Pluto

The Former Planet


Image Credit: NASA/JHUAPL/SwRI

This is a true color mosaic image of Pluto assembled from images taken from the New Horizons spacecraft near the time of its closest approach on July 14, 2015. .

Mass (Earth=1)0.00245
Equatorial diameter (km)2370
Period (years)247.93
Mean distance from Sun, 106 km5,906.4
Density (water=1)2.095
Surface gravity m/s20.62

Pluto was discovered by C. Tombagh in 1930, using the same techniques by which Neptune was found. But Pluto, much smaller than expected, is too tiny to affect faraway Uranus, so it's discovery was an accident.

Pluto's moon Charon is a surprising one-third the diameter of Pluto . Both may be escaped satellites of Neptune.

Pluto's orbit is unique in that it is much more elliptical than the planets, and it is inclined 17° to the plane of the Earth's orbit.

Descriptive data

Image Credit: NASA composite image
Why was Pluto demoted from Planet status?
References:

New Horizons site

New Horizons images

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Moons of Pluto

Pluto is shown to scale with its largest moon, Charon. Both are classified as "ice dwarfs".

Nix, Hydra, Styx and Kerberos are previously unknown moons of Pluto discovered with the use of the Hubble Space Telescope.

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Why Is Pluto No Longer a Planet?

When the International Astronomical Union voted in 2006 to demote Pluto to the status of a "dwarf planet", there was a lot of discussion of the event on the Georgia State University campus. Astronomers Hal McAlister and Todd Henry were both present at the meeting in Prague, Czech Republic when the decision was made.

Dr. McAlister was a member of the Union and voted against the change, partly to honor the discoverer of Pluto, Clyde Tombaugh. Both agreed that Pluto was an oddity as a planet, being smaller than Earth's moon and having a very strange orbit. The 3000 member Union considered the fact that there are other objects in our solar system that are even larger than Pluto and not called planets. Dr. Henry commented that the change clears the way for the study and classification of other objects out in the farthest parts of the solar system.

With all the discussion, the Georgia State Magazine published a humorous Q&A article in the spring of 2007 with the above image which is credited to Meg Buscema.

One of the other terms being used to describe Pluto is "ice dwarf", so it remains to be seen what the long-term designation will be. But it is a planet no longer. It may be classified as part of the Kuiper belt of icy objects out past the orbit of Neptune.

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