Dwarfs - Pluto and Charon
Above: a Pov-Ray simulation of an orange ice dwarf like Pluto.
Below: The Ice Mountains of a Pluto-like planet. A Sun-like star is visible in the Plutonian sky at a an equivalent distance of Sol from Pluto to illustrate the size of Sol as seen from 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),
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
Some key tourist attractions:
The surface of Pluto is relatively crater-free, suggesting that it is relatively young and geologically active. Large uncratered plains occur, such as Sputnik Planum.
Color - Pluto's surface varies in hue and brightness, but has an orange color due to organic compounds such as tholins (polymers formed by the action of the Sun's UV light on hydrocarbons such as methane).
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 a water ice core (water ice is stronger than hydrocarbon ice). The tallest mountain range on Pluto is 6.2 km (Tenzing Montes).
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.
Fields - large plains rippled with regular
dune systems thought to be made chiefly of grains of methane ice
acting like silica sand. this implies that winds on Pluto can
cause significant geological activity, at least during certain
times of the year. These transverse dunes are regularly spaced,
with a wavelength of 0.4 to 1 km and are thought to have been
deposited by winds between 1 m/s and 10 m/s in speed. (Pluto's
cold and thin atmosphere is generally thought not to support
higher wind speeds).
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).
Blades of Ice - jagged ridges of methane ice
near Pluto's equator several hundred feet tall. These are thought
to form when methane ice deposits are eroded by sublimation
(evaporation directly into methane gas) due to relatively warm
periods in Pluto's climate. Material can be seen that has flowed
around hills and through breeches in crater rims.
Impact craters - though not especially frequent do occur and "many appear to be substantially degraded or infilled, and some are highlighted by bright ice-rich deposits on their rims and/or floors" (Stern et al. 2015) such as those in the dark equatorial region called Cthulhu Regio (CR).
Features are generally named as follows:
Planum - large plain
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?
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 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.
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
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).
Above:Pluto itself as imaged by NASA's New Horizons space probe:
The bright heart-shaped region in the south is
Tombaugh Regio. To the left of Tombaugh Regio is the dark region:
Cthulhu Regio. The upper left lobe of Tomaugn Regio is Sputnik
Planitia (Sputnik Planum). The region of bright and dark streaks
near the equator on the right is Tartarus Dorsa - an area of large
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 center 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 center or 17 540 km from the common center of mass (barycenter). The Earth-Moon system is sometimes considered a double or binary planet, however, the center 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 color. One theory is that an impact with Pluto vaporised part of its icy 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.
Is Pluto a Planet?
Scientists disagree on whether Pluto is a planet or not. The IAU (International Astronomical Union) decided to reclassify Pluto as a 'dwarf planet', which is somehow not a 'planet'. Yes, Pluto is a dwarf planet as in a planet that's small but it can still be considered a planet. Why? Because science does not work by diktat: unions and 'official' bodies can define their own definitions and classification systems but they are not absolutely official as the Media have portrayed them to be. Many scientists still regard Pluto as a planet and they have the official capacity to do so. Geologists and interplanetary scientists are among those who study planets, apart from 'astronomers' per se and they have just as much say in the matter. The Media have been disingenuous in explicitly stating the IAU definition as set in stone and official, as if in some legal capacity: they are wrong! Science is not politics!
agrees with many planetary scientists in defining planets on
their own merits, as objects in their own right, rather than on
orbital properties. Other scientists may agree or disagree. A
planet can be considered as an object with sufficient gravity to
pull itself into an approximate sphere. Large moons can be
considered secondary planets. Pluto is clearly sufficiently
spherical and has interesting geological features showing clear
signs of current geological activity. Cronodon considers Pluto
to be a true planet of the dwarf class.
and Charon are almost Perfect Spheres
terms of sphericity, measurements suggest that the difference
between the equatorial and polar diameters of Pluto are less
than 12 km different, so the polar diameter is at least 99% of
the equatorial diameter. Generally, planets with liquid
interiors or extensive gaseous envelopes extend along their
equator due to the centrifugal force of the planet's rotation.
Tidal bulges due to the gravitational pull of other nearby
bodies can also deform a planet. In Pluto's case no oblateness
(polar flattening) has been detected. Similarly, Charon has no
detectable oblateness, placing a maximum limit of a 1%
difference in its polar and equatorial radii.
is tidally locked with Charon: both bodies present the
same face to one-another (their rate of spin on their own axes
is equal to the average orbital period of the two bodies about
their common center of mass or barycenter which is
outside the surface of Pluto). The orbits of the two bodies
about the barycenter are circular. Furthermore, the equators and
orbital plane are coplanar. This situation could have arisen in
two probable ways: either two bodies collided, forming Pluto and
Charon from the impact debris in circular orbits, or Charon was
captured by Pluto, perhaps after a grazing impact, in which case
the orbit of Charon may have been elliptical or eccentric to
begin with. Two such bodies orbiting in close proximity exert
gravitational pulls on one-another, resulting in tidal
bulges on each body: the bulges move as the bodies spin,
but because of friction within the deforming bodies, the bulges
lag behind the gravitational pull. This results in tugs acting
on the two bodies to slow their spins and transfer angular
momentum to their orbits. The total angular momentum must
be conserved in an isolated system, so as angular momentum is
transferred, the spins slow as the orbits speed up, until the
two become equal and the bodies become tidally locked. The
orbits also become circular as the tidal forces resist eccentric
orbits in which resistive tidal forces increase sharply when the
bodies are closer together: circular orbits offer less
fact that very little or no polar flattening / equatorial
extension has occurred in Pluto suggests that Pluto remained
warm, soft and deformable during the early evolution of the
Pluto-Charon binary system either during or after the tidal
locking. It may also suggest that Pluto was not spinning very
fast to begin with.
References / Further Reading
Stern, S.A., Bagenal, F., Ennico, K., et al. 2015. The Pluto system: Initial results from its exploration by New Horizons. Science 350(6258): aad1815. DOI: 10.1126/science.aad1815
Telfer, M.W., Parteli, E.J.R., Radebaugh, J., et al. 2018.
Dunes on Pluto. Science 360(6392): 992-997.
26 July 2015
30 June 2020