Solar System Our Solar System Is the System of Planets and Other Objects

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Solar System

Our solar system is the system of planets and other objects that orbit our sun. It has many parts. Our sun, the star of our solar system, holds everything in place by its gravity. There are eight planets and many moons. There are asteroids and comets. There are meteoroids and tiny particles of rock and dust. The sun, which is actually a star, is the largest object in the solar system. It contains more than 99% of the mass of the solar system.

What is a star? A star is a huge body in space made of gases that is held together by its own gravity. It makes heat and light by the chemical process of nuclear fusion. This process combines hydrogen atoms by joining them or fusing them together. When this happens, a huge amount of energy is released. Our sun is an average-sized star that is about half way through its life cycle. It should continue making heat and light for another 5 billion years!

Our sun provides all the energy for life on Earth. Without its heat, Earth would be too cold to live on. Without its light, plants could not grow, and there would be no food for people and animals.

Our solar system formed about 4.6 billion years ago. It formed from dust and gas in the universe left over from the Big Bang. The sun began as a cloud of gases and dust, which began condensing until it was big enough to begin nuclear fusion. As its heat and light burst forth, more matter began to clump together. The gravity of the sun pulled the matter around it. These clumps became our planets. Between the sun and each of the planets, there are vast distances of empty space.

We have eight planets in our solar system. Recently the IAU (International Astronomical Union) changed the definition of a planet, so Pluto no longer qualifies. Except for Mercury and Venus, all of the planets have at least one moon or natural satellite of their own. The planets are divided into two kinds: the terrestrial planets and the gas giants.

The first four planets outward from the sun are classified as terrestrial planets. They are Mercury, Venus, Earth, and Mars. They are called terrestrial because they have a solid, rocky surface with metal cores. They are also sometimes called the rocky planets or inner planets.

Beyond Mars lies a doughnut-shaped mass of asteroids. The asteroid belt contains millions of chunks of rocky-metallic material. Scientists think this is leftover material that was not able to group itself into another planet. Pieces of asteroids drift into Earth’s atmosphere and become meteors.

Solar System (cont’d)

Past the asteroid belt lie four more planets called the gas giants. Jupiter, Saturn, Uranus, and Neptune do not have a solid surface. They are large balls of gases with small rocky cores. They are much larger than the rocky planets. All of them have rings around them. They are also called the outer planets.

Beyond Neptune lies the Kuiper Belt (pronounced KY per). Over 800 objects have been found in the Kuiper Belt since 1992. The largest is called Eris, and it is actually larger than Pluto! This led to the decision that Pluto is no longer a planet. Pluto and Charon also lie within this belt. Pluto was, until recently, classified as a planet, and Charon was classified as its moon. All of these objects are now called Kuiper Belt Objects (KBO) or Trans-Neptunian Objects (TNO).

The planets move in two ways. First, they orbit or go around the sun. The time it takes a planet to make one complete orbit around the sun is known as its year. Earth’s year is 365 ¼ days. The second way planets move is that they rotate or spin on an imaginary axis that runs through the center of the planet. The time it takes a planet to make one complete turn on its axis is known as a day. Earth’s day is 24 hours.

Our solar system is a vast area in space, and most of it is empty space. Scientists are still learning about the objects that are in the far reaches of our solar system. Only one planet is known to have life on it – our home, planet Earth.

Solar System Questions

1. What is the solar system? a. the sun and the planet that the sun orbits b. the system of planets and objects that orbit the sun c. the group of stars that we can see in the night sky d. the moon and the Earth

2. Why do the planets orbit the sun? a. the sun’s gravity holds the planets in place b. the planets’ gravity c. the moon’s gravity d. They can’t escape the dark matter in space.

3. How many planets are in the solar system? a. six b. seven c. eight d. nine

4. How much of the mass of the solar system does the sun have? a. more than 50% b. more than 75% c. more than 90% d. more than 99%

5. What is the process by which the sun makes light and heat? a. burning b. nuclear fission c. nuclear fusion d. chemical reaction

6. How old is the solar system? a. 46 million years b. 46 billion years c. 4.6 billion years d. 4.6 million years

7. Which body used to be called a planet but now is not? a. comets b. Asteroid Belt c. Pluto d. Kuiper Belt

Solar System Questions (cont’d)

8. What two ways do we group the planets? a. inner and outer b. terrestrial and gas giants c. warm and cold d. both a and b are correct

9. Planets move in how many ways? a. They move in one way. b. They move in two ways. c. They move in three different ways. d. They don’t move at all.

10. How many planets in the solar system are known to have life? a. one b. two c. three d. all of them

Just For Fun! To remember the planets in order from the sun outward, fill in the blanks below to make a silly saying. For example: My Very Elderly Monkey Just Stood Up Naked!

M______V______E______M______

J______S______U______N______

Solar System Questions (cont’d)

True or False? 1. ______The sun gives us heat and light. 2. ______The sun gives us oxygen to breathe. 3. ______The sun orbits the Earth. 4. ______The sun is about the same size as the moon. 5. ______Most of the solar system is empty space. 6. ______The sun is the only star that exists in space. 7. ______The sun is the only star in our solar system. 8. ______All of the planets are about the same size. 9. ______The way we measure time is related to the way the planets move. 10. ______All of the planets are made about the same.

Ordering the Planets

Number the planets in the order that they orbit the sun. Number one should be the planet that is closest to the sun, and number eight should be the planet that is farthest away.

______Earth ______Jupiter ______Mars ______Mercury ______Neptune ______Saturn ______Uranus ______Venus

Solar System Answers

Multiple Choice:

1. b 2. a 3. c 4. d 5. c 6. c 7. c 8. d 9. b 10. a

True/False: 1. true 2. false 3. false 4. false 5. true 6. false 7. true 8. false 9. true 10. false

Ordering the Planets: 3 5 4 1 8 6 7 2