4.2 Planets of the Solar System

Can humans take a field trip through the solar system?

A field trip through the solar system would take a long time. It took 12 years for the Voyager spacecraft to get from Earth to Neptune. If a human was on board, he or she would probably want to come back! Fortunately, unmanned spacecrafts can send back images of far distant places in the solar system.

Solar System Objects

Astronomers now recognize eight planets (Mercury, Venus, Earth, Mars, Jupiter,Saturn, Uranus, and Neptune), five dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris), more than 150 moons, and many, many asteroids and other small objects (Figure below ). These objects move in regular and predictable paths around thesun.

Relative sizes of the sun, planets, and dwarf planets and their positions relative to each other are correct. The relative distances are not correct.

Planet Sizes

The sun is just an average star compared to other stars. But 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! Listed below is data on the sizes of the Sun and planets relative to Earth ( Table below ).

Object Mass (Relative to Earth) Diameter (Relative to Earth)
Sun 333,000 109.2
Mercury 0.06 0.39
Venus 0.82 0.95
Earth 1.00 1.00
Mars 0.11 0.53
Jupiter 317.8 11.21
Saturn 95.2 9.41
Uranus 14.6 3.98
Neptune 17.2 3.81

Distances in the Solar System

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 (93 million miles). Listed below is the distance from the sun to each planet in AU ( Table below ). The table shows how long it takes each planet to spin once on its axis. It also shows how long it takes each planet to complete an orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus!

Planet Average Distance from Sun (AU) Length of Day (in Earth days) Length of Year (in Earth years)
Mercury 0.39 56.84 0.24
Venus 0.72 243.02 0.62
Earth 1.00 1.00 1.00
Mars 1.52 1.03 1.88
Jupiter 5.20 0.41 11.86
Saturn 9.54 0.43 29.46
Uranus 19.22 0.72 84.01
Neptune 30.06 0.67 164.8

The Size and Shape of Orbits

Figure below shows the relative sizes of the orbits of the planets, asteroid belt, and Kuiper belt. In general, the farther away from the sun, the greater the distance from one planet’s orbit to the next. The orbits of the planets are not circular but slightly elliptical ( Figure below ).

The relative sizes of the orbits of planets in the solar system. The inner solar system and asteroid belt is on the upper left. The upper right shows the outer planets and the Kuiper belt.


  • astronomical unit (AU) : Unit of measure; 1 AU is the distance from the Sun to Earth.


  • The solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn,Uranus, and Neptune. There are also five known dwarf planets: Ceres, Pluto, Makemake, Haumea, and Eris.
  • Solar system distances are measured as multiples of the distance between Earth and Sun. This is one astronomical unit (AU).
  • All planets and dwarf planets orbit the sun. All planets and dwarf planets rotate on their axes.
  • The planets make slightly elliptical orbits around the sun.


Use the resource below to answer the questions that follow.

  1. What did early astronomers believe about planet Earth?
  2. How many planets did early astronomers know about?
  3. What did Kepler discover?
  4. How did our solar system form?
  5. How many planets are in the solar system?
  6. What is the Kuiper Belt?
  7. What is the Oort Cloud?
  8. What is found in the Oort Cloud?
  9. What is the outer boundary of the solar system?
  10. Why are scientists interested in the other plants?


  1. What are the names of the planets and dwarf planets?
  2. Where are the most massive planets? Where are the least massive planets?
  3. What is an astronomical unit? Why is this unit used to measure distances in the solar system?

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