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18.1: Introduction to the Solar System

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    Lesson Objectives

    • Describe historical views of the solar system.
    • Name the planets, and describe their motion around the sun.
    • Explain how the solar system formed.


    • geocentric model
    • heliocentric model
    • moon
    • nebula
    • nebular hypothesis
    • solar system

    Changing Views of the Solar System

    Humans’ view of the solar system has evolved as technology and scientific knowledge have increased. The ancient Greeks identified five of the planets and for many centuries they were the only planets known. Since then, scientists have discovered two more planets, many other solar-system objects and even planets found outside our solar system.

    The Geocentric Universe

    The ancient Greeks believed that Earth was at the center of the universe, as shown in Figure below. This view is called the geocentric model of the universe. Geocentric means “Earth-centered.” In the geocentric model, the sky, or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster than the stars, the Greeks placed them closer to Earth.


    Model of a geocentric universe. This diagram of the universe from the Middle Ages shows Earth at the center, with the Moon, the Sun, and the planets orbiting Earth.

    The geocentric model worked well, by explaining why all the stars appear to rotate around Earth once per day. The model also explained why the planets move differently from the stars and from each other.

    One problem with the geocentric model is that some planets seem to move backwards (in retrograde) instead of in their usual forward motion around Earth.

    A demonstration animation of retrograde motion of Mars as it appears to Earth can be found here:

    Around 150 A.D. the astronomer Ptolemy resolved this problem by using a system of circles to describe the motion of planets (Figure below). In Ptolemy’s system, a planet moves in a small circle, called an epicycle. This circle moves around Earth in a larger circle, called a deferent. Ptolemy’s version of the geocentric model worked so well that it remained the accepted model of the universe for more than a thousand years.


    According to Ptolemy, a planet moves on a small circle (epicycle) that in turn moves on a larger circle (deferent) around Earth.

    An animation of Ptolemy’s system is seen here:

    The Heliocentric Universe

    Ptolemy’s geocentric model worked but it was not only complicated, it occasionally made errors in predicting the movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric model or “sun-centered” model of the universe (Figure below). Copernicus’ model explained the motion of the planets as well as Ptolemy’s model did, but it did not require complicated additions like epicycles and deferents.


    Unlike the geocentric model, the heliocentric model had the Sun at the center and did not require epicycles.

    Although Copernicus’ model worked more simply than Ptolemy’s, it still did not perfectly describe the motion of the planets because, like Ptolemy, Copernicus thought planets moved in perfect circles. Not long after Copernicus, Johannes Kepler refined the heliocentric model so that the planets moved around the Sun in ellipses (ovals), not circles (Figure below). Kepler’s model matched observations perfectly.

    Animation of Kepler’s Laws of Planetary Motion:


    Kepler’s model showed the planets moving around the sun in ellipses. The elliptical orbits are exaggerated in this image.

    Because people were so used to thinking of Earth at the center of the universe, the heliocentric model was not widely accepted at first. However, when Galileo Galilei first turned a telescope to the heavens in 1610, he made several striking discoveries. Galileo discovered that the planet Jupiter has moons orbiting around it. This provided the first evidence that objects could orbit something besides Earth.

    An animation of three of Jupiter’s moons orbiting the planet is seen here:

    Galileo also discovered that Venus has phases like the Moon (Figure below), which provides direct evidence that Venus orbits the Sun.


    The phases of Venus.

    Galileo’s discoveries caused many more people to accept the heliocentric model of the universe, although Galileo himself was found guilty of heresy for his ideas. The shift from an Earth-centered view to a Sun-centered view of the universe is referred to as the Copernican Revolution.

    Watch this animation of the Ptolemaic and Copernican models of the solar system. Ptolemy made the best model he could with the assumption that Earth was the center of the universe, but by letting that assumption go, Copernicus came up with a much simpler model. Before people would accept that Copernicus was right, they needed to accept that the Sun was the center of the solar system (1n – I&E Stand.): (0:47).

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    The Modern Solar System

    Today, we know that our solar system is just one tiny part of the universe as a whole. Neither Earth nor the Sun are at the center of the universe. However, the heliocentric model accurately describes the solar system. In our modern view of the solar system, the Sun is at the center, with the planets moving in elliptical orbits around the Sun. The planets do not emit their own light, but instead reflect light from the Sun.

    Extrasolar Planets or Exoplanets

    Since the early 1990s, astronomers have discovered other solar systems, with planets orbiting stars other than our own Sun (called “extrasolar planets” or simply “exoplanets”) (Figure below).


