Scientists decided to create a new classification for celestial objects in 2006.
This classification was announced by the IAU (International Astronomical Union) and addressed the need to further identify planets in a subcategory known as dwarf planets.
| Name | Discovered | Diameter | Distance from the Sun (times Earths distance) | Orbit Period (Earth Years) |
|---|---|---|---|---|
| Ceres | 1801 | 950 km | 2.8 | 4.6 years |
| Pluto | 1930 | 2,372 km | 39.5 | 248.0 years |
| Haumea | 2003 | 1,960 km - 1,518 km × 996 km | 43.1 | 283.3 years |
| Makemake | 2005 | 1,434 km × 1,422 km | 45.3 | 309.9 years |
| Eris | 2003 | 2,326 km | 68 | 560.9 years |
According to the IAU, a dwarf planet is defined by the following characteristics:
- It is in orbit around the Sun
- It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- It has not cleared the neighborhood around its orbit; and
- It is not a satellite (a moon)
Another area that makes dwarf planets different is that regular planets have cleared the neighborhood around their orbits, whereas dwarf planets have not always done so.
Since the announcement, there have only been five celestial bodies defined as dwarf planets in our solar system: Ceres, Pluto, Haumea Makemake, and Eris.
Poor Pluto
At one time, Pluto was considered to be our 9th planet, and then it was downsized to a dwarf planet.
All of the dwarf planets except Ceres are located in the outer solar system near or in the Kuiper belt. Ceres is inside the asteroid belt that is between Jupiter and Mars’ orbit.
There are other objects that are waiting to become officially classified, and it’s believed that there may be around 10,000 dwarf planets in our solar system.
Dwarf planets are also smaller in size, and since they are so far out in our solar system, only some information has been gathered.
New Horizons spacecraft mission
The New Horizons spacecraft mission is the single mission that collected the most data, including excellent images of Pluto.
Most of the pictures that we have had were blurred, without any detail. New Horizons cruised by Pluto, its moons, and various objects in the Kuiper belt.
Eris Biggest Dwarf Planet
It was once thought that Pluto was the largest of the dwarf planets but thanks to the New Horizons mission, we have learned that the largest dwarf planet is Eris.
Eris is only a little bit bigger than Pluto at 1,445 mi in diameter. Eris takes around 561.4 Earth years to complete an orbit around the sun.
Smallest Dwarf Planet
The smallest of the dwarf planets include Makemake, Haumea, and Ceres. The orbital path of both Pluto and Eris actually cross each other.
One of the important things to note is that Pluto had been listed as the ninth planet in our solar system since 1930.
For some strange reason, people had an affection for this little planet and when it was demoted, there was a worldwide outcry.
Children, especially, wrote letters to astrophysicist Neil deGrasse Tyson, to register their complaints.
It does show that people had a connection with that little planet that was both wonderful and confusing.
Facts about Dwarf Planets
- Up until 1992, little information was known about the objects orbiting beyond Neptune.
- University of Hawaii astronomers discovered over 1,000 objects orbiting beyond Neptune and called them “TNOs” (trans-Neptunian Objects).
- Although they exist in varying sizes, TNOs seem to be smaller than Pluto.
- In 2003, Palomar Observatory in California astronomers found a new TNO that was more massive than Pluto and this led to the controversy as to whether Pluto or the new discovery were planets.
- In 2006, astronomers from all over the world gathered to debate the topic of planets and dwarf planets at the IAU’s General Assembly in Prague.
- By the end of the 2006 meeting, the IAU had agreed on a definition of a planet as well as a dwarf planet.
- A new classification of dwarf planet was created to show that it was distinctly different from the rest of the planets.
- The meeting in 2006 didn’t go completely well as many planetary scientists still objected to the qualities that defined a planet and dwarf planet.
- There are still scientists that hold onto their objections, but since time has passed, most of the scientific community has settled into the agreement of the IAU’s decision.
What are the 4 characteristics that define a dwarf planet?
- It is in orbit around the Sun
- It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- It has not cleared the neighborhood around its orbit; and
- It is not a satellite (a moon)
Q&A:
- Which celestial body was downgraded from a planet to a dwarf planet?
Pluto - How many dwarf planets do scientists think exist in our solar system?
10,000 - How many dwarf planets have been officially defined?
5 - What is the largest official dwarf planet?
Eris - What is another characteristic that is different between planets and dwarf planets?
planets have ‘cleared the neighborhood’ around their orbits
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