THE KUIPER BELT, OUTER SPACE (WHTM) — Pluto has a most unusual orbit. Because it’s so far from the Sun (3.7 billion miles on average), it takes the planet 248.09 years to complete one orbit.
While the larger planets in the solar system all revolve around the Sun at about the same angle (Called the ecliptic plane if you want to delve into the matter), the orbit of Pluto is tilted about 17 degrees. And while most of the planetary orbits are almost (but not quite) circular, the orbit of Pluto is more of an oval-an ellipse. (In fact, all orbits are elliptical, but some are more elliptical than others.)
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As a result of its more elliptical orbit, something interesting happens to Pluto as it makes its trip around the sun. Most of the time it will be farther from the sun than the planet Neptune, but for about 20 years, it will cross the orbit of Neptune, and be the closest to the sun of the two bodies. This happened most recently on Feb. 7, 1979-the first time since Pluto was discovered on Feb. 18, 1930.
This composite of enhanced color images of Pluto (lower right) and Charon (upper left), was taken by NASA’s New Horizons spacecraft as it passed through the Pluto system on July 14, 2015. This image highlights the striking differences between Pluto and Charon. The color and brightness of both Pluto and Charon have been processed identically to allow direct comparison of their surface properties, and to highlight the similarity between Charon’s polar red terrain and Pluto’s equatorial red terrain. Pluto and Charon are shown with approximately correct relative sizes, but their true separation is not to scale. The image combines blue, red and infrared images taken by the spacecraft’s Ralph/Multispectral Visual Imaging Camera (MVIC). Credit: NASA/JHUAPL/SwRI 
This image of haze layers above Pluto’s limb was taken by the Ralph/Multispectral Visible Imaging Camera (MVIC) on NASA’s New Horizons spacecraft. About 20 haze layers are seen; the layers have been found to typically extend horizontally over hundreds of kilometers, but are not strictly parallel to the surface. For example, scientists note a haze layer about 3 miles (5 kilometers) above the surface (lower left area of the image), which descends to the surface at the right. NASA/JHUAPL/SwRI 
New Horizons scientists use enhanced color images to detect differences in the composition and texture of Pluto’s surface. When close-up images are combined with color data from the Ralph instrument, it paints a new and surprising portrait of the dwarf planet. The “heart of the heart,” Sputnik Planum, is suggestive of a source region of ices. The two bluish-white “lobes” that extend to the southwest and northeast of the “heart” may represent exotic ices being transported away from Sputnik Planum. NASA/JHUAPL/SwRI 
This panchromatic image of Pluto’s small satellite Nix taken by the Multispectral Visible Imaging Camera (MVIC) aboard New Horizons is one of the best images of Pluto’s third-largest moon generated by the NASA mission. Taken on July 14 at a range of about 14,000 miles (23,000 kilometers) from Nix, the illuminated surface is about 12 miles (19 kilometers) by 29 miles (47 kilometers). The unique perspective of this image provides new details about Nix’s geologic history and impact record. NASA/JHUAPL/SwRI 
NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto’s big moon Charon just before closest approach on July 14, 2015. The image combines blue, red and infrared images taken by the spacecraft’s Ralph/Multispectral Visual Imaging Camera (MVIC); the colors are processed to best highlight the variation of surface properties across Charon. Charon’s color palette is not as diverse as Pluto’s; most striking is the reddish north (top) polar region, informally named Mordor Macula. Charon is 754 miles (1,214 kilometers) across; this image resolves details as small as 1.8 miles (2.9 kilometers).
On Feb. 11, 1999, Pluto moved back outside Neptune’s orbit, once again becoming the furthest planet in the Solar System.
On July 14, 2015, NASA’s New Horizons spacecraft sent back amazing close-up pictures of Pluto and its moon Charon, which was discovered in 1978. Pluto has five moons, Charon, Nix, Hydra, Kerberos, and Styx.
But by then, Pluto was no longer considered a planet. It was increasingly obvious the Kuiper Belt had a lot of worlds, worldlets, or maybe large iceballs. In the early 2000s, The International Astronomical Union (IAU) tackled the problem of defining what actually makes a planet a planet.
They ultimately came up with three criteria:
- It is in orbit around the Sun.
- It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
- It has “cleared the neighborhood” around its orbit.
Pluto didn’t meet that third requirement. So, when the IAU adopted the criteria on August 24, 2006, it was downgraded to a “dwarf planet”, joining Eris, Ceres, Makemake, Haumea, Gonggong, Quaoar, Sedna, and dozens, perhaps hundreds, possibly even thousands waiting to be discovered.
Pluto and its moons continue on their way, regardless of how humans try to define them. They will complete their first full orbit since their discovery on Monday, March 23, 2178. It will be closer to the Sun than Neptune again in 2227.
