  Detection of radio emission from Jupiter was identified quickly as being due
to its planetary-scale magnetic field. Subsequent spacecraft investigations
have revealed that many of the planets, and even some moons, either have or
have had large-scale magnetic fields. In the case of the Earth, Jupiter,
Saturn, Uranus, and Neptune, the their magnetic fields are generated by dynamo
processes within these planets, and an interaction between the solar wind and
their magnetic fields generates intense radio emission via the electron
cyclotron maser instability. In the case of Jupiter, its magnetic field
interacts with the moon Io to result in radio emission as well.
  Extrasolar planets reasonably may be expected to generate large-scale
magnetic fields and to sustain an electron cyclotron maser instability. Not
only may these radio emissions be a means for discovering extrasolar planets,
because magnetic fields are tied to the properties of planetary interiors,
radio emissions may be a remote sensing means of constraining extrasolar
planetary properties that will be otherwise difficult to access. In the case of
terrestrial planets, the presence or absence of a magnetic field may be an
indicator for habitability. Since the first edition of the Handbook, there have
been a number of advances, albeit there remain no unambigous detection of radio
emission from extrasolar planets. New ground-based telescopes and new
possibilities for space-based telescopes provide promise for the future.
