The Cold Star Discovered by Its Radio Auroras

By the end of this article, you will understand how astronomers use radio telescopes to find freezing, invisible ‘failed stars’ by hunting for their powerful magnetic auroras.

For decades, astronomers found brown dwarfs—objects too big to be planets but too small to be stars—by looking for their faint heat using infrared telescopes. But in 2020, a team tried something completely new. They used the LOFAR radio telescope to scan the sky for specific, spiraling radio waves. They found a Surprise: a strong radio signal from a completely blank, dark patch of sky. When they followed up with telescopes in Hawaii, they confirmed it was a freezing ‘failed star’ named BDR J1750+3809. They didn’t find it by seeing it; they found it by ‘listening’ to its massive magnetic field. This is the first time a sub-stellar object was discovered directly through radio waves!

Original Paper: ‘Direct radio discovery of a cold brown dwarf’

BDR J1750+3809 is the first radio-selected substellar object, which demonstrates that such objects can be directly discovered in sensitive wide-area radio surveys.
— H. K. Vedantham

To understand this discovery, we need to know what a brown dwarf actually is. This is NOT a normal star, because it lacks the mass to ignite hydrogen fusion in its core. But it is NOT a normal planet either, because it forms freely from collapsing gas clouds in space rather than growing inside a debris disk around a sun. The Salient Idea here is that these ‘failed stars’ are extremely cold. BDR J1750+3809 is so chilly it has methane in its atmosphere! Because it doesn’t shine with starlight, it is almost entirely invisible to regular optical telescopes. The only way it announces its presence is by shooting out intense, highly polarized radio beams from its poles.

Here on Earth, the Northern Lights are beautiful visual displays caused by the solar wind hitting our magnetic field. But on gas giants like Jupiter, and on brown dwarfs, this same process creates invisible, incredibly powerful radio waves. This is called the Electron Cyclotron Maser Instability (ECMI). The radio waves we detected from BDR J1750+3809 are literally the sound of its auroras. Its magnetic field is about 25 Gauss—comparable to a giant planet’s. In fact, scientists think this brown dwarf’s auroras might be supercharged by an invisible moon orbiting close by, similar to how Jupiter’s moon Io powers Jupiter’s intense radio auroras!

Our discovery suggests that low-frequency radio surveys can be employed to discover sub-stellar objects that are too cold to be detected in infrared surveys.
— The Research Team

How do you prove a random radio beep is a brown dwarf? It comes down to circular polarization. The LOFAR telescope didn’t just measure the brightness of the radio waves; it measured their ‘spin’. Normal stars and galaxies emit messy, unpolarized radio waves. But BDR J1750+3809 had a highly polarized radio fraction of almost 100%. This is the ‘fingerprint’ of an ECMI aurora. The team had to use massive supercomputers to sift through 8 hours of radio data, ruling out pulsars and normal stars, before turning massive infrared telescopes toward the exact coordinates to confirm the cold methane dwarf hidden in the dark.

Searching for circularly polarized radio sources has proved to be a powerful technique to identify coherent stellar radio emission.
— The Discovery Team

Q: If brown dwarfs are failed stars, could they have planets of their own?
A: Yes! Astronomers actually think the incredibly bright radio auroras on this brown dwarf might be caused by an undiscovered, orbiting planet or moon generating electrical currents, much like the Jupiter-Io system.