Unravelling sub-stellar magnetospheres

TL;DR

This paper introduces a new tomographic method to analyze magnetic field structures of ultracool dwarfs using radio observations, offering insights into planetary magnetospheres and potential applications to exoplanets.

Contribution

The authors develop a novel technique for reconstructing magnetic field geometry and viewing angles of ultracool dwarfs from radio burst data, advancing planetary magnetic field studies.

Findings

J0623 is likely viewed pole-on.

J0623's magnetic and rotational axes are significantly misaligned.

J0623 may have a magnetic cycle longer than 6 months.

Abstract

At the sub-stellar boundary, signatures of magnetic fields begin to manifest at radio wavelengths, analogous to the auroral emission of the magnetised solar system planets. This emission provides a singular avenue for measuring magnetic fields at planetary scales in extrasolar systems. So far, exoplanets have eluded detection at radio wavelengths. However, ultracool dwarfs (UCDs), their higher mass counterparts, have been detected for over two decades in the radio. Given their similar characteristics to massive exoplanets, UCDs are ideal targets to bridge our understanding of magnetic field generation from stars to planets. In this work, we develop a new tomographic technique for inverting both the viewing angle and large-scale magnetic field structure of UCDs from observations of coherent radio bursts. We apply our methodology to the nearby T8 dwarf WISE J062309.94-045624.6 (J0623) which was recently detected at radio wavelengths, and show that it is likely viewed pole-on. We also find that J0623's rotation and magnetic axes are misaligned significantly, reminiscent of Uranus and Neptune, and show that it may be undergoing a magnetic cycle with a period exceeding 6 months in duration. These findings demonstrate that our method is a robust new tool for studying magnetic fields on planetary-mass objects. With the advent of next-generation low-frequency radio facilities, the methods presented here could facilitate the characterisation of exoplanetary magnetospheres for the first time.