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Our Solar System

Science, Grade 6

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Table Of Contents: Our Solar System

1. Formation of Our Solar System

2.1. Nebulae
Solar systems begin in dusty gas clouds called nebulae that are found between stars. The dust is composed of elements like iron and carbon. Nebula gases include hydrogen and helium.
2.2. Planetesimals
Within a nebula, dust particles collide and stick together to form larger particles. This process is called accretion. The particles grow from pebbles, to rocks, to boulders. The larger particles are called planetesimals.
2.3. Rocky and Gaseous Planet Formation
Rocky planetesimals near the Sun grew by accretion to become the rocky planets—Mercury, Venus, Earth and Mars. In the colder outer parts of the solar system, frozen gases accreted to rocky cores to form the gas giant planets—Jupiter, Saturn, Uranus and Neptune.

2. Geocentric and Heliocentric Systems

3.1. Geocentric Universe Concept
Before the invention of the telescope, the solar system was described in relation to the Earth. In 140 AD, the Greek astronomer Ptolemy believed that the Earth was the center of the universe, and the Sun and planets revolved around the Earth. This is the geocentric concept of the universe.
3.2. Heliocentric Universe Concept
In 1543 Copernicus published a theory about the universe. He claimed that the Sun is the center of the universe and all the known planets revolved around the Sun. This is the heliocentric concept of the universe.
3.3. A Heliocentric Solar System
Copernicus believed that the Sun was the center of the universe. Today we know that the Sun is the center of our solar system—a collection of planets orbiting a single star, the Sun. Our solar system is within the Milky Way galaxy, which is within a universe of galaxies.
3.4. Parts of Our Solar System
Our solar system includes the Sun, eight planets, the moons of each planet and other objects that revolve around the Sun. The four rocky planets closest to the Sun are known as the inner planets, and the four gaseous planets in the outer solar system are called the outer planets. The relative size of the inner planets is much smaller than the outer planets.

3. The Sun

4.1. What Is the Sun Made of?
The Sun is a massive burning ball of gases held together by gravity. It is primarily composed of helium and hydrogen gases.
4.2. Structure of the Sun
The Sun has three main layers—the core, the radiative zone and the convective zone. The Sun’s atmosphere is made up of the photosphere, chromosphere and corona.
4.3. The Sun’s Core and Nuclear Fusion
The core at the center of the Sun produces the Sun’s heat. This energy is produced by hydrogen nuclear fusion reactions. During nuclear fusion, two or more protons fuse to form helium, and large amounts of energy are released.
4.4. The Sun’s Radiative Zone
Energy produced in the Sun’s core is carried to the Sun’s surface in the form of light energy called photons. This energy moves from the core into a very dense radiative zone. It can take thousands of years for the light energy to move through this layer and into the convective zone.
4.5. The Sun’s Convective Zone
The outer layer of the Sun is called the convective zone. Hot gases in this layer rise toward the surface, where they become cooler and denser. The gases then descend back into the Sun. This circulating movement is called convection, and is similar to currents in the Earth’s mantle.
4.6. The Sun’s Photosphere
The photosphere is the inner layer of the Sun’s atmosphere. This gaseous layer is thick enough to be visible. When we look at the Sun, we are seeing the photosphere.
4.7. The Sun’s Chromosphere
Just beyond the photosphere is a thin layer called the chromosphere, which has a reddish color. The chromosphere can be seen during a total solar eclipse when the Moon covers the Sun.
4.8. The Sun’s Corona
The outer atmosphere of the Sun is called the corona. It extends for millions of kilometers beyond the Sun, and is composed of gases. It can only be seen during a total solar eclipse.
4.9. Sunspots
Sunspots are cooler, darker areas of gas on the Sun’s surface. Cooler gases emit less light than hotter gases. These areas are caused by the Sun’s magnetic field affecting the convection heat. Scientists have discovered that the number of sunspots varies over an 11-year cycle.
4.10. Solar Flares
Solar flares are enormous areas of extreme temperature on the Sun’s surface, caused by magnetic fields. Solar flares typically occur near sunspots. When a solar flare erupts, streams of charged gas particles extend thousands of kilometers into space.

4. Pause and Interact

5.1. Review
Use the whiteboard tools to complete the activity.
5.2. The Sun
Click on the Terms button. Then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.

5. Measuring Distances in Space

6.1. Astronomical Units
The distance between the Earth and the Sun is approximately 150 million kilometers, or one astronomical unit (AU). Scientists use astronomical units to measure and describe distances in space. Another measurement is called a light-minute, which is the distance light travels in one minute, or approximately 18 million kilometers.
6.2. Distance Between the Planets and Sun
The dimensions of our solar system are vast. The four inner planets are much closer to the Sun than the four outer planets. There are numerous orbiting comets found beyond the planets in the outer solar system.

