Dwarf Planets


What do you do when you find something in space that kind of looks and acts like a planet, but doesn’t have all of the qualifications to be a planet?  As we explore further into our solar system, we are discovering that there are all kinds of objects in space that are similar, but not the same.

The International Astronomical Union (IAU) has set up classifications for various celestial objects and anything that they found that was “almost” a planet, but not quite, is called a “dwarf planet.”

Most people hadn’t heard that much about dwarf planets until Pluto was downsized from a planet to a dwarf planet. For many years, Pluto was the ninth planet in our solar system and there was a lot of public outcry (especially from kids) when they changed Pluto’s classification.

Dwarf Planets

Dwarf planets are worlds that are too large to be considered asteroids, too small to be planets, and don’t have all of the qualifications of a planet. A planet must be a celestial body that:

  • Is in orbit around the sun
  • Has sufficient mass for self-gravity to overcome rigid body forces so that it assumes a nearly spherical/round shape
  • Has cleared the neighborhood around its orbit
  • Isn’t a satellite

A dwarf planet is usually missing at least one of these characteristics, most often that it hasn’t cleared the neighborhood around its orbit.

The controversy about dwarf planets became a topic that more people were aware of when the New Horizons spacecraft flew by Pluto in 2015.

Scientists realized that Pluto probably originated in the Kuiper Belt and although it did qualify for most of the definition of a planet, it hadn’t “cleared the neighborhood around its orbit.”

Pluto also didn’t really qualify as just a “KBO” (Kuiper Belt Object) like so many of the asteroids.

The problem with trying to research and define the dwarf planets is that they are so far away that little information had been gathered about them.

There are scientists that are examining some of the dwarf planets and discovering that they are so close to planets, including possible past geological activity, that other classifications should be created beyond the words “dwarf planets.”

So far, astronomers have identified five dwarf planets in our solar system: Ceres, Pluto, Haumea, Makemake, and Eris.

Dwarf Planet Statistics:


Discovered By: Giuseppe Piazzi
Date Discovered: January 1, 1801
Diameter: 950 km
Mass: 8.96 × 10^20 kg (0.01 Moons
Orbit Distance: 413,700,000 km (2.8 AU)
Orbital Period: 1,680 days (4.6 years)
Surface Temperature: -105 degrees C
Moons: None


Discovered By: Clyde W. Tombaugh
Date Discovered: February 18, 1930
Diameter: 2,372 km
Mass: 1.31 × 10^22 kg (0.17 Moons)
Orbit Distance: 5,874,000,000 km (39.26 AU)
Orbital Period: 248 years
Surface Temperature: -229 degrees C
Moons: 5


Discovered By: Disputed: José Luis Ortiz Moreno & team or Mike Brown & team
Date Discovered: December 28, 2004
Diameter/Equitorial: 1,960-1,518 km
Diameter/Polar: 996 km
Mass: 4.01 × 10^21 kg (0.05 Moons)
Orbit Distance: 6,452,000,000 km (43.14 AU)
Orbital Period: 283.3 years
Surface Temperature: -241 degrees C
Moons: 2 (Hi’iaka and Namaka)


Discovered By: Michael E. Brown, Chad Trujillo, David Rabinowitz
Date Discovered: March 31, 2005
Diameter/Equitorial: 1,434 km
Diameter/Polar: 1,422 km
Mass: 2-5 × 10^21 kg (0.04 Moons)
Orbit Distance: 6,850,000,000 km (45.79 AU)
Orbital Period: 309.9 years
Surface Temperature: -239 degrees C
Moons: 1 (MK 2 – S/2015 (136472) 1)


Discovered By: M.E. Brown C.A. Trujillo & D.L. Rabinowitz
Date Discovered: January 5, 2005
Diameter: 2,326 km
Mass: 1.66 × 10^22 kg (0.23 Moons)
Orbit Distance: 10,120,000,000 km (68.01 AU)
Orbital Period: 560.9 years
Surface Temperature: -231 degrees C
Moons: 1

Pluto is the largest dwarf planet in our solar system and they follow in size order with: Eris, Makemake, Haumea, and Ceres. Eris is the farthest away from the sun and Ceres is the closest to the sun.

