Detecting Magnetospheric Radio Emission from Giant Exoplanets

Abstract

As radio astronomy enters a golden age, ground-based observatories are reaching sensitivities capable of unlocking a new and exciting field of exoplanet observation. Radio observation of planetary auroral emission provides unique and complementary insight into planetary science not available via orthodox exoplanet observation techniques. Supplying the first measurements of planetary magnetic fields, rotation rates, and orbital obliquities, we gain necessary and crucial insight into our understanding of the star–planet relationships, geophysics, composition, and habitability of exoplanets. Using a stellar-wind-driven Jovian approximation, we present analytical methods for estimating magnetospheric radio emission from confirmed exoplanets. Predicted radio fluxes from cataloged exoplanets are compared against the wavelengths and sensitivities of current and future observatories. Candidate exoplanets are downselected based on the sky coverage of each ground-based observatory. Orbits of target exoplanets are modeled to account for influential orbit-dependent effects in anticipating time-varying exoplanet radio luminosity and flux. To evaluate the angular alignment of exoplanetary beamed emission relative to Earth’s position, the equatorial latitude of exoplanetary auroral emission is compared against Earth’s apparent latitude on the exoplanet. Predicted time-dependent measurements and recommended beamformed observations for ground-based radio arrays are provided, along with a detailed analysis of the anticipated emission behavior for τ Boo b.

ROME. IV. An Arecibo Search for Substellar Magnetospheric Radio Emissions in Purported Exoplanet-hosting Systems at 5 GHz

“…Recently, Ashtari et al (2022) sought to improve upon previous theoretical efforts to compute the maximum fundamental frequencies and flux densities of ECM emissions from 671 exoplanets. This effort combined scaling laws of exoplanet magnetic properties (i.e., convective zone sizes, magnetic moments, magnetopause standoff distances, surface magnetic fields, and maximum frequency of ECM emission) with GAMERA 3D magnetohydrodynamic simulations of stellar wind activity to produce potentially ultraprecise radio spectra of targets.…”

“…Although the reasoning presented in (Ashtari et al 2022, Appendix B) directly follows prior work by Lazio et al (2004), the omission of the earlier work's disclaimer that the maximum frequency of emission may vary by a factor of 3 due to statistical uncertainties in their empirical relation can underestimate uncertainty in modern models. Ashtari et al (2022) also omit other important factors in the detection of substellar magnetospheric radio emissions. In a number of systems, unless the exoplanet ECM frequency is greater than the local plasma frequency, no ECM signal will be detected.…”

“…For example, Kavanagh et al (2019) determined that the exoplanet in the HD 189733 system requires ν peak > 21 MHz for the its magnetospheric emissions to escape the stellar wind plasma frequency. Exoplanet radio emissions with ν peak  10 MHz would be reflected by the terrestrial ionosphere (de Pater & Lissauer 2015) and hence, undetectable by lowfrequency ground-based surveys (e.g., 51 Peg and AU Mic b in their Table 5; Ashtari et al 2022). Both phenomena are not accounted for in Ashtari et al (2022).…”

Detecting Technosignatures from Earth-scale Civilizations

“…While the conservative targets in Tables 1 and 3 are chosen using more selective demographics, even these candidates have their own notable cautions. M dwarfs, such as that hosting the TRAPPIST-1 planetary system, are predicted to have high levels of stellar activity, making planetary atmospheric and magnetospheric retention planets within their theorized habitable zones difficult (Ashtari et al 2022). Furthermore, the stellar ages for several targets are either unknown or much less than the ∼4.5 billion yr age of the Sun (Carroll & Ostlie 2017), making it difficult or unlikely to conclude associations between stellar age and the technological maturity or age of a possible civilization.…”

“…Complementary to technosignatures, these radio observations also allow for measurement of stellar flares and stellar flare echoes from exoplanets (Schneider 2005), as well as exoplanetary radio emission (Ashtari et al 2022). For planets orbiting M dwarfs, the impact of the flare rate on atmospheric and magnetospheric retention is a huge debate in the discussion of exoplanet habitability (Schulze-Makuch et al 2011).…”