The Structure of the Solar System

What is a Planet?

In 2006, the International Astronomical Union defined a planet as an object that meets 3 criteria:

1. It orbits a star.

2. It has enough mass (and enough gravity) to be spherical.

3. It has cleared the areas of its orbit of planet-sized objects.

All 8 planets meet these criteria!

Multiple Select

What are the criteria that must be met for an object to be considered a planet? Select ALL answers that apply.

• Has moon(s)

• Orbits a star

• Is spherical in shape due to its size and gravity

• Has an oblong orbit

• Has an orbital path cleared of similar sized objects

Multiple Choice

Examine Pluto in this image. Why is Pluto no longer considered a planet?

• It doesn't orbit the Sun

• It isn't a sphere

• It has debris and objects in its orbital path

• It's too far from the Sun

Order within the Solar System

The bodies of the Solar System are ordered and arranged due to the way the Solar System formed (according to the Solar Nebula Theory, condensation, and accretion).

Arrangement of Planets in the Solar System

• Denser, rocky materials settled near the Sun at the center of the Solar System. Thus, the inner "terrestrial" planets are solid and rocky

• Less dense materials (liquid, ice, and gas) were pushed outwards, away from the young Sun. Thus, the outer "jovian" planets are like "gas giants".

Inner/Terrestrial Planets

• Closer to the Sun ("inner" Solar System)

• Smaller, rocky and solid

• 0 to 2 moons per planet (3 total)

• No rings

• Warmed by solar radiation

• Mercury, Venus, Earth, and Mars

Outer/Jovian Planets

• Farther from Sun ("outer" Solar System)

• Larger and mostly gas and ice

• Many moons

• All have rings

• Cooler due to distance from the Sun

• Jupiter, Saturn, Uranus, and Neptune

• Note Earth in picture for scale!

Asteroid/Comets

​Asteroids

• ​Millions of small, rocky objects called asteroids orbit the Sun in the asteroid belt between the orbits of Mars and Jupiter.

• Asteroids are not spherical​

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Comets ​

• A comet is made of gas, dust, and ice and moves around the Sun in an oval-shaped orbit​.

• Comets come from outer parts of the solar system​

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Astronomical Unit (AU)

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• Objects in the solar system are so far apart that astronomers use a larger distance unit.

• AU is the average distance from Earth to the sun – about 150 million km (93 million miles)

Multiple Choice

True or False: The Solar System's structure has nothing to do with its formation.

• True

• False

Multiple Select

Terrestrial planets have which characteristics? Select ALL that apply.

• Rocky

• Larger

• No rings

• Few moons

• Cooler

Multiple Select

Jovian planets have which characteristics? Select ALL that apply.

• Made mostly of gas/ice

• Farther from the Sun

• Warmer

• Smaller

• Have more moons

Multiple Select

What is true of all planets? Select ALL that apply.

• They have gravity

• They orbit the Sun

• They are round

Multiple Select

True or False: Objects in the solar system are so far apart that astronomers use meters to measure their distance.

• False

• True