Dwarf planets are bodies that are similar to planets in that they orbit the sun, and they have enough gravity to maintain their spherical shape. Dwarf planets are small, and, unlike planets, they are not large or strong enough to clear debris out of their paths.
The inability to clear their own path is a defining characteristic of dwarf planets.
How Many Dwarf Planets are There in the Solar System?
There are currently five dwarf planets in the Solar System: these are Eris, Pluto, Haumea, Makemake, and Ceres. Currently, astronomers are working on officially classifying up to seven more dwarf planets in the Solar System.
Eris is the biggest dwarf planet, and it is even bigger than Pluto. Discovered in 2005, Eris is believed to have ice under its surface as well as large amounts of methane. It has one moon, named Dysnomia. It is around 2300 km in diameter.
Ceres is the smallest dwarf planet, but it was first considered a planet when it was found in 1801. It is about 950 km in diameter, and it makes up nearly 1/3 of the mass of the entire asteroid belt in which it lives.
Why did Pluto Change from Being a Planet to Dwarf Planet?
In 2006, the International Astronomical Union (IAU) agreed that they needed to give clear definitions to help astronomers categorize objects found in space. One of the definitions they decided to clarify was the difference between planets and other objects found.
This need for setting criteria came from the fact that astronomers found a celestial body past Pluto that was bigger than Pluto. Astronomers didn’t know whether to call this a planet or if they should call it something else. Many other objects found were similar to Pluto and this new body, so the IAU decided that they needed to set clear guidelines as to what a planet is or does.
When creating their definition, which you will find in the next question, the IAU found that their definition of a planet did not fit the definition or characteristics of Pluto. Therefore, Pluto, which was categorized as a planet in 1930, was suddenly placed in the group of dwarf planets.
When making a decision about Pluto, the IAU found that Pluto simply was not big enough or strong enough to be considered a planet in the Solar System. The decision was met with shock, concern, and sadness. While it was controversial at the time, the anger or excitement around the decision has calmed.
What is the Difference Between a Planet and Dwarf Planet?
Scientists categorize planets by these three characteristics:
- The celestial body has enough gravity to maintain its shape, which is usually spherical.
- The celestial body orbits the sun and remains on the same plane as the sun
- The celestial body can clear the path of dust and debris that is in its path as it moves.
If a celestial body fits the three descriptors above, then the body is called a planet. Dwarf planets are the bodies that fit the first two numbers (meaning they have gravity to keep their shape and they orbit the sun), but they do not fit the third number.
Dwarf planets are different than planets because they simply are not big enough or strong enough to clear the dust and debris they meet as they move around the Sun. Many scientists also say those dwarf planets must not be satellites, meaning they do not orbit a larger, planetary body.
Other than this, dwarf planets share most of the same characteristics as planets in the Solar System.
What is the Size of a Dwarf Planet?
There is no set size standard for dwarf planets. The IAU suggests that objects 800 km in diameter are most more likely to sustain their shape, which remember is a feature of a planet. Anything below 800 km may not be able to keep its shape to be considered a dwarf planet.
The smallest dwarf planet is currently 950 km in diameter. Pluto is 2,377 km in diameter. The largest dwarf planet is 2, 326 km in diameter.
Rather than size, the characteristics listed above by the IAU are the most important aspects of defining a dwarf planet.
Interesting Facts About Dwarf Planets
- Aside from Pluto, the currently classified dwarf planets are found in the Kuiper Belt.
- There could be nearly 200 possible dwarf planets in the Kuiper Belt.
- Ceres is the dwarf planet closes to the Sun.
- Sedna is the dwarf planet furthest from the Sun.
- Pluto is 2/3 the size of our own moon.
- It takes dwarf planet Sedna 11,000 years to fully orbit around the Sun.
- The dwarf planet Haumea is named after a goddess in Hawaiian culture.
- Haumea has a shape that makes it look squashed as if someone sat on it.
- We do not know what Haumea is made of.
- Pluto has three moons, with the largest being named Charon.
- Ceres is the largest boy in the asteroid belt.
- 7 celestial bodies are currently waiting to be classified as dwarf planets.
- The IAU has not given specific sizes to help define dwarf planets.