The article was originally published at The Conversation.
Vahe Peroomian is a professor of physics and astronomy at USC.
Myranda was 8 years old when she asked "What is a dwarf planet?"
The ancient Greek words "wandering star" mean "planet", which makes sense because for thousands of years people have watched planets change position in the night sky.
The ancients discovered five planets: Mercury, Venus, Mars, and Jupiter. Astronomers used telescopes to find the planets in 1781, 1846 and 1930.
I am a space scientist with a passion for astronomy and the exploration of the solar system. I received my PhD in physics in 1994 at a time when astronomer began to find more and more objects beyond Neptune. There is a place in space that holds the leftovers of the solar system.
The bodies were discovered in the early to mid 2000s. All of them are roughly the same size as Pluto.
Astronomers thought there were more icy bodies in the belt. They wondered how many planets would we end up identifying in our solar system. Twenty? Thirty? A hundred? More?
The definition of a planet was changed in 2006 after much debate. The term "dwarf planet" was used for the first time.
The IAU said that a planet has to circle the sun. It must be large enough to have a spherical shape.
The planet cannot share its path with other objects of comparable size because it must clear its neighborhood.
A dwarf planet is an object that only meets the first two criteria, but not the last.
That is the reason why Pluto is now a dwarf planet. Other icy belt bodies are within its orbital path, and it failed the final item on the checklist. The decision is debated by scientists to this day.
At the same time that Pluto was demoted, another solar system object was promoted. The main asteroid belt is between Mars and Jupiter and it is not near the Kuiper belt.
There are five dwarf planets in our solar system. That list is going to grow. Hundreds of candidates, almost all of them in the Kuiper belt, have the potential to be a dwarf planet.
Earth is nothing like a dwarf planet.
They are much smaller than their name suggests. The dwarfs have less than 20% of the Earth's diameter.
They have less mass as well. Earth has about 6,400 times more mass than the other planet. It's like comparing two killer whales to a guinea pig.
The average temperature on dwarf planets is around minus 400 degrees.
Liquid water, an energy source and organic molecule are needed for life.
An enormous ocean of liquid water may exist more than 100 miles below the surface of the dwarf planet.
There are lots of organic molecules in our solar system.
A source of energy is missing from the dwarf planets.
The Kuiper belt dwarfs are too far away from the sun to benefit from sunlight. The light must travel more than four billion km to reach the belt. By the time the sunshine arrives at these distant worlds, it is too weak to heat them.
All the dwarf planets are too small to hold the heat from the solar system.
Near the bottom of the ocean, miles deep in the soil, and even inside an active volcano, scientists have discovered life. Don't say never when it comes to life in our solar system.
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