In August 2006 the International Astronomical Union (IAU) declared Pluto is no longer to be classified as a planet. This was the result of a vote on their
first official definition of a planet. According to the new rules, a planet must meet three criteria: 1) it must orbit the Sun, 2) it must be big enough for
gravity to have forced it to become round, and 3) it must have cleared other things out of the way in its orbital neighbourhood. Pluto fails on the third
count. In the new solar system, there are now three official classes of objects in the Solar System, which are eight planets, at least three dwarf planets
Pluto is now a Dwarf Planet, the qualification for which is only that it must be round. Dwarf planets are not
considered to be true planets. For now, the dwarf planets include Pluto, Ceres (the largest asteroid) and an object known as UB 313, or Xena, which was
discovered in 2003. Xena is larger than Pluto and, as Pluto, orbits out beyond Neptune in a zone of icy debris known as the Kuiper Belt. There are dozens
more potential dwarf planets known in that zone and the number is likely to rapidly increase. Orbiting within this debris is the reason Pluto fails on the
third criteria of the new classification for a planet. Over many orbital cycles, a planet should cause small bodies either to accrete with it or to be
disturbed to another orbit. As a consequence it does not then share its orbital region with other bodies of significant size, except for satellites
governed by its own gravitational influence.
Inscriptions on the space probes Pioneer 10 and 11 and images sent with Voyager 1 and 2, each launched in
the 1970s and intended to give information about the origin of the craft for any future encounters with aliens, all show Pluto as the 9th planet. The
element plutonium is named after Pluto as a planet, as are uranium and neptunium named after Uranus and Neptune.
It is estimated, within the Kuiper Belt, there are at least 500,000 objects larger than 30 kilometres across
and many of these will be at least as large as Pluto. The decision to `downgrade` Pluto because of criteria 3 has caused further controversy since Earth,
Mars, Jupiter and Neptune share their orbits with asteroids.
It is known that at least twelve Kuiper Belt Objects (KBOs) have companions. These are often referred to as
binary KBOs because, where the objects have similar sizes, it can be unclear as to which is the KBO and which its moon. Pluto and its moon Charon are
considered a binary KBO, with Charon in principal orbiting Pluto at a distance of 17,000 kilometres. In fact the centre of mass, around which they orbit
each other, is several thousand kilometres outside of Pluto, which qualifies them as binary objects.
Like our own Moon, Charon`s spin period (6.4 Earth days) is the same as its orbital period and from Pluto
you would only see one side of Charon. Pluto also spins at the same rate, so that from Charon you would also only see one side of Pluto. In addition, from
any particular location on either object, the other object would always appear stationary in the sky (or not be visible ever).
Pluto`s orbit is more elliptical than those of the eight planets and can be seen in the diagram overlapping
Neptune`s orbit, as discussed in the notes on Neptune. Pluto takes 248 years to orbit the Sun at distances ranging from 4.4 billion kilometres to 7.4
billion kilometres. Due to this changing distance from the Sun, the amount of sunlight varies by 280% over the Pluto year compared to 5% during Earth`s
The eight planets orbit the Sun close to the same plane which is called the ecliptic, but Pluto`s orbit is
tilted against this by 17.14º. At 7º, Mercury`s inclination is the highest of the eight planets.
Pluto has a radius of about 1,200 kilometres and Charon about 630 kilometres. Pluto`s surface temperature is
estimated to be about -233ºC with an atmosphere of carbon, methane and carbon monoxide. Pluto has two other known moons, Nix and Hydra, discovered with
Hubble in 2005.
No spacecraft has yet flown by or sent rovers to Pluto. In January 2006 the New Horizons spacecraft was launched to study Pluto,
which was then the last unvisited planet in the Solar System. This spacecraft will cross the orbits of all the planets to fly by Pluto and Charon in July
2015 and on to investigate KBOs from 2016 to 2020. One of the major goals of this mission is to image and map Pluto and Charon, revealing details that have
been impossible to see to date. Currently even the best ground based telescopes can only see these objects as dots of light, and the Hubble images are
Pluto was officially declared a planet in 1930 and was named after the Roman god of the underworld and of the dead who was able
to become invisible. In Greek mythology he was known as Hades. Dead souls were escorted to the shores of Styx, the River of the Dead, by Mercury from where
Charon, the ferryman, collected a toll in the form of a coin buried with the dead, in order to cross the river to Pluto`s kingdom.
The goddess Venus playfully instructed her companion Cupid to shoot an arrow through Pluto`s heart, causing
him to fall in love with the first maiden he saw. This was Persephone, daughter of Ceres the Earth goddess. Pluto carried Persephone off to the underworld
whilst Ceres searched everywhere to find her. Eventually Ceres refused to allow plants to grow and everyone begun to die of hunger. Pluto then admitted to
holding Persephone and said she could return home if she had eaten none of the food of the Dead. Sad at being kept underground, she had only eaten six
pomegranate seeds. She was allowed to go home for six months every year, but had to return to her husband Pluto for the other six. Ceres stopped everything
growing when this happened and this is the season of winter.
In terms of astrology, Pluto is the planet of great change, latent power and untapped potential. Expressed in its positive form,
Pluto may change your life forever with a blinding insight or personal transformation. It can fill a person with a need to lead others and change the world,
or at least their part in it, and urges them to make discoveries or uncover secrets of the past. In its negative form, Pluto's influence results in a
controlling nature and opens the door for treachery, sadism and violence.