Pluto IS a Dwarf Planet!

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A Tiny World Comes into View

A view of Pluto from New Horizons
The New Horizons spacecraft on its way to Pluto took this image of the dwarf planet. It shows what looks like a polar ice cap. NASA

Meet Pluto's Polar Ice Cap!  

The dwarf planet Pluto is coming into sharper focus as the New Horizons mission sweeps closer on its way to the outermost reaches of the solar system. This image was taken in mid-April, 2015, from a distance of just under 111 million kilometers (64 million miles). There are clearly bright and dark areas on the planet (called "albedo markings"), and scientists think the bright region on the lower left part of the planet is a polar ice cap. 

Pluto is 70 percent rock with an icy surface containing frozen nitrogen, carbon dioxide, and methane. The bright regions may be a "snow" that fell to the surface of this tiny world. 

02
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A Quick Peek at Pluto

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An artist concept of what Pluto's surface might look like. The Sun is in the distance. L.Calcada and ESO

Because of its great distance from the Sun, Pluto has been extremely difficult to observe. Hubble Space Telescope revealed dark and light patches on the surface, leading astronomers to suspect that the surface experiences some kind of change. They also know that Pluto has a very thin atmosphere which thickens up when it's closest to the Sun during the 247.6-year orbit. Pluto spins on its axis once every 6.4 Earth days, and is one of the coldest worlds in the solar system. 

No spacecraft has been sent to Pluto; that changed when the New Horizons mission was launched on a multi-year trajectory out to the outer solar system. Its tasks: to study Pluto and its moons, study the environment Pluto moves through, and then move out to explore one or two other Kuiper Belt objects. (The Kuiper Belt is the region of space where Pluto orbits.) 

03
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Happy Discovery Day to Pluto!

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The photographic plates used by Clyde Tombaugh to dicover Pluto. Lowell Observatory

Pluto is the only planet discovered by an American, and its finding took the world by storm. It happened in 1930, when young astronomer Clyde Tombaugh began observations at Lowell Observatory in Flagstaff, Arizona. Tombaugh's job was to take plates of the sky and look for what was (85 years ago) nicknamed "Planet X", which astronomers thought might exist "out there" somewhere. Tombaugh's nightly plates were examined carefully for any hint of a planet.

On February 18, 1930, the work paid off. Tombaugh spotted a small object that seemed to jump in position between two plates. It turned out NOT to be the mysterious Planet X, but it was labeled a planet and eventually named Pluto by a young woman named Venetia Phair.

04
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Pluto: Planet or Not?

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An artist's conception of what Pluto might be like as New Horizons swings by. SWRI

With the discovery of other worlds larger than Pluto, astronomers debated the question "what is a planet?" This led them to question their definition of the word "planet". It comes from the Greek word planetes, which means "wanderers", which the planets seemed to do as they appeared to move across our sky. Later on, astronomers put more scientific meaning into the definition, requiring that a planet have its own orbit around the Sun (for example).  

The debates came to a head in 2006 when the International Astronomical Union, in a controversial vote (that didn't include many planetary scientists), decided to take away Pluto's planetary status because it didn't fit what some thought of as the definition of planet. By most accounts, the vote was a  mess and many planetary scientists felt that their professional opinions had not been heeded.

Pluto is a good example of what's called a "dwarf planet". It's not alone: there are several other dwarf planets: Haumea, Makemake and Eris and Ceres—which is actually in the Asteroid Belt between Mars and Jupiter

"Dwarf planet" is a scientific definition, and much more descriptive than the term "planet". When you see "dwarf planet" it indicates a world's characteristics. And, the idea of dwarf planet is not so terribly different from "dwarf star" or "dwarf galaxy", in terms of more precise definitions and descriptions of objects in space.

Think about this: the solar system is much  more extensive and interesting than we ever thought possible back in the days of the dwarf planet's discovery. Today, we've explored the Sun, the rocky worlds, the gas giants, moons, comets, and asteroids. And, we've figured out that Pluto is a special case of "planet": a dwarf planet with mysteries of its own to be solved.