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Dwarf Planet – Definition and Facts

Jan 20, 2023
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Dwarf Planets

The universe comprises many unique things that might not always fit into specific categories, like planets, stars, and galaxies. Dwarf planets in our solar system are one such entity that has unique characteristics making them harder to classify traditionally. 

What are Dwarf Planets?

Dwarf planets in our solar system are celestial entities that are roughly spherical, orbit a star, and often have other bodies surrounding them like comets, asteroids, or other dwarf planets.

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Dwarf Planets in our Solar System

Wondering how many dwarf planets are there in the solar system? The authority responsible for naming and classifying celestial objects is the International Astronomical Union. It officially identifies five dwarf planets in our solar system, and those include Pluto, Ceres, Eris, Makemake, and Haumea. Several more celestial bodies are being considered to be classified into this category, and it is estimated that thousands of dwarf planets may be present in the solar system.

1. Pluto

Pluto was discovered in 1930 when it was named the ninth planet in the solar system. However, it was recognized as a full-fledged planet in the 1990s. In 2006, Pluto has officially renamed a dwarf planet.

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Apart from Pluto being a popular dwarf planet, it is the biggest dwarf planet in size and the second largest in terms of mass. It consists of five moons, the largest of which is the Charon. Pluto does not have a circular orbit like other planets do, and its orbit crosses that of Neptune. This means that sometimes Pluto is closer to the Sun than Neptune is. Nearly 250 years are taken by Pluto to complete one orbit around the Sun. 

Very little was known about Pluto before the New Horizons mission by NASA. The spacecraft was launched in 2006 and took around nine years to complete its target. The mission proved that the surface of Pluto has plains and mountains composed of water ice and nitrogen ice. 

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2. Eris

Eris lies outside the orbital space of Neptune, and it completes its one orbit circling the Sun in 556 years. It is a little smaller compared to Pluto but has around 25% more matter. Because Eris is very far away, its surface details cannot be seen with the instruments available in current times. However, the presence of methane ice on its surface has been detected by astronomers, and its surface is believed to be very similar to that of Pluto. 

3. Ceres

Ceres is the largest celestial body in the asteroid belt and lies between the orbits of Jupiter and Mars. It alone has about one-third of all the matter present in the belt. Its shape is near-spherical, which means that it is not considered to be an asteroid. 

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Most dwarf planets revolve around the Sun at the outer edges of the solar system, but Ceres is the only one that lies inside the orbit of Neptune. It takes 4.6 years for Ceres to complete one orbit around the Sun. Scientists suspected this unique dwarf planet might even possess an ocean of liquid water masked by a layer of ice. 

The Dawn spacecraft of NASA was launched in 2006 and went into orbit around Ceres after observing the nearby asteroid Vesta for the first time. This spacecraft allowed astronomers to visualize the detailed images of Ceres for the first time and get more information regarding its composition and surface. 

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4. Makemake

This dwarf planet was discovered in 2005, a few months post the discovery of Eris. It was discovered by the same team of astronomers who discovered Eris. It lies in the Kuiper Belt, a ring of icy debris outside Neptune’s orbit. It is around 30-50 times more distant from the Sun than the Earth. Astronomers believe that, like Pluto, Makemake is reddish. In 2015, a moon named MK2 was found to be orbiting around Makemake. It takes around 300 years for Makemake to complete one orbit around the Sun.

5. Haumea

Discovered in 2004, Haumea is located in the Kuiper Belt outside Neptune’s orbit. Although it takes Haymea 285 Earth years to complete one orbit around the Sun, it completes a spin on its axis in less than four hours. It is believed by astronomers that because of this fast rotation, Haumea has been deformed into an egg shape. Haumea has two moons— Namaka and Hi’iaka. It is believed to be the only object in the Kuiper Belt that has its ring.

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Dwarf Planets as ‘Plutoids’

How many dwarf planets are there that are considered plutoids? The dwarf planets Eris, Pluto, Makemake, and Haumea, are called plutoids as they have an orbit outside Neptune. Plutoids are also called ice dwarfs as they are small in size and have cold temperatures on their surfaces. There is evidence of the outer planets showing interaction with plutoids. The largest moon of Neptune, called Triton, is considered a captured plutoid. Even the odd tilt of Uranus on its axis is believed to be because of its collision with a plutoid. Like dwarf planets, hundreds of plutoids exist in the solar system yet to receive official status.

The Planets

A true planet is defined by the IAU as a celestial body circling the Sun without being a satellite of some other object. It is large enough to be surrounded by its gravitational force but not so big as a star to be able to undergo nuclear fusion. This restrictive definition aided in determining what can and can not be considered a planet. As astronomers discovered more and more planet-like celestial bodies in the universe, problems started arising. Pluto, which was earlier classified as a planet, was reclassified as a dwarf planet as it didn’t fulfil the definition. 

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There are nine planets in the solar system, namely Venus, Mars, Saturn, Mercury, Earth, Jupiter, Uranus, and Neptune. The solar system extends from what is called Sol by the ancient Romans and the Sun in modern times. It goes past the inner four planets to the four gas giants through the asteroid belt onto the Kuiper Belt and far beyond to the heliopause, shaped like a teardrop.

It is estimated by scientists that the solar system’s edge is around 15 billion kilometers away from the Sun. The giant, spherical Oort Cloud is located beyond the heliopause, which is believed to surround the solar system. Since Pluto was discovered in 1930, kids used to memorize that there are nine planets in the solar system. This changed in the 1990s when arguments and debates started about whether Pluto was fit to be called a planet. In 2006, the International Astronomical Union, in a highly controversial decision, finally decided to demote Pluto to the status of a dwarf planet. This reduced the number of planets in the solar system to eight.

Astronomers are still on the hunt for another potential planet in the solar system. The evidence of a ninth planet was revealed on 20th Jan 2016. This ninth planet is also known as Planet X and is believed to be around ten times as massive as Earth and five thousand times as massive as Pluto.

Venus, Earth, Mercury, and Mars, the four planets that lie closest to the Sun, are sometimes called terrestrial planets. This is because of their rocky surfaces. The surface of Pluto is also rocky, but it has not been grouped with the four terrestrials.

Saturn, Jupiter, Neptune, and Uranus, the four large outer worlds, are sometimes known as the Jovian or Jupiter-like planets. This is because they are enormous compared to terrestrial planets. They also mostly have gasses like helium, ammonia, and hydrogen rather than rocky surfaces. Astronomers, however, debate whether some or all of them have solid cores.

Conclusion

As the name indicates, the main difference between a planet and a dwarf planet is size. Dwarf planets are smaller, and because of that, they lack the gravitational forces required to attract and build up all the materials in their orbits toward them. In our solar system, each dwarf planet is smaller than the Moon of the Earth.

Frequently Asked Questions 

1. How was the discovery of Eris important?

The discovery of Eris in 2005 is believed to be the turning point that compelled astronomers to reconsider the classification of Pluto as a planet and determine how many dwarf planets there are. The name ‘Eris’ is based on a Greek goddess of discord. 

2. What are gas giants and gas ice?

Saturn and Jupiter are sometimes known as gas giants, while Neptune and Uranus are sometimes called ice giants. This is because Neptune and Uranus have more water and ice-forming substances in their atmospheres.

3. Why was Pluto demoted to a dwarf planet?

Apart from its small size and offbeat orbit, the problem with Pluto is that it does not clear its surrounding debris and shares space with several other objects in the Kuiper Belt. The demotion of Pluto is still a controversial topic.

Dwarf Planets

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