While some dwarf planets have almost all of the characteristics of a planet, including having one or more moons, none discovered so far have rings.

The important thing to know about a planet is that it has strong enough gravity to either push away or attract smaller objects or bodies that enter its orbit, and a dwarf planet doesn’t have a strong enough gravity to accomplish a complete clearing of it orbital “neighborhood.”


The term “dwarf planet” was accepted in 2006 when researchers and spacecraft discovered that there were objects outside of Neptune’s orbit that looked and acted almost like planets, only smaller.

This had a great effect on Pluto that was once considered to be the ninth planet in our solar system. At that time, the IAU had to rethink the requirements to define a planet and little Pluto just didn’t measure up.

In 2014, the scientists at the IAU recognized five dwarf planets with the possibility of adding six more in the coming years. They believe that there is a potential for having as many as two hundred dwarf planets that exist in the Kuiper Belt region.

As the debate about dwarf planets continues, they may consider both Quaoar and Sedna, who are both located beyond the orbit of Pluto, as well as 2012 VP113, which is an object that is one of the most distant at our solar system’s edge. In 2017, NASA scientists also think that an additional dwarf planet of DeeDee could be recognized.

Scientists in the dwarf-planet debate continue to protest against the single requirement of “clearing its neighborhood” as a way to define the difference between and planet and a dwarf planet.

Of the scientists and astronomers that were in attendance for the vote, around 400 out of the 9,000 consider having such a fine line of “clearing the neighborhood of orbit” as ridiculous, and they feel very strongly about getting Pluto added back in as the ninth planet.

This argument may continue on for decades, but those that are objecting to the dwarf planet classification are a very small percentage.

NASA announced that it will comply with the IAU guidelines in defining both a planet and a dwarf planet.


Dwarf planets that have been discovered so far seem to originate in the outer areas of the solar system such as the Kuiper Belt. Objects in this region are considered to be leftovers from the time when our solar system was first created.

Surface and Structure:

One of the characteristics that planets and dwarf planets require is that they have a sufficient mass that will allow its own gravity to be a force that is dominant enough.

The mass creates internal pressure causes its surface to become more flexible so that it fills in the hollows and elevations can sink in so that they can be spherical or rounded.

This doesn’t happen with smaller bodies that are smaller than a few kilometers such as comets and asteroids. Those objects are affected by forces outside of their own gravity and they have a tendency to be oddly shaped.

There are some objects that are larger than a few kilometers across and do have a significant gravity but not a dominant one. In this case, these bodies are more potato-shaped or spheroid.

Pluto and its moon Charon

The larger the object is, the higher the internal pressure will be until the pressure is strong enough to dominate the strength of internal compression and it achieved “hydrostatic equilibrium.” This is the ultimate point where the object becomes as round as it can be based on the tidal effects and rotation.

It’s important to remember that a dwarf planet can be affected by its rotation. If the object doesn’t rotate, it will become a sphere. However, if it does rotate, the faster it goes, the more it will be flattened at its poles or even closer to a triangle.

This is called being “oblate” or “scalene.” One of the dwarf planets is an extreme example of the rotation affecting its shape. Haumea is twice as long on its major axis as it is at both of its poles.

Tidal forces can also slow the rotation and it becomes tidally locked so that only one side always faces its companion, such as the Pluto-Charon partnership where both objects are tidally locked to face each other.

The IAU hasn’t established a lower or upper mass limit for a dwarf planet. Although the lower limit is scientifically defined by the accomplishment of hydrostatic equilibrium shape, the mass or size that the object gets to for this accomplishment will depend upon its thermal history and composition.

Objects that are made up of rocky compositions, such as asteroids have a larger diameter and mass requirement.

However, those that are made up of less rigid compositions, such as water ice are more flexible and have a smaller diameter and mass requirement.

This is why there is no set standard for defining the mass or size of a dwarf planet, and instead, they use the shape as the definition.

