3: Our Solar System
- Page ID
- 55003
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- 3.1: Overview of Our Planetary System
- Our solar system currently consists of the Sun, eight planets, five dwarf planets, nearly 200 known moons, and a host of smaller objects. The planets can be divided into two groups: the inner terrestrial planets and the outer giant planets. Pluto, Eris, Haumea, and Makemake do not fit into either category; as icy dwarf planets, they exist in an ice realm on the fringes of the main planetary system. The giant planets are composed mostly of liquids and gases.
- 3.2: Composition and Structure of Planets
- The giant planets have dense cores roughly 10 times the mass of Earth, surrounded by layers of hydrogen and helium. The terrestrial planets consist mostly of rocks and metals. They were once molten, which allowed their structures to differentiate (that is, their denser materials sank to the center). The Moon resembles the terrestrial planets in composition, but most of the other moons—which orbit the giant planets—have larger quantities of frozen ice within them.
- 3.3: Formation of Planetary Systems
- This section examines how stars and planetary systems form from clouds of gas and dust. It describes the collapse of a solar nebula, the formation of a rotating disk around the young Sun, and the processes by which small particles gradually combined to form planetesimals and planets. The resulting structure of our solar system provides important evidence for understanding how planetary systems form and evolve.
Chapter 3: Our Solar System — Introduction
Our solar system contains a remarkable variety of worlds, from small rocky planets to giant planets surrounded by extensive systems of moons and rings. Despite these differences, the planets share patterns in their composition, structure, and motion that provide important clues about how the solar system formed.
In this chapter, we will compare the major types of planets and examine the characteristics that distinguish terrestrial worlds from the giant planets. We will then use these observations as evidence for the origin and formation of the solar system. The leading model describes the solar system as forming about 4.5 billion years ago from a rotating cloud of gas and dust. As this material collapsed, the Sun formed near the center while the remaining material in a surrounding disk gradually assembled into planets and other smaller bodies.
Our solar system is also only one example of a planetary system. Discoveries of planets around other stars allow astronomers to test ideas about planetary formation and show us that planetary systems can take many different forms. By comparing these systems with our own, we can better understand both how our solar system formed and what makes it distinctive.