Pluto - A Planet in Name Only
How many planets are there? "Nine. Everyone knows that, right?" Well, yes and no. A few years ago the International Astronomical Union reclassified Pluto as "minor planet #1589". The IAU partially relented when the wife of Clyde Tombaugh's [discover of Pluto in 1930] made an appeal to retain Pluto as a planet in honor of her husband. Today Pluto is officially both a major and a minor planet, whatever that means.
In addition to the planets and their moons, the Sun has two belts and a clouds of tiny bodies. The innermost is the familiar Asteroid Belt that lies between Mars and Jupiter for the most part. It is made up of many irregular bodies and a few small worlds all composed largely of iron and rock like the inner planets. The next out is the Kuiper Belt composed of comets and comet like small worlds all composed of icy materials. The outermost region far from the Sun contains a second halo of comets called the Oort Cloud. The Asteroid Belt was discovered in 1801 when its largest member [Ceres] was spotted for the first time. The Oort cloud is too distant to see with telescopes but we know it is there from the frequent long period comets which can be backtracked to this region. The Kuiper Belt's first member was spotted in 1930. It was named Pluto.
Pluto is a typical Kuiper Belt Object [KBO]. It is the largest such object discovered to date, but its satellite Charon and two other large KBOs [Ixion and Varuna] rival it in size. We may yet discover a KBO larger than Pluto.
Comets are icy objects that have temporary atmospheres (comas and tails) when the approach the Sun closely enough. Pluto has develops an atmosphere during its passages inside the orbit of Neptune. Comets all have elongated elliptical orbits, as does Pluto. Comets frequently enter the inner solar system at steep angles. Pluto is tilted almost 25 degrees to the plane of the planets. Comets are composed of frozen volatile materials [chiefly water, carbon dioxide ammonia and methane] with small amounts of dust, sand and pebbles. Everything we tell about Pluto suggests this is its composition. Planets are either rocky bodies with iron cores or large gas giants composed mainly of hydrogen and helium like the Sun. Pluto is unlike either type of planet.
Many KBOs have been pulled into rings surrounding the solar system with large gaps between the rings. The source of these gaps is tidal forces created by Neptune. These rings orbit the Sun with periods which are integer ratio multiples of the period of Neptune. (Similar rings exist in the Asteroid Belt caused by Jupiter. The bands in Saturn's Rings are arranged in integer ratio of Saturn's satellite's periods as well.) The KBOs which share the integer ratio 3 to 2 with Neptune's are called plutinos after their largest member Pluto. Every time Neptune completes three trips around the Sun, Pluto completes two trips.
Sentiment aside, the case for demoting Pluto to a minor planet like Ceres or Vesta seems very strong. It isn't popular. The Rose Planetarium dropped Pluto from the list of planets, only to find an outcry so strong that they had to add an explanation. Still Pluto looks very little like any other planet and very much like a huge comet which just hasn't entered the inner solar system.
According to myth, Pluto is on firmer ground or more or precisely "IN" firmer ground. He is the Roman's god of the underground where the "shades" of the dead reside. The Roman concept of "shade" is not exactly the Christian concept of "soul". Unlike "souls" who remember their earthly life, the "shades" are stripped of their memories as they are ferried across the River Styx by Charon.
When the three great son's of Saturn (Jupiter, Neptune and Pluto) divided up the universe, Pluto must have been out having a sandwich. Jupiter took the heavens and the land, Neptune took the rivers and oceans, leaving only the underground for Pluto. Needless to say, Pluto wasn't thrilled. While he had access to Mount Olympus like all the major deities, he wasn't exactly cordially received. He took the daughter of Ceres the beautiful maiden Persephone. Ceres the goddess of crops and grains (from which we get the word "cereal") mourned and crops failed everywhere. Ceres prevailed on Jupiter and Neptune who read the riot act to Pluto. Since none of the three major gods were conquerable in their domains, it was a bit of a standoff. Finally Pluto relented by allowing Persephone to return to her mother every Spring on the promise that she would return to Pluto in the Fall. Crops now would grow half the year.