SC.5.E.5.3 The Objects of the Solar System

SC.5.E.5.3: Distinguish among the following objects of the Solar System—Sun, planets, moons, asteroids, comets—and identify Earth’s position in it.

What are Moons, Comets, and Asteroids?

The Earth is the third planet from the star at the center of our solar system, the Sun. Stars are objects that produce energy, some of it in the form of light. Some of the energy produced by the Sun is felt as heat on Earth. The Earth is a planet. Planets are smaller objects than stars. Planets do not produce energy in the form of light. Planets reflect the light of the Sun. Planets travel in a path called an orbit around the Sun. The motion of one object around another is called revolution. As planets revolve around the Sun, they also spin on their axis. An axis is an imaginary line through the center of an object on which it spins. The spinning of a planet on its axis is called rotation.

It is the rotation of the Earth on its axis that causes the cycles of day and night. The Sun appears to rise, move across the sky, and set. This motion is caused by the spinning of Earth on its axis. It is the Earth that is moving, not the Sun. The rotation of the Earth also causes the stars to move across the sky.

A moon is an object that is smaller than the planet that it orbits. The Earth has one moon. Saturn has 83 moons. All moons orbit their planet. Saturn’s moon Titan is larger than the planet Mercury. As our moon orbits our planet, it appears to change shape. These changes in the moons shape are called phases. The moon reflects the light of the Sun. As the moon orbits the Earth, the shape of the moon appears to change as the side of the moon facing the Sun changes every night. Sometimes the moon is visible during the day.

The Earth is the only planet with liquid water. There is water on other planets in the Solar System, but it is too cold on those planets to be a liquid. Earth is the only planet with evidence of life. Scientists have been exploring Mars for evidence of life for nearly seventy years, but so far have not discovered any clear evidence of life there.

This photograph of Earth was taken by the last astronauts to visit the moon in 1972. It is easy to see why the picture became known as “the blue marble!” This was the first time a photograph was taken of the “full Earth.” People began to realize how important it is to take care of our planet.

There are eight planets in the Solar System. Other objects in our solar system include asteroids and comets. Asteroids are irregularly shaped objects of rock and metal. Cometsare frozen objects of gas and dust. Both asteroids and comets are occasionally visible from Earth, but not very often.

Asteroids are found in a “belt” between Mars and Jupiter. Sometimes rocks in the asteroid belt get knocked out of their orbit, and can fall toward the Sun. They may pass close to the Earth. Sixty-five million years ago, an asteroid struck the Earth and caused the extinction of all large dinosaurs. Most asteroids pass safely past Earth. Sometimes a smaller rock will enter the Earth’s atmosphere and burn up. This is visible as a meteor.

This meteor crater in Arizona was formed about 50,000 years ago. The meteor that formed this crater was about 160 feet across: that’s the size of four school busses in a line. There are many meteor craters on Earth, but most have been weathered down so much they are not recognizable.

Comets are found far out in the Solar System, beyond the orbit of Pluto, a dwarf planet. Like asteroids, sometimes comets fall in toward the Sun. As they get closer to the Sun, the Sun’s heat begins to melt the gas and dust in the comet. This material streams out from the comet, and reflects the light of the Sun. The dust left behind by a passing comet will stay in the path of the Earth, and cause meteor showers. Meteor showers are when sand and dust sized particles from a passing comet burn up in Earth’s orbit.

The Solar System we live in is one of many in the Milky Way Galaxy. Many of the stars you see in the night sky have their own solar systems. Is there life on any of those planets? Currently, there is no evidence for life on any planet except Earth. Each solar system has a star at the center. There are some solar systems with two stars. The star at the center of our solar system in the Sun. The Sun, like all stars, produces energy, some of it light. Planets are smaller objects that orbit a star. Moons orbit a planet. An orbit is the path that a moon or planet takes as it revolves around a larger object. Comets and asteroids are also part of the Solar System.