Beyond the vast depths of Neptune's oceans, over the edge of the world, lay the underworld - Tartarus - the land of the dead, from which no-one returned. The dark lord that ruled this dark place was Hades, another son of Saturn, brother to Jupiter and Neptune. He was the most feared of all the gods. Afraid that saying his name might draw his attention, he was referred to as Pluton, which meant the rich one, alluding to tales of his netherworld being filled with the riches of the dead. Over time, his name became Pluto.

So, in 1930, 84 years after the discovery of Neptune, when an object was discovered even farther away, in the true netherworld of the solarsystem, it seemed only appropriate to call it the ninth planet in the solar system, and name it after Pluto, god of the netherworld. Below is a trio of photos taken by the Hubble Space Telescope in 2010.

The Dwarf Planet Decision

And so it remained for the next 70 years. We lived in a solar system with nine planets. The word Planet is Greek, and means wanderer, alluding to the fact that the planets appear to wander among the fixed stars. Pluto was discovered when technology developed telescopes able to detect the wandering movement of such a small object so far away. But technology is nothing if not relentless, and telescopes eventually moved out of Earth's atmosphere into space, where they could see a lot more clearly, and lo and behold, more "wanderers" were discovered beyond Pluto, one or two of which were just about the same size as Pluto. So now what? Should these new wanderers be classified as planets as well? By 2005 there were five of them. Was the number of planets just going to keep going up as new wandering objects were discovered out in the icy cold far-flung fringes of our solar system? In scientific communities the debate raged, from one extreme to the other.

Enter the International Astronomical Union. The IAU was founded in 1919, for just this purpose: to bring together distinguished astronomers from all nations of the world to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU is recognised world wide as the official spokes-organization for the global astronomical community, and in August 2006 they voted, and made their decision. The decision was not to keep adding planets to the solar system, but to define specific conditions which would limit the number of planets. To be an official planet, an object now has to do more than just wander. It must meet three criteria.

All of the original nine planets do all these things except Pluto. Pluto does not fulfill condition three. It has not cleared the neighbourhood of its orbit, as all the other planets have. The neighbourhood of Pluto's orbit is full of debris left over from the formation of the solar system, some of which is close to the size of Pluto itself. Pluto is in fact a member of a large group of objects traditionally referred to as Trans-Neptunian, or Kuiper belt, objects that all circle the Sun in highly elliptical orbits, like comets. The IAU has designated a new category for the largest of these objects. They are to be called Dwarf Planets, of which the former planet Pluto is to be the first.

But although Pluto may not be an official planet anymore, there is no doubt in this writer's mind that it is still the ruler of the netherworld of the solar system. It is still the largest object found in the Kuiper belt, that ring of small, dead bodies that circle the edge of the solar system (like the lost souls of Tartarus,) and extends out to a distance of 9.3 billion miles from the Sun. Much like asteroids, Kuiper belt objects are remnants of the early solar system that never coalesced to form planets. While asteroids are largely rocky however, Kuiper belt objects, being so far from the heat of the sun, are composed largely of ices, sometimes with rocky cores, like comets.

With a diameter of only 1,430 miles (2,288 kms), Pluto is smaller than seven of the moons in the solar system. Its orbit is highly elliptical, bringing it as close as 2.7 billion miles from the Sun (inside the orbit of Neptune), and taking it as far away as 4.6 billion miles. Pluto's orbit also does not lie in the same plane as the other planets, as shown in the graphic below.

Moons of Pluto

In 1978 Pluto was found to have a companion. It was given the name Charon, for the boatman who ferried the souls of the dead to the underworld. Originally classed as a moon of Pluto, Charon is now considered as part of a double planetary system, since it is fully half the size of Pluto, and the two bodies whirl about each other in a mutually synchronous orbit, always keeping the same face towards each other. The pair take 248 Earth years to orbit the sun. The photo below was captured by the Hubble Space Telescope.

In May, 2005, astronomers using the Hubble Space Telescope discovered two new moons orbiting Pluto. The innermost of the new moons is named after the mother of Charon, the goddess of night, Nyx. Since there was already an asteroid with the same name, the Egyptian spelling - Nix - was used. The outermost moon is named after the nine headed sea serpent, Hydra, who guards the gates of the underworld.

Mission To Pluto

Pluto has never been visited by a spacecraft, and because of its great distance, even the resolving power of the Hubble Space Telescope can only return an indistinct image. The Sun from Pluto would appear as only a point of light, like any other star, although it would definitely be the brightest star in the sky.

On January 19, 2006, NASA launched its New Horizons spacecraft, 1,050 lbs. of cameras and scientific equipment that will make a one-way journey to the underworld of the solar system, the Kuiper Belt, and bring us the first up close and personal look at Pluto and some of the other bodies that reside there. New Horizons is the fastest spacecraft ever built, attaining speeds approaching 70,000 miles per hour (112,000 kph) and passing the orbit of the Moon a scant nine hours after launch. But even at such an unprecedented velocity, it will still take over nine years to travel the billions of miles to Pluto. The NASA graphic below shows the position of the spacecraft as of June, 2006, just beyond Mars, and its projected course (in red) out into the outer solar system. Regular updates of the spacecraft's progress can be viewed at http://www.nasa.gov/newhorizons.

Needless to say, if all goes well, and the spacecraft survives its nine year journey and arrives at Pluto as planned in the year 2015, there will be more than a little celebrating going on at NASA and at other astronomical venues around the world. In the words of Alan Stern, New Horizons principal investigator, "Exploring Pluto and the Kuiper Belt is like conducting an archeological dig into the history of the outer solar system, a place where we can peek into the ancient era of planetary formation."

In 2002, another large Kuiper belt object was found about a billion miles further out from Pluto. It is about half the size of Pluto, and named Quaoar (Kwa-whar), after a native American creation god.

On November 14, 2003, an even more distant planet-like object was discovered three times further from the sun than Pluto. Named Sedna, after an Inuit goddess of the sea, it is the most distant object discovered in the solar system. It is so far away it takes 10,500 years to orbit the sun. The NASA graphic below accurately shows the relative sizes to scale.

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