On January 21, 1979, a rare phenomenon occurred in our solar system: Neptune, the 8th planet from the Sun, took over Pluto and became the outermost planet as Pluto moved closer due to their highly elliptical orbits. Pluto was still a planet back then (good old days!).

Today’s (January 21) story of what happened this day in Science, Technology, Astronomy, and Space Exploration history.

Neptune becomes the outermost planet (temporarily)

Neptune‘s average distance from the Sun is about 30 AU (Astronomical Unit). The astronomical unit (AU) is a unit of length, roughly the distance from Earth to the Sun. Since 2012 it has been defined as exactly 149,597,870,700 meters or about 150 million kilometers (93 million miles).

Pluto’s average distance from Sun is 39 AU.

But, From January 21 1979 to March 1999, Pluto was near perihelion (the point when it is closest to the Sun). During this time, Pluto was actually closer to the Sun than Neptune.

Between 1979 and 1999, Neptune was the outermost planet in the Solar System. Pluto and Neptune orbits
For a 20-year period between January 1979 and March 1999, Pluto’s highly elliptical orbit brought it closer than Neptune to the Sun, hence Neptune became the outermost planet in the Solar System for that period. Please note that Pluto, Neptune, and Sun are not to scale in this image.

Neptune becomes the outermost planet (permanently)

Then, on August 24, 2006, Pluto lost its status as a planet, and Neptune became the outermost planet again, this time permanently (unless there is no Planet Nine).


M. Özgür Nevres
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