A resource document which gives teachers relevant and essential background knowledge on the science concept being addressed.
Our solar system resides in the Milky Way galaxy and is made up of the Sun, eight planets, and many moons, asteroids, meteoroids, and comets. All of the celestial bodies in the solar system move in predictable patterns known as orbits, and this motion is controlled by gravity. Gravity is a force of attraction between two or more masses.
Everything that has mass has gravity. The more mass you have, the stronger the gravitational force you exude. For example, Earth has more mass than the Moon, so its gravitational field is stronger. In addition, as the distance between two masses increases, the gravitation attraction between them decreases. For example, Earth’s gravitation pull on us is strongest when we are standing on its surface compared to if we were floating in space, thousands of miles away. The celestial body in our solar system with the most mass and hence strongest gravitational force is the Sun. Because the Sun has the strongest gravitation field, it has the strongest influence on the motion of the other bodies in the solar system (i.e., planets).
Celestial objects, such as planets, moons, asteroids, comets, meteors, and even satellites, move within the solar system, around more massive objects (e.g., the Sun), along paths known as orbits. These paths are determined by the momentum with which these objects are travelling and by the gravitational force they experience from other, more massive objects. For example, Earth’s orbit around the Sun is determined by the balance of the Sun’s gravitational pull on Earth and Earth’s momentum as it travels around the Sun. Without the Sun’s gravitational pull, Earth would not move in a circle around the Sun, but would continue moving straight throughout the Milky Way. Without
Earth’s momentum, the Sun would pull Earth into it.
Planets have orbital paths around the Sun that are nearly circular, while other bodies, particularly comets, have paths around the Sun that are more elliptical in shape.
Comets are small bodies of ice and dust that formed near Neptune when the solar system was first forming. Neptune has a very high mass that influences the orbits of these comets, making them elliptical. When comets are closest to the Sun in their orbit, their ice vaporizes, creating the tail we see as they pass over us in the night sky. Other celestial bodies include meteoroids (small pieces of rock travelling in space) and asteroids (larger, rocky, planet-like objects orbiting the Sun). The only difference between a meteoroid and an asteroid is size. Asteroids orbit the Sun in a belt between
Mars’ and Jupiter’s orbits. A meteor is the object we say we see when a meteoroid enters Earth’s atmosphere and lights up as the friction it experiences in our atmosphere causes it to “burn up.” If a meteoroid makes it through our atmosphere to Earth’s surface, it is called a meteorite. Because comets, meteoroids, and asteroids have smaller masses than the planets and Sun in our solar system, their orbits are strongly influence by the gravity of all of these larger celestial bodies.

The Sun is the largest body in our solar system, larger than any of the planets. It is a star, which is made up of hot gases (~92% Hydrogen and ~8% Helium) and is extremely dense (1410 kg/m3). There are billions of stars (estimates between 100-500 billion) just in the Milky Way galaxy, and the Sun is one of them. Scientists classify stars based on a number of properties including their size (radius), luminosity (the amount of energy they release), brightness, temperature, and color. Temperature and color are related t o each other. As temperature increases, a star’s color will change from red, to orange, to yellow, to white and even to blue. (Think of your gas stove. The flame closest to the burner is white almost blue (hottest) and gets redder (cooler) farther away from the burner.) When compared to other known stars, the Sun is an average star, falling in the middle of the range for size, temperature, and luminosity.
There are eight planets in our solar system, all of which revolve around the Sun in their orbits. Each planet also rotates, or spins, along its axis (like a top spinning). Mercury,
Venus, Earth, and Mars are terrestrial planets; that is, they are mostly solid, and are known as the Inner Planets because they are closest to the Sun. The Outer Planets are
Jupiter, Saturn, Uranus, and Neptune, and they are made up mostly of gases with liquid cores.
All of the planets, except Mercury and Venus, have moons. Moons are smaller celestial objects that orbit planets. Scientists collect data about their physical properties and motions using telescopes and satellites. Out of the six largest moons in our solar system, four belong to Jupiter (Io, Europa, Ganymede, and Callisto) and are known as the Galilean Moons because Galileo Galilei discovered them in the 1600s by using a telescope. The largest moon is Titan, which orbits Saturn. Earth’s moon is the fifth largest in our solar system. All of these moons are larger than Pluto, which is no longer classified as a planet; instead it is considered a dwarf planet.
Earth is a special planet within our solar system in that it has the right conditions for life to exist. The combination of Earth’s distance from the Sun and its atmosphere allows for a stable temperature capable of supporting higher life forms, such as humans. The two planets neighboring Earth, Venus and Mars, are either too hot (464 °C) or too cold (-65
°C) to support life, respectively. Earth’s average surface temperature is 15 °C, or 60 °F, and is suitable for life, due largely to Earth’s atmosphere. Earth’s atmosphere is mainly composed of nitrogen (~78%), oxygen (~21%), argon (~1%), and small amounts of other compounds such as water vapor, ozone, and carbon dioxide. Without greenhouse gases, such as carbon dioxide and water vapor, keeping Earth warm, Earth’s temperature would be -
33 °C. Earth’s surface, including vegetation, ice, and cloud cover, also reflects a lot of the Sun’s radiation, preventing Earth from getting too hot for life. In addition, Earth’s atmosphere and magnetic field act as a shield protecting life on
Earth. The ozone layer in the atmosphere protects life from harmful radiation from the
Sun, such as ultraviolet radiation and gamma rays. Earth’s magnetic field protects life on Earth by deflecting the charged particles (protons and electrons) constantly flowing from the Sun, known as solar wind.

Water is a requirement for life as we know it. Most of the organisms on Earth, including humans, need water to survive. Though there is no evidence of liquid water or water vapor on Mars, scientists have geologic evidence suggesting that liquid water used to exist on Mars’ surface. Pictures of Mars’ surface from orbiting satellites and Mars rovers show land features that look like lakes and rivers that have dried up. Scientists also speculate that water exists on other celestial bodies in our solar system in various states of matter. Recently, scientists have found evidence that there could be a liquid ocean of water underneath the frozen surface of Europa, one of Jupiter’s moons. Many celestial objects, such as planets and moons, make up our solar system, and although many of them have some of the components necessary for life to flourish, only Earth has the ideal combination.