Radio emission from exoplanets

A way to directly detect exoplanetary magnetic fields would be via their radio emission. Jupiter is the brightest source of radio signals below 40 MHz. The observed radio emission is thought to come from the cyclotron-maser instability, which acts at a maximum frequency of about 2.8 B MHz (where B is the intensity of the magnetic field in Gauss), and depends on the Jupiter’s magnetospheric interaction with the Solar wind.

Jupiter Magnetic field

The searches of exoplanetary radio emissions have so far targeted several dozens of nearby hot Jupiters, setting upper limits of sub-mJy at best. Nevertheless, there are reasons to be optimistic and expect detections within the next decade. Looking at the planetary diversity, we could expect larger values of magnetic fields, and simple estimates (scaling with rotation) could be violated. New observations have recently allowed to indirectly infer unexpectedly large values of magnetic fields (up to 100 G) for a few hot Jupiters. Such highly-magnetized planets would emit at higher luminosity and frequencies (approaching GHz), so that deep observations could have high chances of detection. Even within a few parsecs the sample of exoplanets is far from being complete, and only a few have been targeted.

The quest for radio emission of exoplanets is therefore gaining momentum, with an increasing number of campaigns aiming at detecting it in the last years. A first tentative detection, to be confirmed after on-going fllow-up studies, has been claimed in 2021. We will propose observations with long-wavelength radio facilities like GMRT and LOFAR, in order to get new upper limits, or, luckily, first detections.

The electron cyclotron maser instability is the mechanism operating for Jupiter. It is classically understood, but is difficult to quantify due to uncertain key factors: emission region, local density of plasma and magnetic field, stellar wind properties. We will build a simplified but quantitative emission model, relying on plasma dynamics under certain conditions, parametrized effectively by magnetic field intensity, plasma density and stellar wind. We will then consider a broad range of values for the distribution of plasma, magnetic field, incoming stellar wind and emission regions and we will estimate the emitted luminosities. A sensitivity analysis is performed to capture the fundamental characteristics that can enhance the radio detection. This will help us to identify the best candidates for radio detections.

We are proposing several targets with the Giant Metrewave Radio Telescope (India) and the Very Large Array, mostly aiming to understanding the connection between the magnetism of brown dwarfs and giant exoplanets.

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