    The extrasolar planet Fomalhaut is surrounded by a large disk of gas. The disk is not centered on the planet, suggesting that another planet may be pulling on the gas as well.

    Some extrasolar planets have been directly imaged, but most have been discovered by indirect methods. One technique involves detecting the very slight motion of a star periodically moving toward and away from us along our line-of-sight (also known as a star’s “radial velocity”). This periodic motion can be attributed to the gravitational pull of a planet or, sometimes, another star orbiting the star.

    This is in line with the plane of the system:

    A planet may also be identified by measuring a star’s brightness over time. A temporary, periodic decrease in light emitted from a star can occur when a planet crosses in front of (or “transits”) the star it is orbiting, momentarily blocking out some of the starlight.

    More than 3,600 extrasolar planets have been identified and the rate of discovery is increasing rapidly.

    Extrasolar Planet from the ESA discusses extrasolar planets and particularly a planetary system very similar to our solar system (1g): (3:29).

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    An introduction to extrasolar planets from NASA is available at (1g): (3:14).

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    KQED: The Planet Hunters

    Hundreds of exoplanets have now been discovered. To learn something about how planet hunters find these balls of rock they usually can’t even see, watch this QUEST video. Learn more at: and

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    Planets and Their Motions

    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), four dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris), more than 150 moons, and many, many asteroids and other small objects.

    (Figure below) shows the Sun and the major objects that orbit the Sun. There are eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and the five known dwarf planets and the five known dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris).


    Relative sizes of the Sun, planets and dwarf planets. The relative sizes are correct and their position relative to each other is correct, but the relative distances are not correct.

    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.

    Sizes of Solar System Objects Relative to Earth
    Object Mass (Relative to Earth) Diameter of Planet (Relative to Earth)
    Sun 333,000 Earth’s mass 109.2 Earth’s diameter
    Mercury 0.06 Earth’s mass 0.39 Earth’s diameter
    Venus 0.82 Earth’s mass 0.95 Earth’s diameter
    Earth 1.00 Earth’s mass 1.00 Earth’s diameter
    Mars 0.11 Earth’s mass 0.53 Earth’s diameter
    Jupiter 317.8 Earth’s mass 11.21 Earth’s diameter
    Saturn 95.2 Earth’s mass 9.41 Earth’s diameter
    Uranus 14.6 Earth’s mass 3.98 Earth’s diameter
    Neptune 17.2 Earth’s mass 3.81 Earth’s diameter

    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 with the Sun located at one of the foci (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.


    The planets orbit the Sun in regular paths.

    While studying the solar system, Johannes Kepler discovered the relationship between the time it takes a planet to make one complete orbit around the Sun, its “orbital period,” and the distance from the Sun to the planet. If the orbital period of a planet is known, then it is possible to determine the planet’s distance from the Sun. This is how astronomers without modern telescopes could determine the distances to other planets within 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, or 93 million mi. 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.

    Distances to the Planets and Properties of Orbits Relative to Earth’s Orbit
    Planet Average Distance from Sun (AU) Length of Day (In Earth Days) Length of Year (In Earth Years)
    Mercury 0.39 AU 56.84 days 0.24 years
    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

    How old are you on Earth? How old would you be if you lived on Jupiter? How many days is it until your birthday on Earth? How many days until your birthday if you lived on Saturn?

    Scaling the solar system creates a scale to measure all objects in solar system (1i – I&E Stand.): (4:44).

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    The Role of Gravity

    Isaac Newton was one of the first scientists to explore gravity. He understood that the Moon circles the Earth because a force is pulling the Moon toward Earth’s center. Without that force, the Moon would continue moving in a straight line off into space. Newton also came to understand that the same force that keeps the Moon in its orbit is the same force that causes objects on Earth to fall to the ground.

    Newton defined the Universal Law of Gravitation, which states that a force of attraction, called gravity, exists between all objects in the universe (Figure below). The strength of the gravitational force depends on how much mass the objects have and how far apart they are from each other. The greater the objects’ mass, the greater the force of attraction; in addition, the greater the distance between the objects, the smaller the force of attraction.


    The force of gravity exists between all objects in the universe; the strength of the force depends on the mass of the objects and the distance between them.

    The distance between the Sun and each of its planets is very large, but the Sun and each of the planets are also very large. Gravity keeps each planet orbiting the Sun because the star and its planets are very large objects. The force of gravity also holds moons in orbit around planets.

