Even early explanations for Earth’s magnetic field linked it to Earth’s interior structure. After the discovery of Jupiter’s radio emission, it was determined that this radiation is due to Jupiter’s magnetic field, which was then tied to the planet’s interior structure. Remote sensing and in situ measurements have since shown that the Earth, Mercury, Ganymede, and the giant planets of the Solar System all contain internal dynamos that generate planetary-scale fields; Mars and the Moon show residual magnetism indicative of past dynamos.
The stellar wind, a super- or transonic magnetized plasma, when incident on a planet's magnetosphere is an energy source to the magnetosphere. The radio emission from an electron cyclotron maser, resulting from this magnetosphere-solar wind interaction, has been detected from the Earth and all of the giant planets in the Solar System.
Detecting magnetospherically-generated radio emission from extrasolar planets provides a ready means to address a broad range of questions, two of which are
What characteristics of an extrasolar planet might contribute to it being habitable? A planet's magnetic field shields its atmosphere from its host star's stellar wind, which may be a factor in terrestrial planet habitability.
What constraints can be placed on the interior structures and compositions of extrasolar planets? Internal dynamos arise from differential rotation, convection, compositional dynamics, or a combination of these processes. Knowledge of extrasolar planetary magnetic fields has the potential to constrain internal compositions and dynamics, which will be difficult to determine by other means.
A combination of ground- and space-based telescopes will be required, with ground-based telescopes likely focusing on Jovian-mass planets and space-based telescopes likely focusing on ice giants and terrestrial planets. This paper draws heavily on the W. M. Keck Institute for Space Studies report Planetary Magnetic Fields: Planetary Interiors and Habitability (Lazio, Shkolnik, & Hallinan 2016), and it incorporates topics discussed at the AAS Topical Conference “Radio Exploration of Planetary Habitability.”
Among the factors expected to affect habitability, the Exoplanet Science Strategy identified “[t]he presence and strength of a global scale magnetic field, which depends on interior composition and thermal evolution ...” Further, “... the persistence of a secondary atmosphere over billion-year time scales requires low atmospheric loss rates, which in turn can be aided by the presence of a planetary magnetic field ....”
Spacecraft observations confirm that the solar wind stagnates at the bow of a planet's magnetosphere, with the bulk of the plasma deflected around the magnetospheric cavity (Figure 1). As such, it seems plausible that a global field reduces a planet's atmospheric loss, in particular helping to retain the hydrogen and oxygen ions (i.e., water), and dramatic evidence for atmospheric erosion has been provided by the Mars Atmosphere and Volatile Evolution (MAVEN) mission (Jakosky et al. 2015). Unshielded from the solar wind due to the lack of a global magnetic field, Mars' atmosphere was observed to be eroded during an interaction with a coronal mass ejection (CME).Surprisingly, however, Venus, Earth, and Mars have similar present-day average atmospheric losses, of order 1025 O+ s-1 from their polar regions (Moore & Khazanov 2010). Ideally, a large sample of planets, with a range of atmospheric compositions and magnetic field properties would be available to test the extent to which the presence of a magnetic field protects an atmosphere.
The detection of even a single extrasolar planetary magnetic field could provide essential information on planetary interiors and dynamos. A limiting factor in understanding planetary dynamos is the small sample in the Solar System (Stevenson 2010; Schubert & Soderlund 2011). Just as the discovery of hot Jupiters gave crucial insights to the diversity of planets, the detection of extrasolar magnetic fields likely will improve our understanding of magnetic dynamos, including in our Solar System.
Inferring planet compositions is an under-constrained inversion problem because planets with disparate compositions can have similar masses and radii. For instance, similar bulk densities could be obtained for a planet with a rock-ice interior and primordial H-He envelope or a water planet or a super-Earth with a H-rich outgassed atmosphere. Magnetic field measurements, providing information about interior structures and compositions, would complement measurements of upper atmosphere compositions obtained by spectroscopy.
The absence of magnetic fields in either ice giants or gas giant would challenge our understanding of their interiors. For ice giants, water is electrically conducting above a few thousand Kelvin, and detections of their magnetic fields would confirm their compositions as being substantially volatiles. Similarly, in Jovian planets with massive H-He envelopes, hydrogen is metallic above about 25 GPa, and they are expected to be convective at depth.
The presence of magnetic fields might be most informative for rocky planets, which are not guaranteed to have electrically conducting liquid iron cores. Partial core solidification may limit the range of planet masses that can sustain dynamos, and the extent to which an iron core solidifies is sensitive to the presence of volatiles. Further, the energy budget for convection in Earth's core is marginal. Higher temperature (> 1500 K), stronger tidal heating, higher concentrations of radioactive nuclei, the presence of a thick H-He envelope, or a stagnant lid tectonic regime could turn off convection (and a dynamo) in the core of an otherwise Earth-like planet. It is even possible that different mechanisms operating at different times have been responsible for generating Earth's magnetic field (Ziegler & Stegman 2013). The inference of convection via a magnetic field measurement would constrain the planet's thermal evolution and energy budget and may serve as an indirect indication of plate tectonics.