6. The Inner Planets

7.1. What Are the Inner Planets?
The four planets closest to the Sun, Mercury, Venus, Earth and Mars, are grouped together as the inner planets. They have rocky surfaces, and are often called the terrestrial planets. These planets are small and closely spaced, as compared to the outer planets.
7.2. Mercury
Mercury is the smallest planet, located closest to the Sun. Mercury revolves on its axis very slowly. A single Mercury day is about 59 Earth days long. Mercury does not have any moons or atmosphere. Its surface is covered with craters. A magnetic field suggests that it has an iron core.
7.3. Venus
Venus is the second closest planet to the Sun. It is often called Earth's sister or twin planet due to similar composition, gravity and size. Venus rotates very slowly in a clockwise direction, called retrograde rotation. Similar to the planet Mercury, Venus does not have any moons.
7.4. Atmosphere and Surface of Venus
Venus is surrounded by a very dense atmosphere composed of carbon dioxide and sulfuric acid clouds. Due to the greenhouse effect, Venus has the hottest surface temperature of any planet. The air pressure on Venus is 92 times that on Earth. Craters and active volcanoes cover the surface of the planet. From a geologic perspective, Venus is considered a young planet.
7.5. Earth
The Earth is the third planet from the Sun. The Earth's distance from the Sun allows for moderate temperatures that keep most surface water in liquid form. The presence of liquid water and an oxygen-rich atmosphere make Earth suitable for millions of living organisms.
7.6. Mars
Mars, the fourth planet from the Sun, has two moons. A day on Mars is slightly longer than a day on Earth, and it takes about 687 days for Mars to revolve once around the Sun. Space probes and rovers have successfully gathered much information about Mars.
7.7. Atmosphere and Surface of Mars
Mars has a very thin atmosphere composed primarily of carbon dioxide. The surface is cold, dry, and dusty, with frozen ice caps found at the poles. Landforms and rover samples indicate that in the past there were large amounts of liquid water on the surface. Mars has a volcanic history, and the largest volcano in the solar system, Olympus Mons, is found there.

7. Pause and Interact

8.1. Review
Use the whiteboard tools to complete the activity.

8. The Outer Planets

9.1. What Are the Outer Planets?
The outer planets beyond Mars are the gas giant planets, Jupiter, Saturn, Uranus and Neptune. They are also known as the Jovian planets. These planets are much larger and spaced further apart than the inner planets. They are primarily composed of hydrogen and helium.
9.2. Jupiter
Jupiter, the fifth planet from the Sun, is the largest planet in the solar system. Although primarily composed of gases, its magnetic field indicates that it may have a rocky core. It has a giant red spot that is actually an enormous storm in its atmosphere. There are 64 known moons that orbit Jupiter.
9.3. Saturn
Saturn, the sixth planet from the Sun, is the second largest planet in the solar system. Winds can blow as fast as 1,800 km on this planet. Saturn's rings are primarily composed of ice particles with some dust and rock particles mixed in. There are 62 known moons that orbit Saturn.
9.4. Uranus
Uranus, the seventh planet from the Sun, is the first planet to be discovered with the use of a telescope. Its cloudy atmosphere is primarily made of helium and hydrogen, but also has higher concentrations of frozen ammonia, water and methane. Uranus is sometimes referred to as an ice giant. This planet has two sets of rings and 27 known orbiting moons.
9.5. Rotation and Tilt of Uranus
Similar to Venus, Uranus spins on its axis in a clockwise direction, called retrograde rotation. It also has a rotation axis that is tilted almost parallel to its orbital plane. This makes it appear that Uranus is rotating on its side.
9.6. Neptune
Neptune is the furthest planet from the Sun. This gas giant is similar to Uranus in that it has a higher amount of frozen gases like methane, water and ammonia, and therefore it is also called an ice giant. The highest known wind speeds in the solar system, measuring up to 2,100 kilometers per hour, are found on Neptune. Thirteen moons orbit Neptune.

9. Pause and Interact

10.1. Review
Use the whiteboard tools to complete the activity.

10. Comets, Asteroids, and Meteors

11.1. Comets
A comet is a small body composed of rock and ice that orbits the Sun. A comet's tail of ice particles can stretch for millions of kilometers behind it. Comets originate in the outer regions of the solar system and have a wide range of orbital periods, up to a thousand years long. Scientists have identified thousands of comets, but this is just a fraction of the total number that exist.
11.2. Asteroids
Asteroids are irregularly shaped, rocky bodies that circle the Sun. They are found mostly in an area between Mars and Jupiter called the asteroid belt. Asteroids range in size from quite small to 100 kilometers in diameter. There are over 500,000 known asteroids.
11.3. Meteoroids
Meteoroids are smaller pieces of rock that have originated from asteroids. A streak of light called a meteor is seen when a meteoroid enters and burns in the Earth's atmosphere. When a meteoroid strikes Earth, it is called a meteorite. Meteor Crater in Arizona shows the impact and size of meteorite collision.

11. Pluto

12.1. Pluto and the Kuiper Belt
Up until 2006, Pluto was considered the ninth planet from the Sun. After intensive studies, a group of international astronomers concluded that Pluto was actually a dwarf planet in a region called the Kuiper Belt. Asteroids and other planetesimals are found in this far distant region of our solar system.

12. Pause and Interact

13.1. The Solar System
Click on the Terms button. Then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.

13. Vocabulary Review

14.1. Vocabulary Matching Review
Use the whiteboard tools to complete the activity.

14. Virtual Investigation

15.1. Planet Exploration
In this virtual investigation you will explore our solar system by comparing several characteristics of the planets. Complete the four activities and record your data during this exploration.

15. Assessment

16.1. Our Solar System
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