The Structures of the Solar System

Chapter 11.1p 374-380

What is the solar system?

• When looking at the night sky, you will likely see stars and planets. Sometimes, the first starlike object you see at night is not a star at all.

• A few of the tiny lights that you can see are part of our solar system.

• Almost all of the other specks of light are stars.

Objects in the Solar System

• Ancient observes looking at the night sky saw many stars but only five planets – Mercury, Venus, Mars, Jupiter, and Saturn.– The invention of the telescope in the 1600s led to

the discovery of additional planets and many other space objects.

The Sun• The largest object in the solar system is the Sun, a

star.– Its diameter is about 1.4 million km– Its made mostly of hydrogen gas– Its mass makes up about 99 percent of the entire mass of

the solar system

• Inside the Sun, a process called nuclear fusion produces an enormous amount of energy.– Some of the energy is emitted as light

• The Sun also applies gravitational forces to objects in the solar system.– Gravitational forces cause the planets and other objects to

move around, or orbit, the Sun.

Objects That Orbit the Sun

• Planets, dwarf planets, asteroids, and comets orbit the Sun.– These don’t emit light.

Planets

• An object is a planet only if it orbits the Sun and has a nearly spherical shape.

• The mass of a planet must be much larger than the total mass of all other objects whose orbits are close by.

• Our solar system has eight objects classified as planets.

Inner Planets and Outer Planets

• The four planets closet to the Sun are the inner planets.– Mercury, Venus, Earth, and Mars– These planets are mainly made of solid rocky

materials

Inner Planets and Outer Planets

• The four planets farthest from the Sun are called the outer planets– Jupiter, Saturn, Uranus, and Neptune– These planets are mainly made of ice and gases such as

hydrogen and helium.– Because they are much larger than Earth, we also call them gas

giants.

Dwarf Planets

• A dwarf planet is a spherical object that orbits the Sun.– It is not a moon of another planet and is in a region of

the solar system where there are many objects orbiting near it.

– Unlike a planet, a dwarf planet does not have more mass than objects in nearby orbits

– They are made of rock and ice and are much smaller than Earth.

– They include Ceres, Eris, Pluto, and MakeMake.

Asteroids

• Millions of small, rocky objects called asteroids orbit the Sun in the asteroid belt between the orbits of Mars and Jupiter– They range in size from less than a meter to

several hundred kilometers in length– Unlike planets and dwarf planets, asteroids

usually aren’t spherical

Comets

• A comet is made of gas, dust, and ice and moves around the Sun in an oval-shaped orbit.– Comets come from the outer parts of the solar

system.– There might be 1 trillion comets orbiting the

Sun.

The Astronomical Unit

• On Earth we use meters or kilometer.

• Objects in the solar system, are so far apart that astronomers use a larger distance unit.

• An astronomical unit (AU) is the average distance from Earth to the Sun – about 150 million km.

The Motion of the Planets

• Revolution and Rotation– The time it takes an object to travel once

around the Sun is its period of revolution• Earth’s period of revolution is one year.

– The time it takes an object to complete one rotation is its period of rotation

• Earth’s period of rotation is one day

The Motion of the Planets

• Planetary Orbits and Speeds– Unlike a ball swinging on the end of a string, planets do not

move in circles– Instead, a planet’s orbit is an ellipse – a stretched-out circle– Inside an ellipse are two special points, each called a focus.– These focus points, or foci, determine the shape of the ellipse.– The foci are equal distances from the center of the ellipse.

The Motion of the Planets

• Planetary Orbits and Speeds– A planet’s speed also changes as it orbits the

Sun. The closer the planet is to the Sun, the faster it moves.

– This also means that planets farther from the Sun have longer periods of revolution.

– For example, Jupiter is more than 5 times farther from the Sun than Earth.

• It takes Jupiter 12 times longer than Earth to revolve around the Sun.