solar system


pluto, is the second-largest known dwarf planet in the solar system and the tenth-largest body observed directly orbiting the sun. originally classified as a planet.

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Pluto, is the second-largest known dwarf planet in the solar system and the tenth-largest body observed directly orbiting the sun. originally classified as a planet, pluto is now considered the largest member of a distinct population called the kuiper belt.

Pluto's unique orbit causes it to periodically come within the orbit of neptune. pluto was the eighth planet from the sun from february 7th, 1979 through february 11, 1999. now pluto will remain the ninth planet through the 23rd century.

Physical Characteristics

mpc designation 134340 pluto
minor planet category dwarf planet, tno, plutoid, kbo, plutino
adjective plutonian
mean radius 1,151 km
0.18 earths
surface area 1.665 w 107 km2
0.033 earths
volume 6,39 w 109 km3
0.0059 earths
mass (1.305 1 0.007) w 1022 kg
0.002 1 earths
0.178 moon
mean density 2.03 1 0.06 g/cm3
equatorial surface gravity 6.833 × 1013 km³
63.086 earths
escape velocity 1.27 km/s
sidereal rotation period -6.387 230 day
6 d 9 h 17 m 36 s
equatorial rotation velocity 47.18 km/h
axial tilt 119.591 1 0.0140 (to orbit)
north pole right ascension 133.046 1 0.0140
north pole declination -6.145 1 0.0140
albedo 0.49-0.66 (varies by 35%)
surface temp. min mean max
kelvin 33 k 44 k 55 k
apparent magnitude up to 13.65 (mean is 15.1)
absolute magnitude -0.7[6]
angular diameter 0.065" to 0.115"

Pluto's Interior


The structure of pluto is not very well understood at present. nevertheless, spectroscopic observation from earth in the 1970s has revealed that the planet surface is covered with methane ice.

surface temperature is -230?c, and the frozen methane exhibits a bright coloration. however, with the exception of the polar caps, the frozen methane surface is seen to change to a dark red on the basis of observation of eclipse by its moon charon.

pluto's atmosphere

surface pressure 0.30 pa (summer maximum)
composition nitrogen, methane

Thin Atmosphere

Pluto was discovered in 1930 as the result of some celestial detective work. it has always been mysterious. even the mighty hubble space telescope can only make out the haziest details on its icy surface. its atmosphere was first detected in 1985 when it made a star passing behind the planet, as viewed from earth, appear to waver slightly. the atmosphere is very thin by earth standards - about a millionth of the thickness of our atmosphere.

Observations and calculations suggested pluto's atmosphere was made of nitrogen, which is in a delicate balance with surface ice. orbiting the sun every 248 years, tiny pluto - smaller than our own moon - is usually the most distant planet in our solar system, except for a few years either side of its closest approach to the sun (and then neptune is slightly more distant).

Lucky Break

It last happened in 1989, and since then pluto has been moving away from the sun, presumably getting colder. the last occultation of a star by pluto was in 1988, so astronomers looked forward to two occultations in july and august 2002, setting up camp in north and south america, and hawaii.

A probe will get there in about 13 years time a star designated p126a passed behind pluto on 20 july 2002, and immediately astronomers realised that pluto's atmosphere had changed in some way, but the quality of the observations were not good enough to say exactly how.

The occultation of 21 august, of star p131.1, yielded better results, suggesting that pluto's atmosphere had swelled since 1988, even though it seemed that its temperature had stayed roughly constant. the astronomers report their observations in the journal nature.

Planet Race

The researchers think that thermal lag is the explanation. pluto will inevitably get colder as it moves away from the sun, but there might be a delay to heat accumulated when closest to the sun. because of the delicate balance between pluto's atmosphere and its icy surface, researchers expect the atmosphere to completely contract sometime around 2015, with an uncertainty of a few years.

It was the need to reach planet and make measurements before the atmosphere collapsed that has made pluto-kuiper express an urgent mission. it may, or may not, reach the planet in time. "the observations are timely," says william hubbard writing in nature, "pluto's orbit over the next few years offers an opportunity to learn more about this planet, at a time when technological developments make consideration of a mission to the planet feasible."

Pluto's Orbital Characteristics

outer solar system
aphelion 7,375,927,931 km
49.305 032 87 au
perihelion 4,436,824,613 km
29.658 340 67 au
semi-major axis 5,906,376,272 km
39.481 686 77 au
eccentricity 0.248 807 66
orbital period 90,613.305 days
248.09 years
synodic period 366.73 days
average orbital speed 4.666 km/s
inclination 17.141 750
11.880 to sun's equator
longitude of ascending node 110.303 470
argument of perihelion 113.763 290
satellites 3

Pluto's Moons

pluto moons

These hubble space telescope images reveal pluto, its large moon charon, and the planet's two new candidate satellites. between may 15 and may 18, 2005, charon, and the putative moons all appear to rotate counterclockwise around pluto.

Discovered in mid-2005, pluto's small moons were provisionally designated s/2005 p1 and s/2005 p2. they have now been officially christened nix and hydra. compared to pluto and its large moon charon, at 2,360 and 1,210 kilometers in diameter respectively, nix (inner moon) and hydra (outer moon) are tiny, estimated to be only 40 to 160 kilometers across.

pluto and charon are bright enough to create diffraction spikes in this hubble space telescope image, but nix and hydra are some 5,000 times fainter than pluto and appear only as small points of light. still, their new names are appropriate for the distant pluto system. in mythology, nix was the goddess of darkness and night and the mother of charon, while hydra was a nine headed monster and is now orbiting the solar system's ninth planet. of course nix and hydra also share initials with the pluto-bound spacecraft new horizons. taken on 24/06/2006.

of all the planets and moons in the solar system, pluto and charon are the two which resemble each other the most closely. they are almost the same size, and they are very close together. they are so close together that they orbit the center of mass between them, and the center of mass goes around the sun. pluto and charon are so close that they may even share an atmosphere. molecules may be drawn off pluto and turn to solids on the surface of charon.

The only other planet and moon in the solar system which could be a double planet are the earth and the moon. as in the case of the earth and the moon, the origin of such a system is hard to explain. the explanation which seems best is that some large object struck pluto (and the earth) and tore out a blob which became charon (and the moon) from the crust of the planet. in the case of the moon, the moon seems to be made of material very much like the earth's crust and not like the earth's more metallic interior. likewise charon may be made of more icy material which resembles pluto's surface, rather than the more rocky interior of pluto.


Charon is pluto's only known moon. it really may be more accurate to call pluto and charon a double-planet system. at about 1,186 km (737 miles), charon's diameter is a little more than half of pluto's. no other planetary moon in our solar system is so close in size to its planet. the duo are also our solar system's only planet and moon whose gravity has locked them into a mutually synchronous orbit, which keeps each one facing the other with the same side. many moons - including our own - keep the same hemisphere facing their planet. but this is the only case in which the planet always presents the same hemisphere to its moon. if you stood on one and watched the other, it would appear to hover in place, never moving across the sky.



Seven years later, charon and pluto began a five-year period of eclipsing each other from earth's point of view. that was lucky for us, because it enabled scientists to measure the diameters and masses of both objects as each passed in front of the other.

Charon appears to be covered by water ice, which differs from pluto's surface of frozen nitrogen, methane, and carbon dioxide. one theory is that the materials that formed charon were blasted out of pluto in a collision. that's very similar to the way in which our own moon is thought to have been created.

moons orbiting pluto

plutos moons

pluto's three moons are shown in the image above orbiting pluto

  • Charon
  • Nix
  • Hydra

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