Dwarf Planets of the Solar System: Pluto, Eris, Haumea & Ceres

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  • 0:06 Dwarf Planet
  • 0:53 Definition of Dwarf Planet
  • 1:36 How Pluto Became a Dwarf
  • 2:20 Eris, Ceres, Haumea…
  • 4:15 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Discover why Pluto had to leave the league of planets and was downgraded to a dwarf planet. Learn the definitions for planet and dwarf planet. Find out about the currently classified dwarf planets in our solar system: Pluto, Eris, Ceres, Haumea and Makemake.

Dwarf Planet

Teacher: Today, class, we will be talking about dwarf planets and the history behind them. Anyone know what a dwarf planet is?

Student: Pluto!

Teacher: Yes, that's right, but you just named a dwarf planet, can you describe what one is?

Student: Uh...

Teacher: According to the International Astronomical Union, a dwarf planet is a celestial body in direct orbit of the sun that is massive enough to form an ellipsoid shape from its gravitational force but has not cleared its neighborhood from other objects. All those words are a fancy way of saying that a dwarf planet looks round, like a planet, but isn't big enough to clear the space around it. Let me show you.

Students: Ohhhh.

Teacher: Let's look at a short clip of how this all happened.

Definition of Dwarf Planet

The term dwarf planet is a new one. It was adopted in 2006 when the International Astronomical Union (IAU) defined the bodies that orbit the sun. At this time, they defined planets as bodies large enough to have cleared the neighborhood of their orbit. Any object that isn't big enough to be rounded by its own gravity they termed small solar system bodies. Everything in between is a dwarf planet.

A dwarf planet is not able to clear the objects in the area around it.
Dwarf Planet Cannot Clear Area

At this time, the IAU recognizes five dwarf planets in the solar system, but they suspect they will find at least 100 more in the near future. The dwarf planets they currently know of are Eris, Pluto, Ceres, Haumea and Makemake.

How Pluto Became a Dwarf

Astronomer 1: It's a planet!

Astronomer 2: No, it isn't!

Astronomer 1: Yes, it is!

Astronomer 2: No, it isn't!

Astronomer 1: Of course it is. Pluto has been classified as a planet since its discovery in 1930. No one alive today in 2006 is going to change that.

From its discovery in 1930 until 2006, Pluto was classified as a planet. Since the late 1970s, though, Pluto's planet status has been in question. The discovery in the '70s of a minor planet, called Chiron, in the outer solar system, the fact that Pluto is unlike our other planets and Pluto's relatively low mass caused astronomers to rethink things. It was the discovery of Eris in 2005 that ultimately got Pluto kicked out of the league of planets.

Eris, Ceres, Haumea and Makemake

Eris was discovered in January 2005 and appeared to be larger than Pluto, so it was initially described as the tenth planet. Confusion over this designation and ongoing debate over Pluto (was it really a planet?) led the IAU to define the term 'planet' for the first time. This led to Eris being termed a dwarf planet. It isn't currently known which one, Eris or Pluto, has the larger diameter, but Eris is the most massive. Eris is, at this time, the most massive body, other than the planets, known to orbit the sun. Although Eris was the final name for this object, the original name was Xena, named after the TV show heroine, Xena: Warrior Princess.

Eris is the most massive of the known dwarf planets.
Eris Largest Dwarf Planet

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