High School Earth Science/Introduction to the Solar System

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Lesson Objectives[edit]

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

Changing Views of the Solar System[edit]

People have not always known about all the objects in our solar system. The ancient Greeks were aware of five of the planets. They did not know what these objects were; they just noticed that they moved differently than the stars did. They seemed to wander around in the sky, changing their position against the background of stars. In fact, the word "planet" comes from a Greek word meaning "wanderer". They named these objects after gods from their mythology. The names we use now for the planets are the Roman equivalents of these Greek names: Mercury, Venus, Mars, Jupiter, and Saturn.

The Geocentric Universe[edit]

The ancient Greeks believed that Earth was at the center of the universe, as shown in Figure 25.1. This view is called the geocentric model of the universe. Geocentric means "Earth-centered". The geocentric model also described the sky, or heavens, as having a set of spheres layered on top of one another. Each object in the sky was attached to one of these spheres, and moved around Earth as that sphere rotated. From Earth outward, these spheres contained the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and an outer sphere which contained all the stars. The planets appear to move much faster than the stars and so the Greeks placed them closer to Earth.

Figure 25.1: 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.

Today, powerful telescopes can actually see the surfaces of planets in our solar system. Even though the closest stars have diameters that are hundreds of times larger than the Earth, the distant stars appear as tiny dots that cannot be resolved.

The geocentric model may seem strange to us now, but at the time, it worked quite well. It explained why all the stars appear to rotate around Earth once per day. It also explained why the planets move differently from the stars, and from each other. One problem with the geocentric model was resolved around 150 A.D. by the astronomer Ptolemy. At times, some planets seemed to move backwards (in retrograde) instead of in their usual forward motion around the Earth. Ptolemy resolved this problem by using a system of circles to describe the motion of planets (Figure 25.2). In Ptolemy's system, a planet moved in a small circle, called an epicycle. This circle in turn moved 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.

Figure 25.2: Diagram of an epicycle and deferent. According to Ptolemy, a planet moves on a small circle that in turn moves on a larger circle around Earth.

The Heliocentric Universe[edit]

Ptolemy's geocentric model worked pretty well, but it was complicated and occasionally made errors in predicting the movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed a different model in which Earth and all the other planets orbited the Sun. Because this model put the Sun at the center, it is called the heliocentric model' of the universe. Heliocentric means "sun-centered". Figure 25.3 shows the heliocentric model compared to the geocentric model. Copernicus' model explained the motion of the planets about as well as Ptolemy's model, but it did not require complicated additions like epicycles and deferents.

Figure 25.3: Unlike the geocentric model (top image), the heliocentric model (lower image), 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. The problem was that, like Ptolemy, Copernicus still thought planets moved in perfect circles. Not long after Copernicus, Johannes Kepler refined the heliocentric model. He proposed that planets move around the Sun in ellipses (ovals), not circles. This model matched observations perfectly.

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. He found that the planet Jupiter has moons orbiting around it. This was the first evidence that objects could orbit something besides Earth. He also discovered that Venus has phases like our moon does. The phases of Venus provided direct evidence that Venus orbits the Sun. Galileo’s discoveries caused many more people to accept the heliocentric model of the universe. The shift from an Earth-centered view to a Sun-centered view of the universe is referred to as the Copernican Revolution.

In astronomy, Kepler's laws of planetary motion are three scientific laws describing the motion of planets around the Sun. 1. The orbit of a planet is an ellipse with the Sun at one of the two foci. 2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.[1] 3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

The Modern Solar System[edit]

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—in fact, the universe has no true center. However, the heliocentric model does accurately describe our solar system. In our modern view of the solar system, the Sun is at the center, and planets move in elliptical orbits around the Sun. The planets do not emit their own light, but instead reflect light from the Sun.

Planets and Their Motions[edit]

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

Figure 25.4 shows the Sun and the major objects that orbit the Sun. There are eight planets. From the Sun outward, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 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! Table 25.1 gives more exact data on the sizes of the Sun and planets relative to Earth.

