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51 Pegasi b Fellow Discovers First Extrasolar Radiation Belt: Q&A with Dr. Melodie Kao

A team of scientists led by Dr. Melodie Kao, a 51 Pegasi b postdoctoral fellow based at the University of California, Santa Cruz, has discovered the first extra-solar radiation belt, a zone of energetic charged particles that are captured by and held around a planet by that planet’s magnetosphere. See below for a Q&A with Dr. Melodie Kao to learn more about this important discovery.

Can you tell us about your discovery and why it is important?

We have observed the first radiation belt outside our solar system—around an ultracool dwarf, and this discovery also proves that radiation belts can exist around objects that aren’t just planets. An article published in a May issue of Nature describes the discovery and its importance.

65 years ago, the first radiation belt was discovered – Earth’s! All of the planets in our solar system that have “big” magnetic fields—including Earth, Jupiter, and other giant planets—have radiation belts of these high-energy charged particles trapped by that planet’s magnetic field. And what’s really cool is that, while the solar wind is an important source of radiation belt particles in the solar system, particles can also come from volcanoes or even the planet’s own atmosphere. For instance, Jupiter’s moon Io has over 400 active volcanoes and it’s a major supplier of radiation belt particles in Jupiter’s magnetosphere. 


What is a radiation belt?

Strong magnetic fields form a “magnetic bubble” around a planet called a magnetosphere, which can trap and accelerate particles like electrons, protons, and ions like oxygen and sulfur to really high energies, forming a radiation belt.

The particular properties of each radiation belt tell us something about that planet’s personality: how quickly it’s spinning, how strong its magnetic field is, how close it likes to be to the Sun, if it has moons or rings, and more.

And now we’ve discovered that not just planets have radiation belts. We studied an ultracool dwarf planet that doesn’t have a host star like Jupiter has the Sun. Instead, it’s hanging out on its own about 20 lightyears away. So, one of the questions emerging for me is: Could there be something like a volcanic Io (or a few) in the magnetospheres of the ultracool dwarfs that have radiation belts? If not, where are they getting their electrons from? For the first time, we’re starting to be able to see what sorts of energetic particles surround ultracool dwarfs. 


What are ultracool dwarfs?

The object we studied, LSR J1835+3259 is an “ultracool dwarf” – with a mass of around ~78 Jupiter masses. It’s too massive to be a planet and might not be quite massive enough to be a star. Ultracool dwarfs are  showing us that the phenomenological boundary between stars and planets is more porous than we thought—some low mass stars and brown dwarfs exhibit magnetic behaviors that are more planet-like than star-like.

How did you make this discovery?
Artist’s impression of an aurora on the ultracool dwarf LSR J1835+3259 and its surrounding radiation belt. Image credit: Chuck Carter ( Melodie Kao (

Our team coordinated an array of 39 radio dishes from 4 radio observatories across the world to make an Earth-sized telescope in order to see the structure of this radiation belt. The bigger the telescope, the higher resolution. This is the first time that we’ve imaged the magnetosphere of a planet-sized object outside of the solar system, and we did this by taking a picture of the radio-emitting electrons trapped in its large-scale magnetic field. Actually, I’m really proud of the fact that we used all of the National Radio Astronomy Observatory’s (NRAO) Northern Hemisphere telescopes: the Karl G. Jansky Very Large Array of twenty-seven 25-meter dishes in New Mexico, the Very Long Baseline Array of ten 25-meter dishes that span Hawaii to St. Croix in the Caribbean, and the 100-meter Robert C. Byrd Greenbank Telescope in West Virginia.  Plus, the 100-meter Effelsberg Telescope in Germany and managed by the Max Planck Institute for Radio Astronomy was a major contributor to our observing campaign. Just to put that into context, JWST has a 6.5-meter diameter and a collecting area of 33 square meters.  The collecting area of the array I used was almost 34,000 square meters—or a factor of 1,000 times more collecting area than JWST!

I think one of the fun things about my science is that it really pushes the observational techniques of modern radio observatories, so I get to spend a lot of time dreaming up future cool experiments and instruments.

The ultracool dwarf that you studied also has an aurora. What would the aurora look like from the perspective of an astronaut visiting it?

Realistically, the astronaut would see a solid wall of metal that’s keeping her alive because she’d have to be in a pretty radiation-proof spacecraft. The radiation belt around LSR J1835+3259 is almost 10 million times more intense than Jupiter’s, which itself is already several million times more intense than Earth’s. When NASA sent astronauts to the Moon during the Apollo mission days, they had to be careful to limit astronaut exposure to Earth’s radiation belts.