4 Vesta, one of the largest asteroids

Atmosphere and Magnetosphere:

So far, there have been no dwarf planets that have been found to have an atmosphere.

Not enough information is known about the defined dwarf planets to say that they have a magnetosphere.

Rotation and Orbit:

Besides achieving hydrostatic equilibrium, astronomers have indicated that planets need the ability to “clear the neighborhood around their orbits.”

What this means is that planets must have the ability to get rid of smaller objects within their orbit by capturing them, collision, or creating an orbit that avoids collision.

Dwarf planets don’t have a large enough mass to be able to accomplish this.

To figure out whether a planet is powerful enough to clear its neighborhood, planetary scientists have developed a parameter calculation that can be applied so that an object can be defined as a planet or a dwarf planet.

Could Life Exist?

No dwarf planet has been discovered that has all of the requirements to harbor life as we know it.

Interesting Information:

  • When the IAU downgraded Pluto from our solar system’s ninth planet to dwarf planet status, there was outrage all over the world. However, the IAU stood its ground until 2003, when astronomers at the California Palomar Observatory found an object that was more massive than Pluto and it had a satellite. It was given the name 2003 UB313 and the discovery was accompanied the 1992 discovery that there were thousands of objects orbiting beyond Neptune, called trans-Neptunian Objects (TNOs) that were smaller than Pluto. 2003 UP313 made scientists question whether it should be considered a planet as well as they now knew there were a lot more objects of various sizes past Pluto.
  • The question about planet versus dwarf planet led the IAU to create a committee to get the opinions of all of the professionals and scientists which later led to drafting a resolution about a planet’s definition that is called Resolution B5. It created the new classification of celestial objects that was distinct from planets.
  • After the IAU resolution was approved, 2003 B313 was given the official name of Eris.
  • Scientists that were against the IAU resolution said that they thought the dwarf planets should be given a category of “true planets” in the same way that we separate gas giant planets from terrestrial planets.
  • Scientists such as Alan Stem objected to the dwarf planet requirement of clearing the neighborhood of objects because Pluto, Earth, Mars, Jupiter, and Neptune haven’t completely cleared their orbital zones. Earth has 10,000 near-Earth asteroids and Jupiter has 100,000 Trojan asteroids in its orbital path. The argument from other astronomers is that the major planets have control over the other objects in their orbital zone.
  • Scientists continue to argue the “planet versus dwarf planet” idea and defend against the dwarf planet concept by saying that there are only indirect techniques to identify whether the objects have “cleared out their orbit.” To answer this objection, the IAU added a separate “working definition” that “The minimum mass/size required for an extrasolar object to be considered a planet should be the same as the used in the solar system.”


So far, in 2015, the Dawn spacecraft has orbited the dwarf planet Ceres and the New Horizons spacecraft has visited Pluto.

Facts about Dwarf Planet for Kids:

  • There are other potential dwarf planets that are being considered including Orcus, 2002 MS, Salacia, Quaoar, 2007 OR, and Sedna. Except for Sedna, these are all objects located in either the Kuiper Belt or the Scattered Disc.
  • Scientists believe that there is a possibility that there may be another 40 known solar system objects that could be classified as dwarf planets. They have estimated that up to 200 dwarf planets may be found in the Kuiper Belt and those outside the Kuiper Belt may be as high as 10,000 objects.


  • Pluto

    Of all of the planets in our solar system that have been the topic of controversy and debate, Pluto is at the top of the list.

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  • Ceres

    Of all of the dwarf planets that have been identified and defined, Ceres is the closest dwarf planet to the sun.

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  • Haumea

    This dwarf planet was originally given the identification of 2003 EL61 and astronomers nicknamed it “Santa.”

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  • Makemake

    A majority of people weren’t aware that dwarf planets existed until the former ninth planet, Pluto, was downgraded to a dwarf planet.

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  • Eris

    The discovery of Eris made the IAU (International Astronomical Union) make a decision about the definition of a planet and led to the eventual demotion of Pluto from being a ninth planet to joining Eris as a dwarf planet.

    Read More