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

7
The Giant Planets:
Local Laboratories and Ground Truth for Planets Beyond
Jupiter, Saturn, Uranus, and Neptune are the giants of the solar system. These four planets define
the dominant characteristics of our planetary system in multiple ways, e.g., they contain over 99 percent
of the solar system’s mass and total angular momentum. Their formation and evolution have governed
the history of the solar system. As the last planetary exploration decadal survey articulated, “the giant
planet story is the story of the solar system.” 1
One of the most significant advances (Table 7.1) since the last decadal survey has been the
discovery that giant planets reside in the planetary systems discovered around other stars, as well. To
date, the vast majority of known planets around other stars (exoplanets) are giants close to their parent
stars, though observational bias plays a role in the statistics. This chapter discusses the four local giant
planets, placing them in the context of the growing population of exoplanets and our understanding of our
own solar system. Both remote and in situ measurements of their outer atmospheric compositions are
discussed, as well as external measurements that probe their deeper interiors both through their gravity
fields and through their magnetic fields and magnetospheric interactions with the Sun. This chapter also
includes the ring systems and smaller moons of these worlds, which together with the larger moons,
effectively constitute miniature solar systems. This chapter explicitly excludes discussion of the largest
moons of the giant planets, which are addressed in a separate chapter of this report.
Studying the giant planets is vital to addressing many of the priority questions developed in
Chapter 3. For example, central to the theme, Building New Worlds, is the question “how did the giant
planets and their satellite systems accrete, and is there evidence that they migrated to new orbital
positions?” Formation and migration of the giant planets are believed to have played a dominant role in
the sculpture and future evolution of the entire solar system. The giant planets are particularly important
for delving into several key questions in the Workings of Solar Systems theme. For example, “how do
the giant planets serve as laboratories to understand Earth, the solar system, and extrasolar planetary
systems?” Most of the extrasolar planets that have been discovered to date are giants, with a spectrum of
types that includes our own ice and gas giants; close-up study of the giants of our own solar system
provides crucial insights about what we are seeing around distant stars. Our giant planets, particularly
Jupiter, are central to the question: “what solar system bodies endanger and what mechanisms shield
Earth’s biosphere?”—in fact Jupiter may overall shield Earth from impact. The atmospheres of the giant
planets provide important laboratories in addressing “can understanding the roles of physics, chemistry,
geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of
climate change on Earth?” Finally, harboring most of the mass and energy of our planetary system, the
giant planets are a major element in understanding “how have the myriad chemical and physical
processes that shaped the solar system operated, interacted, and evolved over time?”
SCIENCE GOALS FOR THE STUDY OF GIANT PLANETS
Giant planets dominated the history of planetary evolution: the processes of their formation and
migration sculpted the nascent solar system into the habitable environment of today. The materials that
comprise the giant planets preserve the chemical signatures of the primitive nebular material from which
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