What are Planets? - Facts & Definition

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  • 0:00 What Is a Planet?
  • 1:17 Planet Formation
  • 2:08 Terrestrial Planets
  • 2:48 Gas Giants
  • 3:33 Pluto
  • 4:06 Lesson Summary
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Lesson Transcript
Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

In this lesson, you'll learn the official definition of a planet, how planets are formed, and about the two types of planets found in our solar system: terrestrial planets and gas giants.

What Is a Planet?

In 2006, the International Astronomical Union (IAU) defined a planet as having the following three essential criteria:

  1. A celestial object that orbits a star
  2. Is large enough that its gravity pulls it into the shape of a sphere
  3. Has cleared other objects in its orbital neighborhood

The word planet comes from the Greek word planetes, which means 'wanderer.' This is because the planets appear to wander amongst the fixed stars in the night sky as they orbit the sun. As of June 2013, approximately 900 planets have been identified in the universe. However, it's estimated that there are trillions of them out there. Scientists believe it's very likely that one of these trillions of planets is capable of supporting life.

In our solar system, there are eight official planets. Listed in order of proximity to the sun, they are:

  • Mercury
  • Venus
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune

A great way to remember the order is to use the acronym: My Very Eager Mother Just Served Us Nachos.

Planet Formation

Planets are formed when clouds of gas and dust collapse in a nebula during the formation of a star. As the gravity of the new-born star increases, the surrounding cloud begins to spin and flatten into a protoplanetary disk. The area of the disk which is close to the star contains mostly dust, and the area further from the star contains mostly gas and ice. Some of the contents of the protoplanetary disk will eventually begin to stick together like giant space tumbleweeds and form a pre-planet called a planetesimal. As the gravitational forces increase, it will become more spherically shaped. Finally, the planetesimal will accrue more and more debris until it clears the surrounding space and reaches official planetary status.

Terrestrial Planets

Terrestrial planets, also called rocky planets, are formed in the dusty inner space of the protoplanetary disk. As the bits of dust collide, they collect into a rocky mass of a planet. Our solar system has four terrestrial planets: Mercury, Venus, Earth, and Mars. They range in size from 4,879 kilometers to 12,784 kilometers in diameter and have varying atmospheric conditions. Their orbits are located at distances between 46 million kilometers and 205 million kilometers away from the sun.

Gas Giants

Gas giants are planets formed at the outer edges of the protoplanetary disk outside the zone of the frost line. Outside of this range, gas is able to condense during planetary formation, and as a result, giant planets with gaseous surfaces are formed. In our solar system there are four gas giants: Jupiter, Saturn, Uranus, and Neptune. They range in width from 49,244 kilometers to 139,822 kilometers. Their orbits are located at distances between 779 million kilometers to 4.5 billion kilometers away from the sun.

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