Exodynamos

Exoplanet dynamos, or "exodynamos", have yet to be directly detected, although there presence has been inferred from interactions with their host stars. In Driscoll & Olson (2011) we computed optimistic, or "optimal", magnetic field strengths at the surface of hypothetical exoplanets ranging from 1-10 Earth masses, and either an Earth-like (0.35) or Mercury-like (0.65) core mass fraction (CMF). To maximize the magnetic field strength we assumed there are no heat sources in the mantle and that the temperatures in the mantle thermal boundary layers (which determine the heat loss) are at the local solidus. We use a buoyancy-flux based magnetic scaling law to compute field intensity at the planetary surface. Finally, we estimate the cyclotron emission frequency and power from these exodynamos as shown below. These emissions remain below the detection threshold of modern radio interferometers (e.g. LOFAR), but stronger magnetic fields, temporary enhancements due to flares, and technological advances could make these objects observable in the future.

Optimal dynamos in the cores of terrestrial exoplanets: Magnetic field generation and detectability

Fig. 8. Cyclotron radio emission spectrum for optimal 32% and 65% CMF exoplanets. Shaded region indicates the terrestrial dynamo region for cyclotron emission. Solid curves are the emission from 1–10M_Eoptimal exoplanets orbiting at a = 0.02 AU around a Centauri at s = 1.33 pc and GJ876 at s = 4.72 pc from Earth. Also shown are the expected emissions for nearby exoplanets GJ674b, GJ581b, GJ581e, 55Cnc e, and HD7924b all assuming 65% CMF, and for Earth, Jupiter, and Saturn analogs assuming they orbit a Centauri. The ionospheric cutoff at 10 MHz sets the lower frequency limit for ground-based radio telescopes such as LOFAR, shown for an 8 h exposure detection threshold.

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