Table 25.1: Mass and Diameter of Sun and Planets Relative to Earth
Object Mass (Relative to Earth) Diameter (Relative to Earth)
Sun 333,000 Earth masses 109.2 Earth diameters
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 mass 1.00 Earth diameter
Mars 0.11 Earth's mass 0.53 Earth's diameter
Jupiter 317.8 Earth masses 11.21 Earth diameters
Saturn 95.2 Earth masses 9.41 Earth diameters
Uranus 14.6 Earth masses 3.98 Earth diameters
Neptune 17.2 Earth masses 3.81 Earth diameters
Figure 25.4: Relative sizes of the Sun, planets, and dwarf planets. The largest objects in the solar system are the Sun, the eight planets, and the three known dwarf planets. In this figure, the relative sizes are correct but the relative distances are not correct.

What Is (and Isn't) a Planet?[edit]

So what exactly is a planet? Simply put, a planet is a massive, round body orbiting a star. For our solar system, this star is the Sun. A moon is an object that orbits a planet.

"Isn't Pluto a planet?" you may wonder. When it was discovered in 1930, Pluto was considered a ninth planet. When we first saw Pluto, our telescopes actually saw Pluto and its moon, Charon as one much larger object. With better telescopes, we realized that Pluto had a moon and Pluto was much smaller than we thought! With the discovery of many objects like Pluto, and one of them, Eris, even larger than Pluto, in 2006, astronomers refined the definition of a planet. According to the new definition, a planet must:

  • orbit a star
  • be big enough that its own gravity causes it to be shaped like a sphere
  • be small enough that it isn't a star itself
  • have cleared the area of its orbit of smaller objects

Objects that meet the first three criteria but not the fourth are called dwarf planets. Most astronomers now consider Pluto to be a dwarf planet, along with the objects Ceres and Eris. Even before astronomers decided to change the definition of a planet, there were many aspects of Pluto that did not fit with the other planets in our solar system.

The Size and Shape of Orbits[edit]

Figure 25.4 shows the Sun and planets in the correct relative sizes. However, the relative distances are not correct. Figure 25.5 shows the relative sizes of the orbits. The image in the upper left shows the orbits of the inner planets. The upper left image also shows the asteroid belt, a collection of many small objects between the orbits of Mars and Jupiter. The image in the upper right shows the orbits of the outer planets. This upper right image also shows the Kuiper belt, another group of objects beyond the orbit of Neptune. In general, the farther away from the Sun, the greater the distance from one planet's orbit to the next.

Figure 25.5: This figure shows the relative sizes of the orbits of planets in the solar system. The inner solar system is on the upper left. The upper right shows the outer planets of our solar system.

In Figure 25.5, you can see that the orbits of the planets are nearly circular. In fact, the orbits are not quite circular, but are slightly elliptical. The orbit of Pluto is a much longer ellipse. Some astronomers think Pluto was dragged into its current orbit by Neptune.

Something else Kepler discovered was a relationship between the time it takes a planet to make one complete orbit around the Sun (this is also called an "orbital period") and the distance from the Sun to the planet. So, if the orbital period of a planet is known, then it is possible to determine how far away from the Sun the planet orbits. This is how we can measure the distances to other planets within our own 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 miles. Table 25.2 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.

Table 25.2: 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
Mars 1.52 1.03 1.88
Jupiter 5.20 0.41 11.86
Saturn 9.54 0.43 29.46
Neptune 30.06 0.67 164.8

The Role of Gravity[edit]

Planets are held in their orbits by the force of gravity. Imagine swinging a ball on a string in a circular motion. If you were to let go of the string, the ball would go flying out in a straight line. But the force of the string pulling on the ball keeps he ball moving in a circle. The motion of a planet is very similar, except the force pulling the planet is the attractive force of gravity between the planet and the Sun.

