20.3 Planets of the Solar System

Who is in the Sun's family?

The family includes the Sun, its eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and the five known dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris). In the image above, relative sizes of the Sun, planets, and dwarf planets and their positions relative to each other are correct, but the relative distances are not.

Eight Planets

Since the time of Copernicus, Kepler, and Galileo, we have learned a lot more about our solar system. Astronomers have discovered two more planets (Uranus and Neptune), five dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris), more than 150 moons, and many, many asteroids and other small objects.

Although the Sun is just an average star compared to other stars, it is by far the largest object in the solar system. The Sun is more than 500 times the mass of everything else in the solar system combined! Table below gives data on the sizes of the Sun and planets relative to Earth.

Orbits and Rotations

Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km, or 93 million miles. Table below shows the distances to the planets (the average radius of orbits) in AU. The table also shows how long it takes each planet to spin on its axis (the length of a day) and how long it takes each planet to complete an orbit (the length of a year); in particular, notice how slowly Venus rotates relative to Earth.

Summary

• The planets of the solar system, with increasing distance from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The five known dwarf planets are Ceres, Pluto, Makemake, Haumea, and Eris.
• Solar system distances are measured as multiples of the distance between Earth and Sun, which is defined as one astronomical unit (AU).
• All planets and dwarf planets orbit the Sun and rotate on their axes.

Review

1. Why does the number of dwarf planets recognized by astronomers in the solar system sometimes increase?
2. What is the order of planets and dwarf planets by distance from the Sun?
3. What is an astronomical unit? Why is this unit used to measure distances in the solar system?

Explore More

Use this resource to answer the questions that follow.

1. How old is our solar system?
2. How did the planets form?
3. What are the two main regions of the solar system?
4. List the inner planets.
5. List the outer planets.
6. What are the requirements to be a planet?
7. Why was Pluto demoted?
8. What is the Kuiper Belt?
9. What is the scattered disk?
10. What is the heliosphere?