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Ever wondered why Pluto is no longer a planet?

As we approach an international holiday known as Pluto Demoted Day, an expert explains why the dwarf planet is still super cool.

Pluto, left, is shown with Charon, one of its five moons. Pluto was demoted to a dwarf planet in 2006. (HYPERSPHERE/SCIENCE PHOTO LIBRARY/AP)

At the edge of our solar system, there’s a tiny, icy world with a diameter — or length across at its widest point — only 18.5 percent as large as Earth’s. You know it as Pluto.

When your parents were kids, Pluto was actually considered a planet. But 15 years ago, a group of scientists known as the International Astronomical Union voted to make the definition of “planets” more specific, and Pluto no longer made the cut. According to the IAU, Pluto is technically a “dwarf planet,” because it has not “cleared its neighboring region of other objects.” This means that Pluto still has lots of asteroids and other space rocks along its flight path, rather than having absorbed them over time, like the larger planets have done.

Believe it or not, each year on August 24, the international scientific community recognizes Pluto’s historic downgrade with a holiday called Pluto Demoted Day.

But just because Pluto lost its planet status doesn’t mean it isn’t fascinating, says Cathy Olkin, a planetary scientist at the Southwest Research Institute.

For instance, Pluto can be more than 4 billion miles away from Earth, depending on where it is in its wonky orbit, and the dwarf planet’s average temperature dips to -387 degrees Fahrenheit. That’s so cold, things get a little bit weird.

“Pluto has this huge glacier on its surface, but the glacier is made of exotic ices,” says Olkin, who is also a scientist on NASA’s New Horizons mission to Pluto and beyond. “So, not water-ice, like we have here on Earth, but ice made out of nitrogen and methane, things that are gases in our atmosphere.”

Pluto is also really dark, because the sun is much farther away than it is here on Earth. In fact, NASA has a handy website that allows you to catch a glimpse of what scientists call “Pluto Time.” With an adult, simply enter your location and the website will tell you to look outside at a certain time of the day, usually right at dusk, when the light here on Earth looks almost exactly like it would on Pluto at noon, or its brightest time of the day.

Another interesting fact about Pluto is that it has five moons. One of them, called Charon, is half of Pluto’s size. (For comparison, our moon is just over ­one-quarter the size of Earth.) Charon is so big, Olkin says, that its gravity actually causes Pluto to wobble in its orbit.

The last thing you should probably know about Pluto? Some scientists do not agree with its demotion. One reason is that space is full of objects, and every planet has some in its “neighboring region.”

“There are many different ways to decide what is a planet,” Olkin says. “[Pluto] has an atmosphere. It has moons. It goes around the sun.”

“There are still people who are trying to fight that definition,” she says.

Bittel is a freelance journalist who often writes about animals. His children’s book, “How to Talk to a Tiger . . . and Other Animals,” published in April.

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