But if she could peek past that wall of metal, she would see that its aurora probably looks quite different from Earth’s because almost the whole atmosphere of LSR J1835+3259 consists of hydrogen, whereas Earth’s atmosphere is made of less than 1% hydrogen and instead is mostly nitrogen and oxygen. Earth’s aurorae glow a lot at blue and green wavelengths with some red from nitrogen and oxygen, and it also has faint purple from hydrogen.

For ultracool dwarfs like LSR J1835+3259, Jupiter is a better analog, though we know from recent Hubble Space Telescope observations that they aren’t going to be exactly like Jupiter’s aurorae. Our best guess is that we think that they’ll be glowing deep red from excited hydrogen, as well as at far infrared and ultraviolet wavelengths that are invisible to the naked eye. Beyond that, we need telescopes like James Webb and Hubble Space Telescopes to help us figure out how else LSR J1835+3259’s aurorae look like.

But if our astronaut could stick a radio outside of her spacecraft that was tuned to frequencies that are about 100 times higher than FM radio frequencies, she could hear its aurora and also its radiation belts. Jupiter’s aurorae sound eerie—almost like a mysterious creature singing into space.

For LSR J1835+3259, she’d have to make sure the volume was turned down, since its aurora would be much louder than Jupiter’s. It’d be fun if any readers out there ever wanted to turn any of my data into an audio clip. If you do, please let me know!

What got you personally interested in this research?

When I was in grad school, I was part of the team that established that aurorae occur on ultracool dwarfs. But I always wondered about the radio emission that wasn’t coming from the aurora. What secrets did it hold?

More broadly, I’ve always been interested in learning new ways to see things in a different manner.

I wondered what would happen if I picked something that’s invisible, like magnetism, to be my medium for studying all the wonderful things happening on other worlds far away. The more we learn about magnetism—how interior properties and fluid dynamics of objects generate and sustain magnetic fields, the dynamic plasma environments that they contain and energize, the aurorae that they power, and even the clues they hold about long-forgotten early atmospheres or moons and volcanoes that we haven’t yet seen—the more fully we can tell the stories occurring everyday, everywhere in all the planetary systems that ever were and ever will be.

“At the end of the day, as an astronomer, my job is to help the Universe tell its stories, so I try to design experiments that help me find ways to listen and see more deeply. In this way, we can grow ever closer from our perceptions to the deep yet never fully knowable beauty of reality.” Dr. Melodie Kao

What are you excited about next?

I’m really excited for when the ngVLA and the Deep Synoptic Array 2000 are built. These extra sensitive arrays will make it possible to start doing big statistical experiments that can help us better understand the “recipe” for making extrasolar radiation belts and the factors that help determine their “personalities” like how intense they are, their sizes, and their other observable properties.  I’ve been working on a statistical framework to do exactly that in anticipation of this upcoming era of big radio data.

More broadly, I’m curious to see how we can harness extrasolar radiation belts as “instruments”––in this case, nature-made rather than human-made, for learning more about brown dwarf and exoplanet systems. Will astronomers eventually find moons or exoplanets around one of the objects I’ve studied? Will radiation belts help us one day learn more about what volcanic activity can look like outside our solar system? Will having lots of different radiation belts to study show us that the ones in our own solar system are atypical?

Finally, I’m also really excited for the day that we’ll get to study radio emissions from terrestrial exoplanets. We’ll have to go into space for that, since Earth’s upper atmosphere blocks out the low frequency radio waves that we expect terrestrial planets to emit. There are a lot of mission concepts being developed for this right now, and I’m hopeful that one day radio astronomers can team up with NASA to build our nation’s first radio Great Observatory in space––perhaps on the far side of the Moon, or even freely floating in space along Earth’s orbit.

How has the 51 Pegasi b Fellowship helped in pursuing this work?

The 51 Pegasi b Fellowship gave me the time, funding, and intellectual freedom to take on a big experimental risk and see it through to the end. This fellowship program is incredibly special, because we’re given so much latitude to invest our research dollars and time. At each step of the way, the science team at the Heising-Simons Foundation trusted me and told me that they were investing in me. That sort of trust gave me the freedom to develop and refine my own creative process.

As a scientist, I’m leading expeditions into the wilderness of space and human knowledge. Much like how mountaineers will spend years assembling the right team and acquiring the skills and resources to pull off a first ascent, I needed to do the same for this discovery. For me, the Heising-Simons Foundation, together with NASA, funded my team’s “first ascent” into the terrain of extrasolar radiation belt physics.