Astronomy: Terrestrial Planets, Gas‐Giant Planets

The goal of planetary astronomy is to understand both the differences and the similarities (called comparative planetology) of the major objects in the solar system, including their atmospheres, surfaces, internal structures, and other factors, such as magnetic fields. The study is also aimed at understanding the sequence of events and dates that define the history of the solar system. When studying the planets of the solar system (see Figure 1 ), the first thing one might notice is that the characteristics of the planets suggest a division into two fundamental types: the terrestrial (Earth-like) and the gas-giant (Jupiter-like) planets. These characteristics are compared in Table 1 .

Figure 1

The planets of the solar system, drawn to scale.

TABLE 1

Comparison of Planetary Properties

Property	Terrestrial Planets	Gas-Giant Planets
Location in solar system	Nearest Sun	Outer part of solar system
Size (or radius)	Small	Large
Mass	Non-massive	Massive
Density	Dense	Not dense
Structure	Solid, with iron cores	Mostly gaseous with solid cores
Chemical composition	Heavy elements	Primarily hydrogen and helium
Atmosphere	Thin	Deep and thick
Exterior	Warm	Cold temperature
Rotation	Slow	Fast
Satellites	Few	Many
Ring systems	No	Yes

The outermost planet, Pluto, however, does not fit into either of these classifications: for example, it is small (really too small to be a terrestrial planet), but in the outer part of the solar system.