Vision and Voyages for Planetary Science in the - Solar System ...

Analyze the Properties and Processes in Planetary Magnetospheres
Giant exoplanets orbiting close to their parent stars exist in an extreme regime of physical
conditions. They are expected to have much stronger interactions with the stellar winds than Jupiter or
Earth; in fact detecting exoplanets through their auroral emissions has often been discussed. 34,35 The four
giant planets and Earth provide us with an understanding of the basic physics and scaling laws of the
interactions with a stellar wind needed to understand exoplanets. Exoplanet internal magnetic field
strengths are unknown, but can be roughly estimated if the planet rotation rate equals its orbital period
due to tidal torques. Exoplanets’ hot atmospheres may well extend beyond the magnetopause and be
subject to rapid loss in the stellar wind, important for estimating the lifetime of these objects.
The interaction of an exoplanet magnetosphere with its host star could take many forms. A
Venus-like interaction with rapid mass loss from the top of the atmosphere could result if the planet’s
internal magnetic field is weak. An Earth-like auroral interaction could result if the internal field is
stronger, or a Jupiter-like interaction if the planet is rapidly rotating and its magnetosphere contains a
large internal source of plasma. A much stronger star-planet interaction could result if the star’s rotation
rate is rapid compared with the planet’s orbital period: the planet’s motion through the star’s magnetic
field would generate a large electric potential across the planet, driving a strong current between the
planet and the star. This could result in a “starspot,” analogous to a sunspot. Observations of starspots
may be a promising approach for remotely sensing the electrodynamic interaction of exoplanets with their
stars.
The giant planets in our solar system have strong magnetic fields and giant magnetospheres,
leading to solar wind interactions quite different from what is seen at Earth. The size scales are much
larger and the time scales are much longer, still the aurora on both Jupiter and Saturn are affected by
changes in the solar wind. Unlike Earth, Jupiter’s magnetosphere and aurora are dominated by internal
sources of plasma, and the primary energy source is the planet’s rotation. Saturn is an intermediate case
between Earth and Jupiter: it has a large, rapidly rotating magnetosphere with internal sources of plasma,
yet the aurora and nonthermal radio emissions consistently brighten when a solar wind shock front arrives
at the planet. The ice giants have substantially tilted magnetospheres that are significantly offset from the
planets’ centers, configurations that differ completely from those of Jupiter and Saturn.
Understanding of the magnetospheric environments of Jupiter and Saturn has deepened since the
last decadal survey. The Galileo mission at Jupiter has concluded. Cassini passed Jupiter, entered orbit
around Saturn, and successfully completed its nominal mission. NASA’s Infrared Telescope Facility has
obtained infrared images of Saturn’s aurorae. A large Hubble observing program to observe the
ultraviolet auroral emissions from Jupiter and Saturn has been conducted that—coupled with New
Horizons measurements at Jupiter and Cassini measurements at Saturn—has shown the extent of solar
wind control over giant planet aurora. There have been no comparable missions to Uranus or Neptune,
however, thus knowledge of ice-giant magnetospheres is limited to data from Voyager 2 flybys more than
two decades ago, supplemented by subsequent scant Earth-based observations. The scarcity of closerange
measurements of ice giants has seriously limited the advance of our knowledge of their
magnetospheres and plasma environments. New measurements from Uranus and/or Neptune therefore
have a high priority in the outer planet magnetospheres community.
Some Important Questions
Some important questions concerning the properties and processes in planetary magnetospheres
include the following:
• What is the nature of the displaced and tilted magnetospheres of Uranus and Neptune, and
how do conditions vary with the pronounced seasonal changes on each planet?
• What is the detailed plasma composition in any of these systems, particularly for ice giants?
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
7-13