kao, metallic hydrogen in the interiors of brown dwarfs could possibly result in the generation of magnetic fields.

However, the team found it difficult to determine the strength and shape of magnetic fields in this studied object. These two factors are critical in determining the planet's habitability. 

"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 at Arizona State University, who has been studying magnetic fields for years.

This celestial object was closely examined using a network of 39 radio dishes stretching from Hawaii to Germany, acting as one large radio telescope.

The network of radio dishes is coordinated by the NRAO in the United States and the Effelsberg radio telescope operated by the Max Planck Institute for Radio Astronomy in Germany.

The results have been published in the journal Nature. 

Study abstract:

Radiation belts are present in all large-scale Solar System planetary magnetospheres: Earth, Jupiter, Saturn, Uranus, and Neptune1. These persistent equatorial zones of relativistic particles up to tens of MeV in energy can extend farther than 10 times the planet’s radius, emit gradually varying radio emissions2–4 and impact the surface chemistry of close-in moons5. Recent observations demonstrate that very low mass stars and brown dwarfs, collectively known as ultracool dwarfs, can produce planet-like radio emissions such as periodically bursting aurorae6–8 from large-scale magnetospheric currents9–11. They also exhibit slowly varying quiescent radio emissions7,12,13 hypothesized to trace low-level coronal flaring14,15despite departing from empirical multi-wavelength flare relationships8,15. Here we present high resolution imaging of the ultracool dwarf LSR J1835+3259 at 8.4 GHz demonstrating that its quiescent radio emission is spatially resolved and traces a double-lobed and axisymmetric structure similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf radii separate the two lobes, which are stably present in three observations spanning more than one year. For plasma confined by the magnetic dipole of LSR J1835+3259, we estimate 15 MeV electron energies consistent with Jupiter’s radiation belts4. Our results confirm recent predictions of radiation belts at both ends of the stellar mass sequence8,16–19 and support broader re-examination of rotating magnetic dipoles in producing non-thermal quiescent radio emissions from brown dwarfs7, fully convective M dwarfs20, and massive stars18,21