**2. Planetary overview**

Although Solar System planets have been studied for centuries, the observation of exoplanets is only a few decades old. The first exoplanet orbiting a star outside the Solar System (i.e., 51 Pegasi), discovered by Mayor and Queloz [2], did not occur until 1995. Since that time, progress in exoplanet discovery and characterization has increased rapidly. As of January 2021, 4341 exoplanets have been discovered.

General characteristics of Solar System planets are addressed in Section 2.1 and other Solar System bodies (i.e., planetary moons, the Asteroid Belt, comets, Kuiper Belt Objects, and meteoroids) are discussed in Section 2.2. Section 2.3 is devoted to a discussion of exoplanets. Exoplanet detection methods are provided in Section 3. Section 4 summarizes the variety of space probes utilized to examine planetary and exoplanetary systems.

The presentation of this chapter is designed to facilitate the flow of subsequent material. Selected content provides an overview of subsequent chapters. Since there are numerous references to the characteristics of Solar System planets and exoplanets, the presentation of this chapter is necessarily incomplete. However, the text does provide an overview of the subject and the basis for more detailed study utilizing the accompanying reference list.

**5**

*a*

*b Ref. [28].*

**Table 1.**

*Solar System Planets and Exoplanets*

**2.1 Solar System planets**

tion of gas, rock, and ice.

*2.1.1 Mercury*

density ≤ 1011 particles/m3

Mean density (g/cm3

*General Solar System Planet Characteristics.a,b*

*DOI: http://dx.doi.org/10.5772/intechopen.98431*

are addressed in subsequent discussion.

characteristics are summarized in **Table 1**.

eccentricity). Their surface gravitational acceleration in m/s2

planets, and successfully orbit and land on these worlds [4].

**Planetary Characteristic Terrestrial** 

Structural Form Primarily

There are eight Solar System planets as well as dwarf planets, planetary moons, the Asteroid Belt, comets, Kuiper Belt Objects, and meteorites. These spatial bodies

An overview of the Solar System planets and a general classification scheme is summarized in **Table 1**, and illustrated in **Figure 1**. The Solar System planets can be grouped in terms of structural characteristics with the terrestrial planets being compact objects that are primarily rocky objects. Gas giant planets are a combina-

The terrestrial planets are closest to the Sun and are smaller, warmer, and less massive than the gas giants. Although more massive, the gas giants have a lower density. The gas giants have more moons and exhibit ring structures. These general

**Table 2** provides additional data for the Solar System planets including their orbital characteristics (i.e., distance from the Sun, orbital period, and orbital

data are important for spacecraft and probes attempting to investigate these worlds. Detailed orbital calculations are required to plan missions that would reach these

Mercury is the planet closest to the Sun, the smallest of the Solar System planets,

[15]. Mercury's limited atmosphere consists of oxygen,

**Planet**

rock

) 3.9–5.5 0.69–1.6

and similar in size to Earth's moon. It has no moons, and has a cratered surface that is similar to the topography of Earth's moon. Given its proximity to the Sun and high temperature, Mercury's atmosphere is thin with a constituent number

sodium, potassium, and calcium that evolved from surface material. Thermal effects that increase the temperature of the crust and the impact of solar particles and meteorites on the surface create this atmospheric composition. Limited atmospheric hydrogen and helium are derived from particles emitted by the Sun, and

Mean distance from Sol (AU) 0.39–1.5 5.2–30 Mean surface temperature (K) 220–730 70–170 Mass ( *M*<sup>⊕</sup> ) 0.055–1.0 15–320 Equatorial radius ( *R*<sup>⊕</sup> ) 0.38–1.0 3.9–11

Period of sidereal rotation at equator 24 h – 243 d 9.9 h – 17 h Number of moonsb 0–2 14–82 Ring systems No Yes

*Adapted from Refs. [1, 15]. Rounded to two significant figures due to various values quoted in the literature.*

is also provided. These

**Giant Planet**

Combination of gas, rock, and ice
