Eight Planets

What do you mean Pluto isn’t a planet any more? How could that be? Let’s find out what happened.


Planets

Long, long ago, some people spent more time looking at the night sky than most people do today. Some were out all night to protect their sheep and other animals from wolves and other dangers. Some were on watch at night as their boats sailed on the sea. They noticed the slow motion of the patterns of stars through the night and through the year. They also noticed that some stars seemed to wander from one star pattern to another. 

The Greeks called these odd stars planetai, which meant wandering stars. Today we call them planets. There are five of them that can be seen without a telescope or binoculars. Eventually people figured out that they all went around the sun, and that earth, our home, was doing the same thing.

As telescopes were invented and improved, more planets were discovered until there were eight in all. It was also discovered that many of them had moons that orbited with them, just as our moon travels through space with us. 

Other Objects

Telescopes showed that other objects orbit our sun. There are comets that go deep into space away from the sun and then circle back again. Some of them take many years to do one orbit. Telescopes helped discover pieces of rock that also go around the sun. These are in orbit between the planets Mars and Jupiter. They are called asteroids. 

Pluto

In 1930, a scientist discovered a new planet, out beyond Neptune. He named it Pluto. Eventually it was found that Pluto has several moons. One of them, named Charon, is half as big as Pluto.

Discovery

Things seemed pretty settled then until 2005 when scientists discovered another object out beyond even Pluto and its moons. They found this new object when they looked at some telescope photographs taken in 2003. They discovered the new object was bigger than Pluto. They named the new object Eris.

So, did that mean there are ten planets? What about the fact that seven moons are bigger than Pluto? How big does something have to be to be called a planet? What was the definition of a planet anyhow?

Planet Defined

Lots of things were written about this. Lots of discussions went on. Finally scientists decided that a planet had to have several properties:

  • It had to orbit a star. In our Solar System that would be the sun.
  • It had to be big enough that its gravity could shape it into the shape of a ball or sphere.
  • It had to be big enough to clear its orbit, pulling nearby things into part of itself or into orbiting it as a moon.

Dwarf Planets

There were a couple of objects that did not have all of these properties. These objects were too small to clear their orbits of other objects. Scientists decided these would be put into a new group called dwarf planets. The group would include Pluto, the newly discovered Eris, and Ceres, the largest of the asteroids.

So here we are for now. Our Solar System has eight planets and three dwarf planets, at least until scientists make more new discoveries.

To learn more:

You can download a free computer program called Stellarium. It lets you see what star patterns and planets can be seen on the next clear night. Make sure you have permission from the owner of the computer you are using before getting this software.

ESP has a page that helps you set up scale models of the Solar System. 

To see a drawing that compares the size of the different objects in our solar system, click here.

Starchild has lots of interesting things to read about the Solar System.

Exploring Planets in the Classroom has activities to help you learn about the planets.

Our Astronomy references page and our Past Activities of the Month pages have even more links to help you learn about astronomy and the night sky.

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