**1. Introduction**

Following the birth of the Universe through the Big Bang, a cyclic creation, or another unique event, space was filled with nebulae composed primarily of gas and dust. Stars formed from this primordial material, and the residual mass or interstellar medium (ISM) formed the constituents that led to planet formation. There are numerous papers, references, and books describing the characteristics of Solar System planets as well as exoplanets [1–35].

This crude model for planetary formation is based on the assumption that a star forms from the gravitational attraction and associated collapse of the primordial material. The contraction of the star with its decreasing radius increased the angular momentum of the accretion disk of ISM that formed around the star [3, 7, 10, 15]. The temperature of the material within the accretion disk varied with distance from the star. This temperature dependence caused rocky bodies to form throughout the disk, but icy bodies developed at greater distances. In the Solar System, the icy bodies developed beyond the Asteroid Belt.

Within the Solar System, the terrestrial planets formed from rocky bodies (i.e., preplanetary clusters also known as planetesimals). The terrestrial planets include Mercury, Venus, Earth, and Mars. The larger planets (i.e., Jupiter, Saturn, Uranus, and Neptune) formed from the rocky bodies, icy bodies, gas, and dust that led to their increased size. The higher temperatures and lower masses of the terrestrial planets limited their capture of gases. This was not the case for the giant planets. For Jupiter and Saturn, the larger masses and cooler temperatures led to the capture of significant atmospheres.

The initial planet structures also developed their own accretion disks that led to the formation of planetary moons. These disks were larger for the giant planets, which led to these bodies generally having more moons than the less massive terrestrial planets. Some moons were formed by planetary gravitational capture of rocky structures and asteroid fragments. Other moons (e.g., Earth's moon) formed when a large body collided with the planet.

Following the creation of the initial planets and their moons, the Solar System still contained considerable debris that collided with these bodies. The Moon's craters are an example of the effect of the resulting impact of this debris. Some of this debris, particularly icy structures, formed beyond Neptune's orbit and as is known as the Kuiper Belt. An Asteroid Belt comprised of rocky structures formed between the orbits of Mars and Jupiter.

This simplified model of planetary creation has been supplemented with a bifurcation model. Within the bifurcation model, planet formation occurred in spatially and temporarily distinct domains through a postulated mechanism that was driven by the presence of water [24]. Although the details of this mechanism are unknown, the domains evolved in distinct physical modes with different volatile materials. Model calculations suggest that these physical differences led to the formation of the terrestrial and gas giant planets.

As the capability to observe exoplanetary systems and their atmospheres improves, it will be interesting to determine if the characteristics of the Solar System and life on Earth are unique. Will further exoplanet observations reveal a variety of star and planetary systems having the capability of sustaining life?

The reader should note that the literature provides a range of values for planetary data including their associated composition. Given this consideration, specific references are cited when particular data are noted. Significant figures are usually provided to accommodate the variation in literature values.

This chapter provides a general overview of Solar System planets and exoplanets. It's intended to introduce these systems to readers not well versed in planetary science. Additional planetary details are provided in this chapter's references.
