  Properties of planetary atmospheres, ionospheres, and magnetospheres are
difficult to measure from Earth. Radio occultations are a common method for
measuring these properties, but they traditionally rely on radio transmissions
from a spacecraft near the planet. Here we explore whether occultations of
radio emissions from a distant astrophysical radio source can be used to
measure magnetic field strength, plasma density, and neutral density around
planets. In a theoretical case study of Jupiter, we find that significant
changes in polarization angle due to Faraday rotation occur for radio signals
that pass within 10 Jupiter radii of the planet and that significant changes in
frequency and power occur from radio signals that pass through the neutral
atmosphere. There are sufficient candidate radio sources, such as pulsars,
active galactic nuclei, and masers, that occultations are likely to occur at
least once per year. For pulsars, time delays in the arrival of their emitted
pulses can be used to measure plasma density. Exoplanets, whose physical
properties are very challenging to observe, may also occult distant
astrophysical radio sources, such as their parent stars.
