Pluto is tiny as far as planets go. It is a dwarf planet, but in my classification a dwarf planet is not a distinct class
from a planet. Rather, planets can be divided into dwarf planets, like Pluto, medium planets, like Earth, and giant
planets, like Jupiter. In this scheme, a 'dwarf planet' is a subtype of 'planet'. Anything below 1000 km in radius I
would not classify as a planet but rather as a planetoid. Let us compare Pluto, its largest moon Charon, The
Earth and Earth's Moon. Also included for comparison is Eris, probably the most massive Kuiper Belt object and
all the moons in the Solar system with a radius above 1000 km.
|Ice Dwarfs - Pluto and Charon
Below: The Ice Mountains of Pluto. Sol is visible in the Plutonian sky.
Below: the frozen mud-planes of Pluto. Charon is visible in the sky. Charon is Pluto's largest moon and
although much smaller than Earth's Moon it is also much closer to its parent planet and so appears about 6 or
7 times as large in Pluto's sky as the Moon does on Earth.
Pluto, an orange ice dwarf is aptly named after a mythical god of the Underworld as it is far from the Sun. It is
so far from the Sun that the Sun appears as a bright star when Pluto is furthest away (at aphelion) and the disc
of the Sun is just about discernible when Pluto is at its closest (at perihelion). Light from the Sun is about one to
two thousand times as dim here, but since the eye detects the log of light intensity the perceived light levels are
only about 3 times lower (about one-third) than on Earth and looking at the Sun will still hurt the eyes.
Planet type: carbohydrate and water (?) ices, cold desert planet. The largest known object in the Kuiper Belt.
The Kuiper Belt is a ring or band of objects orbiting Sol beyond Neptune from about 30 to 50 AU (astronomical
units). I consider Pluto to be a planet of the dwarf category (rather than a 'dwarf planet' per se).
Equatorial Diameter: 2372 km (radius 1186 km)
Orbit: Pluto orbits Sol in a very eccentric (elliptical) orbit ranging from 29.657 to 48.871 AU from Sol, with a year
lasting 247.68 Earth years and a day length of 6.387 Earth days
Atmosphere: Very thin (about 1 pa or one hundred thousandth of Earth's atmospheric pressure), consists of
nitrogen, methane and carbon monoxide, a visible hydrocarbon haze extends about 130 km (80 miles) above
the planet's surface; it possible snows hydrocarbon ice. The atmospheric gases are in equilibrium with surface
ices of nitrogen, methane and carbon monoxide. A cycle of ice sublimation and ice snow probably maintains
Surface Temperature: 33 to 55 K (-240 to -218 degrees C)
Magnetic Field: no data
Life: none as yet discovered.
Some key tourist attractions:
The surface of Pluto is relatively crater-free, suggesting that it is relatively young and geologically
Colour - Pluto's surface varies in hue and brightness, but has an orange colour due to organic
compounds such as tholins (polymers formed by the action of the Sun's UV light on hydrocarbons such
Ice Mountains - a layer of water ice probably exists beneath Pluto's surface layers of methane,
nitrogen and hydrocarbon ices. This is thought to account for the existence of mountain ranges on
Pluto, with the mountains consisting of an ice core.
Ice Plains - vast plains with a network of troughs containing darker material (some of which forms hills)
possibly consist of frozen 'mud', the 'mud' consisting of particles of methane ices.
Nitrogen Glaciers - evidence indicates that Pluto possibly has glaciers of frozen nitrogen (and other
ices such as CO and methane). Water ice is too brittle at these low temperatures to flow, behaving
more like silica rock on Earth. Despite the low temperatures frozen nitrogen may remain sufficiently
soft and fluid to slowly flow across Pluto's surface. Nitrogen freezes at about -210 degrees C (63 K)
and boils at about -196 degrees C (77 k).
Above: a Pov-Ray simulation of an orange ice dwarf like Pluto.
Below - artistic renditions of Pluto's ice mountains produced by a computer simulation. Can you see Sol in the
sky on one of these views?
Values listed are: radius, mass relative to the Earth and surface gravity relative to the Earth
Pluto: 1186 km, 0.00218, 0.063g
Charon: 603.5 km, 0.000254, 0.028g
Earth: 63741 km, 1, 1g
Moon: 1737 km, 0.0123, 0.1654g
Eris: 1163 km, 0.0028, 0.084g
Triton: 1353 km, 0.00359, 0.0794g
Titan: 2576 km, 0.0225, 0.14g
IO: 1822 km, 0.015, 0.183g
Ganymede: 2634 km, 0.025, 0.146g
Callisto: 2410 km, 0.018, 0.126g
Europa: 1561 km, 0.008, 0.134g
In our classification the Moon would be a planet of the dwarf type, however, since it orbits the Earth which is
much more massive the Moon can be considered a secondary planet. Objects like Charon and Ceres, a
more-or-less spherical asteroid with a mean radius of 473 km would not be classes as a dwarf planet, but rather
as a planetoid, since it has enough gravity to form a roughly spherical object. Objects with a radius of about
1000 km or greater usually have a richer and more varied geology. The four largest moons of Jupiter: IO,
Ganymede, Callisto and Europa are secondary planets of the dwarf type, along with Titan and Triton. This would
give the Solar System a total of 17 planets, of which 9 are dwarf planets. Furthermore, 7 of the nine dwarf
planets are secondary planets (major moons), leaving ten primary planets in the Solar System. This seems the
most natural and convenient classification to me. Eris is currently about three times as far from Sol as Pluto,
though its highly eccentric orbit means that it is sometimes closer (perihelion of 37.9 AU, aphelion of 97.65 AU).
The Moons of Pluto
Pluto has one large moon, Charon. Indeed, Charon is more than half of Pluto's diameter, though much less
massive, and both Pluto and Charon orbit a common centre of mass which is above Pluto's surface (making this
a binary system of two primary bodies, of which one is a planet and one a planetoid). Charon orbits about 19
570 km from Pluto's centre or 17 540 km from the common centre of mass (barycentre). The Earth-Moon system
is sometimes considered a double or binary planet, however, the centre of mass lies beneath the surface of the
Earth and so the Moon is a secondary planet in our classification.
Pluto has four other moons, which are tiny asteroids captured from the Kuiper Belt. These are Styx, Nix,
Kerberos and Hydra. Of these, Hydra is the largest at 55 by 40 km across (and hence not even a planetoid in
our classification as it is not even roughly spherical). These orbit around the central Pluto-Charon system.
Charon's surface is apparently dominated by water ice rather than by methane or nitrogen ices and is less
orange and more brownish in colour. One theory is that an impact with Pluto vaporised part of its icey mantle
which condensed into Charon. There is some evidence that Charon may have cryogeysers. The surface of
Charon is also relatively crater-free, suggesting that it's surface is relatively young and geologically active.
26 July 2015