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Pluto is a Dwarf Planet, a Plutoid, a Kuiper Belt Object and a Binary

Pluto the Planet

Pluto was discovered in 1930 by Clyde Tombaugh.  He was searching for a planet predicted to exist by Percival Lowell.  So Pluto was called a planet, and most of us were taught about the 9 planets of the Solar System.  That is 9 including Pluto. 

But this century there have been several new discoveries and in 2006 Pluto was demoted.

Dwarf Planet

So far there have been 5 dwarf planets discovered.  In order of size, starting at the biggest they are Eris, Pluto, Makemake Haumea and Ceres. 


Although Pluto is no longer considered a planet in most places, it was given special recognition by the use of its name for a whole class of Trans-Neptunian Objects.  These are now called Plutoids.  Ceres is not a Plutoid, but the other 4 Dwarf Planets are.

Kuiper Belt Object

The Kuiper Belt is similar to the Asteroid Belt, but is beyond the orbit of Neptune.  There are over a thousand known Kuiper Belt Objects.  Pluto is the second biggest.  However, it is confidently expected that there are a very large number of undiscovered objects in this belt. 


After Pluto was discovered a moon was found, and called Charon.  The moons of the Solar system tend to be much smaller than the planet they revolve around.  There are two exceptions.  One of these is the Planet Earth.  The Earth’s moon is unusually large compared with is planet.  It has even been suggested that the Earth Moon system should be considered as a double planet. 

The other exception is the Pluto Charon pair.  Charon is about half the diameter of Pluto, and the pair could be called a Double Dwarf Planet or Binary Dwarf Planet.  Considered as a binary, this pair is the only Binary Dwarf Planet so far discovered.
Some of the reasons Pluto was Demoted


Pluto is the eighteenth most massive object so far discovered in the Solar System.  Of course the Sun is the biggest.  It can be noted that all the planets (excluding Pluto) are more massive than all the other objects except the Sun of our Solar system.  Mercury actually has a smaller diameter than the moons Ganymede and Titan, but is more massive. 

This is not an official criterion, but it seems to have some merit to me. However, I can easily imagine a different planetary system round another star with much bigger moons than our system.


The eight official planets do not cross each other’s orbits, while Pluto crosses Neptune’s.  That is, all the planets have nearly circular orbits while Pluto has a much more elliptical one.


The eight official planets orbit the Sun in a single geometric plane called the plane of the ecliptic.  Pluto’s orbital plane is well outside this plane.


Increasingly we are discovering objects similar to Pluto.  How many of these should be considered to be planets?  The discovery of Eris was one of the things that made the international astronomical community look seriously at Pluto’s status.  Eris shares some of Pluto’s characteristics and is a little bigger both in mass and diameter.

These are some of the reasons for Pluto’s demotion to a dwarf planet.  Of course there are reasons against it:


Most of us were brought up with the idea that Pluto is a planet.  Old people like me can find changes difficult to adjust to.


Historically, the existence of Neptune was predicted by minor variations in the orbits of the outer planets known at the time.  Percival Lovell made a similar prediction of the existence of another planet at roughly the position of Pluto, and when Pluto was eventually found, for some time we had little idea of its mass.  Mr. Lovell had expected it to be a gas giant.

When the mass of Pluto was determined, it was found to be too small to explain the variations in the orbits of the outer planets, but despite its small size it was left with its planetary status until recently.


There is little doubt that some people are offended by the change, and at least one state does not accept it. To see a story incorporating this idea, where the people of Illinois receive help from an unexpected direction, see The Story of Tiffany Rat.
Images of Pluto from the Hubble Space Telescope
By NASA, ESA, and M. Buie (Southwest Research Institute)
via Wikimedia Commons

Pluto and Charon
Picture taken with the Hubble Space Telescope
By Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA [Public domain], via Wikimedia Commons
In 2006 NASA launched the New Horizons probe. 
It is due to fly past Pluto in 2015.
This picture shows the 2010 position of the probe.
By NASA JHU/Applied Physics Laboratory[see page for license], via Wikimedia Commons.
Note that the orbits are not strictly to scale, and not all the planets are represented.