Although, for ease of use, still listed on this site under the heading
'Planets', Pluto was finally stripped of its full planet status and reclassified
as a 'Dwarf Planet' in August 2006. The status of Pluto as a planet had been intermittently called into
question many times before, due to its small size in relation to its neighbouring planets (it's nearly 10,000 times smaller than Neptune!)
and its unique physical make-up of ice and rock. The
factor which finally persuaded the International Astronomical Union (IAU) to declassify Pluto however
was the discovery of a number of Kuiper Belt Objects (KBOs) of a similar if not greater size to Pluto
in the few years preceeding its declassification. Not taking action at that point would have
led to the inevitable classification of a number of other large KBOs as 'planets', eventually resulting in the number
of planets increasing by the tens, with most of these being 'new' planets found in the outer fringes of the
Solar System - small hard bodies bearing little or no resemblance to the 8 objects which we traditionally think
of as 'The Planets'. Such objects, from now on, are to be termed 'Dwarf Planets'.
The search for Pluto (named after the Roman
God of the Underworld), which began shortly after the discovery of Neptune, was most
associated with the astronomer Percival Lowell, a man more famous for his ideas about Mars
than discovery of distant planets. Still, the idea of a 'Planet X' fascinated him and soon
he began searching the heavens for this planet with a team of researchers, using his
private observatory in Flagstaff, Arizona. After many years of trawling through
photographs nothing was found until, 13 years after Lowell's death, a young amateur
astronomer named Clyde Tombaugh finally discovered Pluto in February 1930.
Since then this icy dwarf planet has fascinated many people the world over, yet not that much
is actually known about it, as its sheer distance from us has so far prohibited any space missions
to send probes near it, and makes viewing it from the ground harder too. However, this situation is changing - ever bigger
and more powerful telescopes are helping us to better understand this dwarf planet, and a NASA mission is planned to send a space probe to Pluto. As we keep learning more details about this world, it becomes ever more intriguing.
Pluto has an average Density of between 1.8 and 2.1 g/cm3, and
is estimated to be composed of 50-75% rock mixed with ice. Proof of Pluto's icy surface temperature
comes with the knowledge that its surface is composed 98% of Nitrogen, with traces of Carbon Monoxide and Methane, which can only be present
as a solid under 70°K.
Also noteworthy is Pluto's highly elliptical orbit, because although it's usually
further from the Sun than any of the 8 planets, there is an period of time during its orbit when it's actually
inside Neptune's orbit! This last happened in 1979 and serves to demonstrate just how much Pluto's distance from the Sun varies. At the moment it has recently passed its closest point and is
slowly increasing its distance from the Sun. Pluto also has
a thin atmosphere of Carbon Dioxide and Methane. This atmosphere is not permanent though
and when Pluto gets too far from the Sun, it collapses. This is next expected to happen in
2020 or so, although we can't be sure as this process has never actually been observed.
|Pluto and Charon|
With a diameter of a mere 2,320 kilometres (about two thirds that of
our moon) Pluto is smaller than any of the 8 planets, and yet, rather surprisingly, Pluto has a
moon of its own; Charon. Charon is 1,210 kilometers wide, more than half the diameter of
Pluto itself, and therefore holds the record of being the largest Moon
relative to its planet or dwarf planet in the Solar System.
It is considered very likely that Pluto and Charon were formed independantly of each other,
unlike the Earth and our moon, due to the fact that its surface composition is thought to be mainly water ice, and thus very different to Pluto's, and
also because it has a much lower density (approx 1.2g/cm3 to Pluto's 1.8-2.1g/cm3).
Charon circles Pluto in a slightly eccentric orbit (0.0076),
probably due to a recent collision with a sizeable object. Another peculiar feature had by Pluto
and Charon though is that Charon orbits Pluto in the same 6.387 days that Pluto takes to
rotate, keeping the two locked in synchronous rotation. Charon therefore behaves like our
moon in this respect - it always keeps the same face towards Pluto; the difference in this case
however is that Pluto also always keeps the same face towards Charon.