    Formation of the Solar System

    There are two additional key features of the solar system:

    1. All the planets lie in nearly the same plane, or flat disk like region.

    2. All the planets orbit in the same direction around the Sun.

    These two features are clues to how the solar system formed.

    A Giant Nebula

    The most widely accepted explanation of how the solar system formed is called the nebular hypothesis. According to this hypothesis, the Sun and the planets of our solar system formed about 4.6 billion years ago from the collapse of a giant cloud of gas and dust, called a nebula.

    The nebula was drawn together by gravity, which released gravitational potential energy. As small particles of dust and gas smashed together to create larger ones, they released kinetic energy. As the nebula collapsed, the gravity at the center increased and the cloud started to spin because of its angular momentum. As it collapsed further, the spinning got faster, much as an ice skater spins faster when he pulls his arms to his sides during a spin.

    Much of the cloud’s mass migrated to its center but the rest of the material flattened out in an enormous disk, as shown in Figure below. The disk contained hydrogen and helium, along with heavier elements and even simple organic molecules.


    An artist’s painting of a protoplanetary disk.

    Formation of the Sun and Planets

    As gravity pulled matter into the center of the disk, the density and pressure at the center became intense. When the pressure in the center of the disk was high enough, nuclear fusion began. A star was born—the Sun. The burning star stopped the disk from collapsing further.

    Meanwhile, the outer parts of the disk were cooling off. Matter condensed from the cloud and small pieces of dust started clumping together. These clumps collided and combined with other clumps. Larger clumps, called planetesimals, attracted smaller clumps with their gravity. Gravity at the center of the disk attracted heavier particles, such as rock and metal and lighter particles remained further out in the disk. Eventually, the planetesimals formed protoplanets, which grew to become the planets and moons that we find in our solar system today.

    Because of the gravitational sorting of material, the inner planets — Mercury, Venus, Earth, and Mars — formed from dense rock and metal. The outer planets — Jupiter, Saturn, Uranus and Neptune — condensed farther from the Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out by Jupiter and beyond, where it’s very cold, these materials form solid particles.

    The nebular hypothesis was designed to explain some of the basic features of the solar system:

    • The orbits of the planets lie in nearly the same plane with the Sun at the center
    • The planets revolve in the same direction
    • The planets mostly rotate in the same direction
    • The axes of rotation of the planets are mostly nearly perpendicular to the orbital plane
    • The oldest moon rocks are 4.5 billion years

    This video, from the ESA, discusses the Sun, planets, and other bodies in the Solar System and how they formed (1a, 1d). The first part of the video explores the evolution of our view of the solar system starting with the early Greeks who reasoned that since some points of light – which they called planets – moved faster than the stars, they must be closer: (8:34).

    An interactive or media element has been excluded from this version of the text. You can view it online here:

    Lesson Summary

    • The solar system is the Sun and all the objects that are bound to the Sun by gravity.
    • The solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Ceres, Makemake, Pluto and Eris are dwarf planets.
    • The ancient Greeks and people for centuries afterwards believed in a geocentric model of the universe, with Earth at the center and everything else orbiting our planet.
    • Copernicus, Kepler, and Galileo promoted a heliocentric model of the universe, with the Sun at the center and Earth and the other planets orbiting the Sun.
    • Gravity holds planets in elliptical orbits around the Sun.
    • The nebular hypothesis describes how the solar system formed from a giant cloud of gas and dust about 4.6 billion years ago.

    Review Questions

    1. What does geocentric mean?

    2. Describe the geocentric model and heliocentric model of the universe.

    3. How was Kepler’s version of the heliocentric model different from Copernicus’?

    4. Name the eight planets in order from the Sun outward. Which are the inner planets and which are the outer planets?

    5. Compare and contrast the inner planets and the outer planets.

    6. What object used to be considered a planet, but is now considered a dwarf planet? What are the other dwarf planets?

    7. What keeps planets and moons in their orbits?

    8. How old is the solar system? How old is Earth?

    9. Use the nebular hypothesis to explain why the planets all orbit the Sun in the same direction.

    Further Reading / Supplemental Links

    Points to Consider

    • Would you expect all the planets in the solar system to be made of similar materials? Why or why not?
    • The planets are often divided into two groups: the inner planets and the outer planets. Which planets do you think are in each of these two groups? What do members of each group have in common?
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