An electron cyclotron maser generated by a stellar wind-planetary magnetosphere interaction enables a direct measure of a planet's magnetic field (Figure 2). The emission occurs up to a characteristic frequency determined by the polar cyclotron frequency, which depends upon the planet's magnetic field B, fECM = 2.8 MHz(B/1 Gauss); for Jupiter, BJ ~ 15 G and fECM,J ~ 30 MHz. Further, scaling laws exist, based on the Solar System planets (Zarka et al. 1998; Farrell et al. 1999; Christensen 2010). These relations are predictive, with the luminosities of Uranus and Neptune predicted before the Voyager 2 encounters (Desch & Kaiser 1984; Desch 1988; Millon & Goertz 1988). However, only planets with magnetic field strengths comparable to those of Jupiter are detectable from the ground, due to absorption by the Earth's ionosphere; indeed, even the Earth's electron cyclotron maser emission, or the auroral kilometric radiation (AKR), was not discovered until the advent of the Space Age.
Figure 3 presents a graphical summary of most published limits on the radio emission from extrasolar planets. Not shown are a few observations at frequencies above 1000 MHz and a few observations at frequencies around 20 MHz, observations that provide effectively no constraints on planetary radio emission. The lack of effective constraints below about 20 MHz results from the effects of the Earth's ionospheric absorption.
Until recently, most searches for magnetospherically-generated radio emission from extrasolar planets had been at frequencies sufficiently high that they would have been successful only if giant planets could generate magnetic fields an order of magnitude or more stronger than that of Jupiter. Recent observations of HD 80606b with the Low Frequency Array (LOFAR), however, have obtained an order-of-magnitude improvement in sensitivity at frequencies comparable to those at which Jupiter emits (de Gasperin et al. 2020).
Over the next decade, the following are likely: (i) The Juno mission, and potentially subsequent outer planet missions, will improve our knowledge of the magnetic dynamos of the Solar System's giant planets. (ii) Studies of the solar neighborhood will refine the set of extrasolar planets for which magnetic field measurements would be possible and valuable. (iii) Ground-based telescopes, e.g., LOFAR, the Long Wavelength Array at the Owens Valley Radio Observatory (OVRO-LWA), NenuFAR, will improve upon the sensitivity and techniques for detecting extrasolar planetary magnetospheric emissions, with a likely focus on giant planets, and potential surprises from ice giants if their fields are sufficiently strong. (iv) The Sun Radio Interferometer Space Experiment (SunRISE, Kasper et al. 2020), a space-based radio telescope designed to observe solar radio bursts generated by space weather events such as CMEs, such as those that erode Mars' atmosphere, will prove out technologies related to a future mission for studying extrasolar planetary radio emission.
It is a pleasure to thank the many colleagues who have aided my understanding of planetary magnetic fields, planetary radio emissions, and the possibilities for (and challenges of) future space missions. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
Christensen, U. R. 2010, "Dynamo Scaling Laws and Applications to the Planets," Space Sci. Rev., 152, 565;
Desch, M. D. 1988, "Neptune radio emission: Predictions based on planetary scaling laws," Geophys. Res. Lett., 15, 114;
Desch, M. D., & Kaiser, M. L. 1984, "Predictions for Uranus from a radiometric Bode's law," Nature, 310, 755;
de Gasperin, F., Lazio, T. J. W., & Knapp, M. 2020, "Radio Observations of HD80606 Near Planetary Periastron: II. LOFAR Low Band Antenna Observations at 30--78 MHz," A&A, in press; arXiv:2011.05696
Farrell, W. M., Desch, M. D., & Zarka, P. 1999, "On the possibility of coherent cyclotron emission from extrasolar planets," JGR, 104, 14025;
Jakosky, B. M., et al. 2015, "MAVEN observations of the response of Mars to an interplanetary coronal mass ejection," Science, 350, 0210;
Kasper, J., Lazio, J., Romero-Wolf, A., Lux, J., & Neilsen, T. 2020, "The Sun Radio Interferometer Space Experiment (SunRISE) Mission Concept," in 2020 IEEE Aerospace Conference;
Lazio, T. J. W., Shkolnik, E., Hallinan, G., et al. 2016, Planetary Magnetic Fields: Planetary Interiors and Habitability (Keck Institute for Space Studies: Pasadena, CA)
Millon, M. A., & Goertz, C. K. 1988, "Prediction of radio frequency power generation of Neptune's magnetosphere from generalized radiometric Bode's law," Geophys. Res. Lett., 15, 111;
Moore, T. E., & Khazanov, G. V. 2010, "Mechanisms of ionospheric mass escape," J. Geophys. Res.: Space Physics, 115, A00J13;
Schubert, G., & Soderlund, K. M. 2011, "Planetary magnetic fields: Observations and models," Phys. Earth Plan. Interiors, 187, 92;
Stevenson, D. J. 2010, "Planetary Magnetic Fields: Achievementsand Prospects," Space Sci. Rev., 152, 651;
Zarka, P., Queinnec, J., Ryabov, B. P., et al. 1997, "Ground-Based High Sensitivity Radio Astronomy at Decameter Wavelengths," in Planetary Radio Emissions IV, eds. H. O. Rucker, S. J. Bauer, & A. Lecacheux (Austrian Academy of Sciences: Vienna) p. 101, ISBN: 3700126913;
Ziegler, L. B., & Stegman, D. R. 2013, "Implications of a long-lived basal magma ocean in generating Earth's ancient magnetic field," Geochem. Geophy. Geosy., 14, 4735;