Curious Kids: what is a dwarf planet? | Kiowa County Press

Pluto, the largest of the dwarf planets. This image was taken by NASA’s New Horizons spacecraft. NASA/JHUAPL/SwRI

Vahe Peroomian, USC Dornsife College of Letters, Arts and Sciences

curious children is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected]


What is a dwarf planet? – Myranda, 8, Knoxville, TN


The word “planet” comes from the ancient Greek words meaning “wandering star”. This makes sense, because for thousands of years people have seen planets change position in the night sky – unlike stars, which appear fixed and motionless to the naked eye.

This is how the ancients discovered five of the planets: Mercury, Venus, March, Jupiter and Saturn. Discovery of astronomers using telescopes Uranus in 1781, Neptune in 1846, and Pluto in 1930.

Artist's impression of the dwarf planet Eris, a white and pale gray sphere.
Artist’s impression of the dwarf planet Eris. ESO/L.Calcada and Nick Risinger

Remnants of the solar system

I am a space scientist with a passion for astronomy and solar system exploration. I received my doctorate. in physics in 1994, around the time astronomers began to find more and more objects beyond Neptune, in the Kuiper Belt. It’s a place in space that contains the “remnants” of the solar system – specifically the small icy bodies.

Three of these icy bodies – Eris, haumea and makemake – were discovered in the early to mid-2000s. They seemed large enough to be planets; all are about the same size as Pluto.

Astronomers then surmised that there were likely many more of these icy bodies in the Kuiper Belt. They began to wonder: how many planets could we end up identifying in our solar system? Twenty? Thirty? Hundred? After?

Artist's rendering of the dwarf planet Haumea, an oval-shaped world surrounded by its ring.
Artist’s rendering of the dwarf planet Haumea, surrounded by its ring. Institute of Asrofisica of Andalucia

Dwarf planet set

In 2006, and after many debates, the International Astronomical Union came up with a new definition for a planet. And for the first time the term “dwarf planet” was used.

Here is what the IAU said: A planet must orbit directly around the Sun. It should also be large enough to have a round or spherical physical shape.

And the planet must”clean up your neighborhoodThis means that aside from any moons it might have, the planet cannot share its orbit with other objects of comparable size.

An object that satisfies only the first two criteria – but not the last – is now called a dwarf planet.

Artist's rendering of Makemake, a dwarf planet in the Kuiper belt.  Nearby is its moon, MK 2. In the distance: the Sun.
Artist’s rendering of Makemake, a dwarf planet in the Kuiper belt. Nearby is its moon, MK 2. In the distance: the Sun. NASA/ESA/A. Parker/Southwestern Research Institute

Pluto is retrograde

This is why Pluto lost its status as a planet and is now classified as a dwarf planet. It failed the last item on the checklist – other icy Kuiper Belt bodies lie in its orbital path. The decision, controversial for sure, is debated by scientists to this day.

At the same time Pluto was demoted, another solar system object was promoted. Ceres, once considered an asteroid, is now classified as a dwarf planet. This is far from the Kuiper Belt; instead, Ceres is in the main asteroid beltorbiting between Mars and Jupiter.

Add them up – Pluto, Ceres, Eris, Haumea and Makemake – and that brings the number of dwarf planets in our solar system to five. But this list is sure to grow. Already, hundreds of candidates, almost all in the Kuiper Belt, potentially meet the criteria to be a dwarf planet.

A photo of the dwarf planet Ceres.  To the human eye it appears a sandy brown color and is pockmarked with craters.
This photograph of Ceres, a dwarf planet in the Main Asteroid Belt, was taken by NASA’s Dawn spacecraft. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

About dwarf planets

Dwarf planets have nothing to do with Earth.

As their name suggests, they are much smaller. Pluto and Eris, the largest of the dwarfs, are less than a fifth the diameter of Earth.

They also have less mass. For example, the Earth has about 6,400 times more mass than Ceres. It’s like comparing two killer whales has a guinea pig.

And dwarf planets are cold. The average temperature of Pluto is approximately minus 400 degrees Fahrenheit (minus 240 Celsius).

A photograph of Pluto and one of its five moons, Charon.
A photograph of Pluto and one of its five moons, Charon. With the exception of Ceres, all dwarf planets have at least one moon. Charon is almost half the size of Pluto. NASA/Johns Hopkins University Applied Physics Laboratory/Southwestern Research Institute

Could life exist on a dwarf planet?

Three things are necessary for life: liquid water, a source of energy, and organic molecules, that is to say molecules containing carbon.

More than 100 miles (161 kilometers) below Pluto’s surface, a massive ocean of liquid water may exist; this could also be true for other Kuiper Belt worlds. Ceres also has underground waterremnants of what might have been an ancient world ocean.

Organic molecules, in abundance everywhere in our solar systemhave been found on Ceres and Pluto.

But the one missing ingredient for all dwarf planets is a power source.

Sunlight won’t work, especially for Kuiper Belt Dwarfs; they are simply too far from the Sun. To reach the belt, the light must travel over 2.7 billion miles (4.4 billion km). As the sun comes to these distant worlds, it is too weak to heat up significantly any of them.

And all the dwarf planets are too small to hold the internal heat left over from the formation of the solar system.

Yet scientists have discovered life on Earth in the most hostile places imaginable – near the bottom of the ocean, miles deep in the ground, and even inside an active volcano. When it comes to life in our solar system, never say never.

Circle the dwarf planet Ceres.


Hello, curious little ones! Do you have a question you would like an expert to answer? Have an adult send your question to [email protected] Please let us know your name, age and the city where you live.

And since curiosity has no age – adults, let us know your questions too. We cannot answer all questions, but we will do our best.

The conversation

Vahe Peroomianprofessor of physics and astronomy, USC Dornsife College of Letters, Arts and Sciences

This article is republished from The conversation under Creative Commons license. Read it original article.

About Florence M. Sorensen

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