Every object is attracted to every other object by gravity. The force of gravity between two objects depends on how much mass the objects have and on how far apart they are. When you are sitting next to a friend, there is a gravitational force between you and your friend, but it is far too weak for you to detect. In order for the force of gravity to be strong enough to detect, at least one of the objects has to have a lot of mass. You can feel the force of gravity between you and Earth because Earth has a lot of mass. This force of gravity is what keeps you from floating off the ground. The distances from the Sun to the planets are very large. But the force of gravity between the Sun and each planet is very large because the Sun and the planets are very large objects. The force of gravity also holds moons in orbit around planets.

The moon orbits the Earth, and the Earth-moon system orbits the Sun. But Earth and its moon are not the only things that orbit the Sun. There are also other planets and smaller objects, such as asteroids, meteoroids, and comets that also orbit the Sun. The solar system consists of the Sun and all the objects that revolve around the sun as a result of gravity.

Extrasolar Planets or Exoplanets[edit]

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"). Although a handful of exoplanets have now been directly imaged, the vast majority 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. Another technique involves 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. As of February 2010, over 420 exoplanets have been confirmed with more being discovered at an ever-increasing rate.

Formation of the Solar System[edit]

There are two key features of the solar system we haven't mentioned yet. First, all the planets lie in nearly the same plane, or flat disk like region. Second, 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[edit]

The most widely accepted explanation of how the solar system formed is called the nebular hypothesis. According to this hypothesis, the 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 made mostly of hydrogen and helium, but there were heavier elements as well.

The nebula was drawn together by gravity. As the nebula collapsed, it started to spin. 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 move. This effect, called "conservation of angular momentum", along with complex effects of gravity, pressure, and radiation, caused the nebula to form into a disk shape, as shown in Figure 25.6. This is why all the planets are found in the same plane.

Figure 25.6: The nebular hypothesis describes how the solar system formed from a cloud of gas and dust into a disk with the Sun at the center. This painting was made by an artist; it's not an actual photograph of a protoplanetary disk.

Formation of the Sun and Planets[edit]

As gravity pulled matter into the center of the disk, the density and pressure increased at the center. When the pressure in the center was high enough that nuclear fusion reactions started in the center, a star was born—the Sun.

Meanwhile, the outer parts of the disk were cooling off. Small pieces of dust in the disk started clumping together. These clumps collided and combined with other clumps. Larger clumps, called planetesimals, attracted smaller clumps with their gravity. Eventually, the planetesimals formed the planets and moons that we find in our solar system today.

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 can form solid particles. But in closer to the Sun, these same materials are gases. As a result, the inner planets—Mercury, Venus, Earth, and Mars—formed from dense rock, which is solid even when close to the Sun.

Lesson Summary[edit]

  • The solar system consists of the Sun and all the objects that are bound to the Sun by gravity.
  • There are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Ceres, Makemake, Pluto and Eris are considered dwarf planets.
  • The ancient Greeks believed in a geocentric model of the universe, with Earth at the center and everything else orbiting Earth.
  • 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.
  • Planets are held by the force of gravity 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.
  • The nebular hypothesis explains why the planets all lie in one plane and orbit in the same direction around the Sun.

Review Questions[edit]

  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.
  5. What object used to be considered a planet, but is now considered a dwarf planet?
  6. What keeps planets and moons in their orbits?
  7. How old is the solar system?
  8. Use the nebular hypothesis to explain why the planets all orbit the Sun in the same direction.


geocentric model
Model used by the ancient Greeks that puts the Earth at the center of the universe.
heliocentric model
Model proposed by Copernicus that put the Sun at the center of the universe.
A celestial object that orbits a larger celestial object.
An interstellar cloud of gas and dust.
nebular hypothesis
The hypothesis that our solar system developed from a spinning cloud of gas and dust, or a nebula.
A round, celestial object that orbits a star and has cleared its orbit of smaller objects.
solar system
The Sun and all the objects that revolve around the Sun as a result of gravity.

Points to Consider[edit]

  • 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?

The Solar System · Inner Planets