Chapter 8, Astronomy

LESSON 3 THE SOLAR SYSTEM

Identify planets by observing their movement against background stars.

Explain that the solar system consists of

many bodies held together by gravity.

OBJECTIVES

The solar system consists of the planets, their moons, and many other bodies

orbiting the Sun.

MAIN IDEA

planet moon

solar system inertia

VOCABULARY

asteroid comet

meteoroid meteor

meteorite

• Before telescopes were invented people studied stars by looking up at the night sky.

• People noticed the bright lights in the sky changed.

• A solar system has stars like the sun as well as moons and planets.

• All but 2 planets have one or more than one moon.

WHAT IS THE SOLAR SYSTEM?

• Gravity is the force of attraction among all objects.

• The amount of gravitational force between the sun and a planet depends on mass and distance.

• The greater the mass, the greater the force.

• The greater the distance, the less force.

PLANETS AND ORBITS

• The second property that keeps planets in orbit is inertia.

• Without it we would be pulled into the Sun because of its mass.

• With only inertia the planets would move in a straight line.

• Gravity and inertia are balanced so the planets’ paths are curved.

• Ancient astronomers saw that the planets moved across the field of stars but didn’t know why.

• Over time two explanations emerged.

IDEAS ABOUT PLANETARY MOTION

• The first explanation was that the Earth was the center of the universe.

• The other explanation was that the Earth, the Moon, the stars, and other planets revolve around the Sun.

Classify List the planets in order of increasing

distance from the Sun.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

QUICK CHECK

Critical Thinking Would the pull of the Sun’s gravity on a

space probe be greater near Mercury or near Saturn? Explain.

The Sun’s gravity on a space probe would

be greater near Mercury because it is closer to the Sun.

• Mercury, Venus, Earth, and Mars are closest to the Sun and called the inner planets.

• They have similar sizes and mostly rocky structures.

• They also have closely spaced orbits and few, if any, moons.

• All the inner planets rotate relatively slow and none of them have rings.

• Earth is the largest of the inner planets.

WHAT IS IN THE INNER SOLAR SYSTEM?

• Between the orbits of Mars and Jupiter are asteroids, rocky or metallic objects, that orbit the Sun.

• Most asteroids are located in the asteroid belt. • The largest object is about one fourth the

diameter of the Moon. • Asteroids orbit the Sun just like planets. • Some asteroids travel as far from the Sun as

Saturn’s orbit, other asteroids have orbits that cross Earth’s path.

ASTEROIDS

• Scientists have a lot of knowledge about asteroids by using space probes.

• Galileo, a space probe, passed by Gaspra and Ida, two asteroids.

• NEAR Shoemaker, another space probe, encountered the asteroid Mathilde and landed on Eros.

Classify List the inner planets in order from smallest

to largest.

Mercury, Mars, Venus, Earth

QUICK CHECK

Critical Thinking In what ways are asteroids similar to

planets?

Asteroids are similar to planets because they are also made of rock and metal. They

orbit the sun, as do the planets.

• Beyond the asteroid belt is another group of planets that includes Jupiter, Saturn, Uranus, and Neptune, known as the outer planets.

• They are gas giants which are huge planets with a small, metallic core, and a thick atmosphere.

• The gas giants all have rings and many moons.

• They spin very rapidly, so a day on a gas giant is very short.

WHAT ARE THE OUTER PLANETS?

• Pluto was once known as the ninth planet.

• In 2003, astronomers discovered a similar, slightly larger world beyond the orbit of Pluto.

• In 2006, the International Astronomical Union reclassified Pluto as a dwarf planet.

Classify List the outer planets in order from smallest to

largest.

Neptune, Uranus, Saturn, and Jupiter are the outer planets listed from smallest to largest.

QUICK CHECK

Critical Thinking How is Pluto different from the outer planets?

Differences between Pluto and the outer

planets include size and rings. The outer planets are huge, while Pluto is very

small. The outer planets have rings and Pluto does

not.

• A comet is a ball of ice and rock that orbits the Sun.

• They come from the outer fringes of the solar system.

WHAT ARE OTHER OBJECTS IN OUR SOLAR

SYSTEM?

• Some comets come from the Kuiper belt located just beyond Pluto’s orbit.

• Other comets originate in an area called the Oort cloud, a region surrounding the solar system at a distance of about 30 trillion kilometers from the Sun.

• Meteoroids are small, rocky, or metallic objects that orbit the Sun in both the inner and outer regions of the solar system.

• A meteor is a meteoroid that enters Earth’s atmosphere.

• A meteoroid that strikes Earth’s surface is a meteorite.

Classify How are space objects classified as meteoroids,

meteors, and meteorites?

In space, they are meteoroids; In Earth’s atmosphere, meteors;

And on Earth, they are meteorites.

QUICK CHECK

Critical Thinking Is the tail of a comet in front of or behind the

comet? Explain your answer.

Sunlight and solar radiation push the cloud

(tail) away, forming a tail behind it that always points away from the Sun.