Radio emission from exoplanets: the role of the stellar coronal density and magnetic field strength

TL;DR

This paper models radio emissions from close-in exoplanets, emphasizing the influence of stellar coronal density and magnetic field strength, and predicts detectable signals under certain stellar conditions.

Contribution

It introduces a model considering magnetic reconnection and coronal density effects, highlighting their importance in exoplanet radio emission predictions.

Findings

Radio fluxes of tens of mJy are possible for close-in exoplanets with strong stellar magnetic fields.

Radio emission power depends mainly on the ratio of coronal density to stellar magnetic field strength.

High-density stellar coronae increase the likelihood of detectable exoplanet radio signals.

Abstract

The search for radio emission from extra-solar planets has so far been unsuccessful. Much of the effort in modelling the predicted emission has been based on the analogy with the well-known emission from Jupiter. Unlike Jupiter, however, many of the targets of these radio searches are so close to their parent stars that they may well lie inside the stellar magnetosphere. For these close-in planets we determine which physical processes dominate the radio emission and compare our results to those for large-orbit planets that are immersed in the stellar wind. We have modelled the reconnection of the stellar and planetary magnetic fields. We calculate the extent of the planetary magnetosphere if it is in pressure balance with its surroundings and determine the conditions under which reconnection of the stellar and planetary magnetic fields could provide the accelerated electrons necessary for the predicted radio emission. We show that received radio fluxes of tens of mJy are possible for exoplanets in the solar neighbourhood that are close to their parent stars if their stars have surface field strengths above 1-10G. We show that for these close-in planets, the power of the radio emission depends principally on the ratio (Nc/B^{1/3})^2 where Nc is the density at the base of the stellar corona, and B is the stellar surface magnetic field strength. Radio emission is most likely to be detected from planets around stars with high-density coronae, which are therefore likely to be bright X-ray sources. The dependence of stellar coronal density on stellar rotation rate and effective temperature is crucial in predicting radio fluxes from exoplanets.