In 1958, the first satellites launched by the United States (Explorer 1 and 3) detected a massive radiation belt around planet Earth. This confirmed something that many scientists suspected before the Space Age began: that energetic particles emanating from the Sun (solar wind) were captured and held around the planet by Earth’s magnetosphere. This region was named the Van Allen Belt in honor of University of Iowa professor James Van Allen who led the research effort. As robotic missions explored more of the Solar System, scientists discovered similar radiation belts around Jupiter, Saturn, Uranus, and Neptune.
Given the boom in extrasolar planet research, scientists have eagerly awaited the day when a Van Allen Belt would be discovered around an exoplanet. Thanks to a team of astronomers led by the University of California, Santa Cruz (UCSC) and the National Radio Astronomy Observatory (NRAO), that day may have arrived! Using the global High Sensitivity Array (HSA), the team obtained images of persistent, intense radio emissions from an ultracool dwarf star. These revealed the presence of a cloud of high-energy particles forming a massive radiation belt similar to what scientists have observed around Jupiter.
The research was led by Ph.D. student Melodie M. Kao, a Heising-Simons 51 Pegasi b Fellow at UCSC and a former NASA Hubble Fellow with the School of Earth and Space Exploration at Arizona State University (SESE-ASU), and NRAO researcher Amy J. Mioduszewski. They were joined by Jackie Villadsen & Evgenya L. Shkolnil, two astrophysicists from Bucknell University and SESE-ASU, respectively. Their findings appeared in a recent paper, “Resolved imaging confirms a radiation belt around an ultracool dwarf,” published in Nature.
Strong magnetic fields form a double-lobed bubble around a planet (called a magnetosphere) that can trap and accelerate particles to near the speed of light. To generate one, a planet’s interior must have temperatures high enough to maintain electrically conducting fluids. In Earth’s case, the core region is comprised of a solid inner core and a molten outer core (both composed of iron-nickel), the latter of which revolves in the opposite direction as Earth’s rotation. In the case of Jupiter and Saturn, electrical conduction is caused by a layer of metallic hydrogen rotating in the interior.
These magnetospheres can capture high-energy particles, leading to large donut-shaped radiation belts. In Earth’s case (as noted already), these particles consist of electrons, protons, and alpha particles released by the Sun’s corona. In Jupiter’s case, these particles result from volcanic activity on its moon Io, which can spew magma and gas particles hundreds of kilometers into space. Astronomers have also speculated that stars and brown dwarfs might have magnetic fields that result from ionized or metallic hydrogen in their interiors.
In the hopes of learning more about radiation belts and their relationship with planetary magnetic fields, Kao and her team selected LSR J1835+3259, a dwarf object that straddles the boundary between low-mass stars (M-type red dwarfs) and massive brown dwarfs. This was the only object Kao and her colleagues were confident would yield the high-quality data needed to resolve its radiation belts. They observed this object using the HSA’s network of 39 radio dishes, including the NRAO’s Very Long Baseline Array (VLBA) and Very Large Array (VLA), the Green Bank Telescope (GBT), and the 100-m\eter Radio Effelsberg Telescope, and subsets thereof.
The combined power of these radio antennas allowed the team to capture high-resolution images of the LSR J1835+3259 radiation belt, which allowed them to infer the presence and strength of the object’s magnetic field. As Kao explained in a UCSC News release, this represents a first for astronomers: “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.”
Artist’s view of a cool brown dwarf with magnetic fields and auroral activity visualized. Credit: ASTRON/Danielle Futselaar
What they observed, as noted, was similar in shape to what had been previously observed with Jupiter – a double-lobed radiation belt. However, the belt surrounding LSR J1835+3259 was ten times brighter than Jupiter’s, implying a magnetic field of incredible intensity! This represents a first for astronomers and opens the door to many new opportunities. Said Kao:
“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.”
Using numerical models and a theoretical understanding of how brown dwarf systems work, planetary scientists can predict the shape of a planet’s magnetic field. Before these observations, astronomers did not have an effective means of testing these predictions. Moreover, the images Kao and her team obtained were the first of an object outside our Solar System capable of differentiating between its aurorae and radiation belts. These findings, said Kao, reaffirm that while the process by which planets form may be different, they can still share some key characteristics:
“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. 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 also be an important factor in determining a planet’s habitability. By deflecting energetic particles, Earth’s magnetosphere has prevented our atmosphere from being slowly stripped away by solar wind. This is what took place on Mars, which lost its magnetic field after geological activity largely ceased in its interior about 4 billion years ago. Given their importance to maintaining a stable climate, exoplanet researchers look forward to the day when they can visualize planetary magnetic fields.
This research also showcases the capabilities of modern instruments and partnerships, where observatories worldwide can contribute to the study of faint and distant objects that are otherwise difficult to resolve. Looking ahead, Kao and her colleagues hope to use the Next Generation Very Large Array (ngVLA), a major NRAO project currently under development. This array operates at frequencies of 1.2 to 116 gigahertz (GHz) – ultra-high to extremely-high frequency (UHF to EHF) – in the microwave spectrum and has sensitivity and spatial resolution a full order of magnitude higher than Jansky VLA and ALMA at the same wavelengths.
This instrument will allow astronomers to image many more extrasolar radiation belts. Evgenya Shkolnik, a professor of astrophysics at SESE-ASU and a co-author on the study, has been studying magnetic fields and planetary habitability for many years. As she related, studying dwarf objects like LSR J1835+3259 could lead to more detailed studies of radiation belts and magnetic fields around rocky exoplanets. “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,” she said.
