Neptune was photographed by NASA in 1989, while it was still outside Pluto's orbit. *

Image: Courtesy of NASA * 1979: Neptune begins a 20-year run as the outermost planet in the solar system as Pluto's elliptical orbit carries it closer in to the sun.

While its eccentric orbit around the sun is not among the reasons the International Astronomical Union gave for downgrading Pluto's status to "dwarf planet" in 2006, it probably didn't help. Not only is Pluto's orbit elliptical, which puts it at odds with the circular orbits of the great eight, but its orbital plane is also very different.

The effect of Pluto's orbit takes it inside of Neptune's at its perihelion, or closest distance to the sun, which is roughly 2.7 billion miles. It takes 20 years for the two orbits to cross again.

Neptune passed back inside Pluto's orbit in 1999.

It's not a phenomenon any of us will witness again unless Ray Kurzweil gets his way and we all live to be 500 years old. Pluto's orbit carries it inside Neptune's every 248 years, so unless you're planning to hang around until 2247, you'll miss it.

(Source: Various)

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