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Curious kids: What is a dwarf planet?

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

This article was originally published at The Conversation. (opens in new tab) The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Vahe Peroomian (opens in new tab), Professor of Physics and Astronomy, USC Dornsife College of Letters, Arts and Sciences

"What is a dwarf planet?"Myranda, age 8, Knoxville, Tennessee

The word "planet" came from the ancient Greek words that mean "wandering star." That makes sense, because for thousands of years, people have watched planets change position in the night sky — unlike stars, which appear fixed and unmoving to the naked eye.

That's how the ancients discovered five of the planets: MercuryVenusMarsJupiter and Saturn. Astronomers using telescopes found Uranus in 1781, Neptune in 1846, and Pluto in 1930.

Solar system leftovers

I'm a space scientist (opens in new tab) with a passion for astronomy and the exploration of the solar system. I received my Ph.D. in physics in 1994, about the time astronomers began to find more and more objects beyond Neptune, in the Kuiper belt. That's a place in space that holds the "leftovers" of the solar system — particularly small icy bodies.

Three of those icy bodies — ErisHaumea and Makemake — were discovered in the early to mid 2000s. They seemed large enough to be planets; all of them are roughly 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 might we end up identifying in our solar system? Twenty? Thirty? A hundred? More?

Artist’s impression of the dwarf planet Eris. (Image credit: ESO/L.Calçada and Nick Risinger)

Dwarf planet defined

In 2006, and after much debate, the International Astronomical Union (opens in new tab) came up with a new definition for a planet. And for the first time, the term "dwarf planet" was used.

Here's what the IAU said: A planet has to orbit the sun directly. It also must be large enough to have a round, or spherical, physical shape.

And the planet must "clear its neighborhood." That means, aside from any moons it might have, the planet can't 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.

An artist’s illustration of the dwarf planet Haumea, surrounded by its ring. (Image credit: Instituto de Asrofísica de Andalucía)

Pluto is demoted

That's why Pluto lost its status as a planet (opens in new tab) and is now classified as a dwarf planet. It failed the final item on the checklist — other icy Kuiper belt bodies are within its orbital path. The decision, a controversial one to be sure, is debated by scientists to this very day (opens in new tab).

At the same time Pluto got demoted, another solar system object was promoted. Ceres (opens in new tab), once considered an asteroid, is now classified as a dwarf planet. It's nowhere near the Kuiper belt; instead, Ceres is in the main asteroid belt (opens in new tab), orbiting 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 that list is sure to grow. Already, hundreds of candidates, nearly all in the Kuiper belt, potentially satisfy the criteria to be a dwarf planet.

Artist’s illustration of Makemake, a dwarf planet in the Kuiper belt. Nearby is its moon, MK 2. Off in the distance: the sun. (Image credit: NASA/ESA/A. Parker/Southwest Research Institute)

About the dwarf planets

Dwarf planets are nothing like Earth.

As their name implies, they are much smaller. Pluto and Eris, the largest of the dwarfs, have less than one-fifth the diameter of the Earth.

They have less mass, too (opens in new tab). For example, Earth has about 6,400 times more mass (opens in new tab) than Ceres. That’s like comparing two killer whales (opens in new tab) to a guinea pig (opens in new tab).

And dwarf planets are cold. Pluto's average temperature (opens in new tab) is around minus 400 degrees Fahrenheit (minus 240 Celsius).

This photograph of Ceres, a dwarf planet in the main asteroid belt, was taken by NASA’s Dawn spacecraft. (Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Could life exist on a dwarf planet?

Three things are needed for life: liquid water, an energy source and organic molecules — that is, molecules containing carbon.

More than 100 miles (161 kilometers) below Pluto's surface, an enormous ocean of liquid water may exist; this might also be true for other Kuiper belt worlds. Ceres also has subsurface water (opens in new tab), remnants of what might have been an ancient global ocean.

Organic molecules, in abundance everywhere in our solar system (opens in new tab), have been found on Ceres and Pluto.

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

A photograph of Pluto and one of its five moons, Charon. Except for Ceres, all the dwarf planets have at least one moon. Charon is nearly half Pluto’s size. (Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

Sunlight won't work, particularly for the Kuiper belt dwarfs; they are simply too far away from the sun. To reach the belt, the light must travel more than 2.7 billion miles (opens in new tab) (4.4 billion km). By the time the sunshine arrives at these distant worlds, it's too weak to significantly heat (opens in new tab) any of them.

And all the dwarf planets are too small to hold the inner heat that remains from the solar system's formation (opens in new tab).

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

This article is republished from The Conversation (opens in new tab) under a Creative Commons license. Read the original article (opens in new tab).

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Vahe Peroomian

Vahe Peroomian has been teaching physics and astronomy for over 20 years, with a teaching philosophy centered on the success of not just a select few students in each class, but each and every student that is enrolled in the courses he teaches.

Vahé has 25 years of research experience in space science (specializing in geomagnetic storms and their societal impacts) with funding from NASA and the National Science Foundation.

In addition to his passion for astronomy, Vahe is an avid photographer and an author on the Serway and Jewett series of physics textbooks.

From 2005 to 2020 Vahe served as a member of the Board of Trustees of Glendale Community College (GCC) in Glendale, California, a city-wise elected position to which he was reelected three times.