Since the late 1950s, we have been aware of the existence of radiation belts around Earth and Jupiter. A radiation belt is a doughnut-shaped region around an object created by its magnetic field. Charged particles (mainly electrons, but not exclusively) are trapped in this region and accelerated so much that astronauts need to take Earth’s radiation belt into consideration when leaving our home planet. Years after this discovery, we found the same structure in the other magnetized planets of the Solar System (Saturn, Uranus and Neptune). Our knowledge of these radiation belts came originally from observations at radio frequencies. However, radio observations of objects beyond the Solar System have not been able to conclude that these magnetic structures also exist there… until now.

 

On 15 June 2021, our research group carried out radio observations of LSR J1835+3259:  an object classified as an “ultracool dwarf” (UCDs) with a surface temperature of 2300 K.  It is estimated to be similar in size to Jupiter but 55 times more massive. Only 18.5 light years away from Earth, LSR J1835+3259 represented our best chance of probing the surroundings of UCDs, particularly the region covered by its powerful magnetic field: the magnetosphere. 

 

But to achieve enough “zoom” in our images to discern the magnetosphere, we needed a very special technique: very long baseline interferometry (VLBI). This is the same technique that was employed to obtain the very first images of a black hole back in 2019. In a nutshell, VLBI uses an array of antennas around the globe to create a virtual telescope with the same size as the maximum separation between antennas. For example, in the first black hole image, the maximum separation between antennas was around 10,000 km, which resulted in a virtual telescope so big that one could appreciate, from Earth, the details of a golf ball located on the Moon.

 

In our observations, we used the European VLBI Network (EVN) for 6 hours. This total observing time is important as it covers two full rotations of the object (yes, LSR J1835+3259 rotates once every 2.8 hours!). We detected two types of radio emission: (i) quiescent emission that remained almost constant during the observations and (ii) bursting radio emission that occurred once per rotation. The bursting emission was approximately 10 times greater than the quiescent emission but only lasted for about 30 minutes. The origin of the bursting emission is usually linked to auroras similar to the ones we have on Earth but much more intense. The origin of the quiescent emission is much less known, with one of the hypotheses being a radiation belt.

 

To shed some light on the origin of this quiescent emission, we created images of LSR J1835+3259 during each rotation period and were able to resolve its magnetosphere. This object did not look like a point in our images, but showed a very interesting structure. What is even more interesting is that when the bursting emission occurred, we could separate the quiescent contribution, and those images revealed a pattern of emission very similar to a radiation belt. This was the first time resolving the magnetosphere of a UCD and it showed that, at least, part of its emission came from a radiation belt. Our estimations show that, although the energies of the electrons that populate the radiation belt of LSR J1835+3259 are similar to those that populate the Jupiter one, the total size of this newly discovered radiation belt is 10 times larger and millions of times more powerful. 

 

Perhaps the greater significance of this discovery is that this magnetic structure has not been seen on an exoplanet but rather on a much more massive object. This suggests that UCDs exhibit magnetic behaviors more akin to planets than stars, while also opening up exploration for the search for other radiation belts around even more massive objects.

 

Finally, one may wonder about the origin of the particles of LSR J1835+3259 radiation belt. Although it is still an open question, one possible explanation would be to rely on an exoplanet similar to the one proposed by other authors in 2015 to explain the powerful auroras seen in this UCD (via interaction UCD-exoplanet). This tentative exoplanet could also act as a source of plasma that would populate the radiation belt. This complex yet intriguing scenario awaits further data for validation.