In a groundbreaking discovery, astronomers have detected the first-ever radiation belt outside our solar system. This remarkable finding was made possible through the collaboration of 39 radio dishes spanning from Hawaii to Germany, which captured high-resolution images. By analyzing the persistent and intense radio emissions emitted by an ultracool dwarf, scientists were able to identify a cloud of high-energy electrons confined within the object's potent magnetic field. The resulting imagery revealed a double-lobed structure resembling Jupiter's radiation belts, providing an unprecedented glimpse into the magnetosphere of an extraterrestrial gas giant.
Lead author Melodie Kao, a postdoctoral fellow at UC Santa Cruz, expressed the significance of their achievement, stating, “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.” The study documenting these groundbreaking findings was published in the journal Nature on May 15.
Magnetospheres, which are formed by strong magnetic fields, create a protective “magnetic bubble” around celestial bodies. Within these magnetospheres, particles can become trapped and accelerated to nearly the speed of light. Planets within our solar system, such as Earth and Jupiter, possess magnetic fields and consequently harbor radiation belts consisting of highly energetic charged particles confined by the planet's magnetic forces.
Earth's radiation belts, known as the Van Allen belts, are vast toroidal regions containing high-energy particles captured from solar winds by our planet's magnetic field. Jupiter's radiation belts, on the other hand, derive most of their particles from the volcanic activity of its moon Io. In a fascinating comparison, the radiation belt observed by Kao and her team would outshine Jupiter's belts by a staggering factor of 10 million if placed side by side.
As particles within the radiation belt are deflected towards the poles by the magnetic field, they interact with the atmosphere, giving rise to awe-inspiring phenomena such as auroras, commonly referred to as the “northern lights” on Earth. In another remarkable accomplishment, Kao's team successfully obtained an image that enables the differentiation between an object's aurora and its radiation belts beyond our solar system.
This groundbreaking discovery opens up new avenues for exploring and understanding the complex magnetic environments surrounding celestial objects beyond our own solar system. By shedding light on the existence and characteristics of radiation belts outside our familiar astronomical neighborhood, scientists can expand our knowledge of the universe and further unravel the mysteries of these distant celestial bodies.
The study focused on an ultracool dwarf that exists in a transitional region between low-mass stars and massive brown dwarfs. According to Kao, the physics within this mass range is comparable, despite differences in the formation of stars and planets. Exploring the magnetic fields of such objects remains relatively unexplored territory. While theoretical understanding and numerical models can predict the strength and shape of a planet's magnetic field, there has been a lack of practical methods to validate these predictions.
Kao explained that although auroras can provide insights into the magnetic field's strength, they do not reveal information about its shape. Hence, the researchers designed their experiment to establish a technique for assessing magnetic field shapes in brown dwarfs and, eventually, exoplanets.
Understanding the strength and shape of a planet's magnetic field holds significance in determining its potential habitability. Kao emphasized that when considering the habitability of exoplanets, the role of their magnetic fields in maintaining a stable environment should be taken into account alongside factors like atmosphere and climate.
For a planet to possess a magnetic field, its interior must maintain sufficient heat to generate electrically conducting fluids. Earth's molten iron core serves this purpose, while Jupiter's metallic hydrogen, formed under extreme pressure, fulfills the role. Brown dwarfs likely generate magnetic fields through metallic hydrogen as well. In the case of stars, the conducting fluid is ionized hydrogen within their interiors.
LSR J1835+3259, an ultracool dwarf, was identified as the most promising object by Kao to obtain high-quality data and resolve its radiation belts effectively.
The discovery of radiation belts through steady-state, low-level radio emissions in the large-scale magnetic fields of such objects provides a foundation for future research. Kao explained that when similar emissions are detected from brown dwarfs and, eventually, gas giant exoplanets, scientists can confidently infer the presence of substantial magnetic fields, even if the telescopes are unable to visualize their shapes. Anticipating advancements in telescope technology, such as the Next Generation Very Large Array being planned by the National Radio Astronomy Observatory (NRAO), Kao expressed excitement about the possibility of imaging a greater number of extrasolar radiation belts.
Co-author Evgenya Shkolnik from Arizona State University, a longstanding researcher in magnetic fields and planetary habitability, emphasized the significance of this initial step in locating more objects of similar nature and refining techniques to investigate increasingly smaller magnetospheres. Ultimately, this progress will enable the study of potentially habitable Earth-sized planets.
The study utilized the High Sensitivity Array, a collection of 39 radio dishes coordinated by the NRAO in the United States, in conjunction with the Effelsberg radio telescope operated by the Max Planck Institute for Radio Astronomy in Germany. Through the collaborative efforts of researchers across the globe, highly detailed images with unprecedented resolution were generated, unveiling previously unobserved phenomena. Jackie Villadsen from Bucknell University, another co-author, compared the image obtained to reading the top row of an eye chart in California while standing in Washington, D.C., highlighting the remarkable precision achieved.
Kao acknowledged the essential role played by co-first author Amy Mioduszewski at NRAO, who contributed her expertise in observational planning and data analysis, as well as the valuable insights provided by Villadsen and Shkolnik in the domains of multiwavelength stellar flares and magnetic fields, respectively. This discovery stands as a testament to the collaborative efforts and diverse expertise of the research team.