7 Our Solar System

The Milky Way

A is a collection of billions of stars, gas, and dust held together by gravity in space. Our is located in the Milky Way Galaxy. As seen in the image, below, it got its name because it appears as a milky band of light in the sky.

Image of the Milky Way Galaxy taken in Chile
La Silla Dawn Kisses the Milky Way” by ESO/B. Tafreshi is licensed under CC BY 4.0

As seen in the image, below, the Milky Way is a large spiral-shaped galaxy which contains hundreds of billions of stars. At the center of the Milky Way is a supermassive black hole named Sagittarius A which has a mass of 4 million suns. Our Sun, Earth, and all the planets are located halfway between the center and the outer edge on a small partial arm called the Orion Spur.

Spiral shape of the Milky Way Galaxy. Our solar system is located on the Orion Spur. “The Milky Way Galaxy” by NASA/JPL-Caltech/R. Hurt


Planets in the Solar System “A representative image of the solar system with sizes, but not distances, to scale” by WP is licensed under CC BY-SA 3.0

There are 8 planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. To remember the order of the planets (closest to farthest from the sun), use the acronym “My Very Educated Mother Just Served Us Nutella.”

The four inner planets–Mercury, Venus, Earth, and Mars–are rocky planets because they have a solid surface. The four outer planets–Jupiter, Saturn, Uranus, and Neptune–are gaseous planets because they are composed of gases, mainly hydrogen and helium. Notice that the rocky planets are much smaller in size and the gaseous planets are larger. One theory for this is that when the Sun turned on and became a star, it caused the gas clouds of the four inner planets to blow away. The rocky planets were left with a smaller, solid planet. The gaseous planets are farther from the sun, so they retained their composition. As they increased in mass, their gravity increased which allowed them to attract more and more material from space and grow larger in size.

Sizes and Distances of Planets

The image below shows the huge variance in size between planets in our solar system. Notice the differences in size between the inner, rocky planets and the outer, gaseous planets.

Size planets comparison” by Lsmpascal-Own work is licensed under CC BY-SA 3.0/labels added from original

Measurements of Our Solar System

Element Diameter (km) Distance from the Sun (x106) (km)
Sun 1,392,000 ————————–
Mercury 4,897 57.9
Venus 12,104 108.2
Earth 12,756 149.6
Mars 6,794 227.9
Jupiter 142,980 778.6
Saturn 120,540 1433.5
Uranus 51,120 2872.5
Neptune 49,530 4495.1

Watch the following video to see a size comparison of the planets across Iowa’s campus.

Size comparison of the planets

K-6 Standards

5-ESS1-1. Support an argument that differences in the apparent brightness of the sun
compared to other stars is due to their relative distances from Earth.


Why is Pluto Not a Planet?

Pluto was the ninth planet in our solar system until a controversial 2006 decision when it was reclassified as a dwarf planet. Pluto meets two requirements to be a planet: it orbits around the sun and its gravity formed the planet into a round shape. However, it does not meet the third requirement of “clearing the neighborhood.” Planets must have gravitational dominance and clear the neighborhood around their orbit; this means that large planets (more mass=more gravity) either attract or eject other, smaller bodies from that region of space. Several other dwarf planets and similarly-sized space objects were discovered in the solar system near Pluto’s orbit in the Kuiper Belt. Therefore, Pluto has not cleared the neighborhood and so it cannot be considered a planet.


During the early life of our solar system, dust and rocks in space were pulled together by gravity to form the planets. Not all the dust and rocks were made into planets; smaller, rocky remnants called remain and orbit the Sun in our solar system. Between Mars and Jupiter, you can find the Main Asteroid Belt which is where most of the known asteroids orbit.

Other Objects in the Solar System

  • Comet: A frozen ball of gas, rock, and dust that orbit the Sun. When a comet gets close to the Sun, it heats up and the gas and dust form a tail.
  • Meteoroid: A small rock in space that orbits around the Sun. Most meteors have broken off of larger objects such as asteroids or comets. They can be as small as a grain of sand or as large as a pickup truck.
  • Meteor: The streak of light that is caused when a meteoroid enters Earth’s atmosphere and burns up due to friction. Also known as a shooting star.
  • Meteorite: A meteoroid that survives its trip through the atmosphere and lands somewhere on Earth. The impact of a meteorite can cause a crater on the surface of a planet.
Image of Halley’s Comet by NASA is public domain
Meteorite found in the Nubian Desert in northern Sudan by NASA













Performance Expectations

5-ESS1-1. Support an argument that differences in the apparent brightness of the sun compared to other stars is due to their relative distances from the Earth. [Assessment Boundary: Assessment is limited to relative distances, not sizes, of stars. Assessment does not include other factors that affect apparent brightness (such as stellar masses, age, stage).]
5-PS2-1. Support an argument that the gravitational force exerted by Earth on objects is directed down. [Clarification Statement: “Down” is a local description of the direction that points toward the center of the spherical Earth.] [Assessment Boundary: Assessment does not include mathematical representation of gravitational force.]
MS-ESS1-2. Develop and use a model to describe the role of gravity in the motions within galaxies and the solar system. [Clarification Statement: Emphasis for the model is on gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. Examples of models can be physical (such as the analogy of distance along a football field or computer visualizations of elliptical orbits) or conceptual (such as mathematical proportions relative to the size of familiar objects such as students’ school or state).] [Assessment Boundary: Assessment does not include Kepler’s Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth.]
MS-ESS1-3. Analyze and interpret data to determine scale properties of objects in the solar system. [Clarification Statement: Emphasis is on the analysis of data from Earth-based instruments, space-based telescopes, and spacecraft to determine similarities and differences among solar system objects. Examples of scale properties include the sizes of an object’s layers (such as crust and atmosphere), surface features (such as volcanoes), and orbital radius. Examples of data include statistical information, drawings and photographs, and models.]  [Assessment Boundary: Assessment does not include recalling facts about properties of the planets and other solar system bodies.]


5th Grade


PS2.B: Types of Interactions

ESS1.A: The Universe and its Stars

middle School


ESS1.B: Earth and the Solar System


Crosscutting Concepts



Cause and Effect

Scale, Proportion, and Quantity



Scale, Proportion, and Quantity

Systems and System Models