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The dwarf planet Pluto and its moon Charon

Pluto and its moon Charon: the outermost planet of our solar system

At 2.6 billion miles from the Sun, this is Hubble’s clearest view of Pluto yet.
In July 2015, the NASA New Horizons spacecraft launched in 2006, will reach Pluto and its five moons and send us revealing new photographs.




For tens of thousands of years men had watched seven lights moving across the night skies. These seven "wanderers" (the Greek meaning of planetos) in the sky were the Sun, Moon, Mercury Venus, Mars, Jupiter and Saturn. It is from this retinue of gods that our lucky seven and the seven days in the week are derived. Then in 1781 the astronomer William Herschel discovered a planet beyond Saturn. At first he thought it was a comet, but after a few decades of squabbling over a name it came to be known as the eight planet of the solar system Uranus.

After Herschel's telescopic discovery of Uranus it was noticed that its orbit was not exactly following Newton’s laws of motion. When it was conjectured that an as yet undiscovered even more distant planet must be perturbing Uranus' orbit, many great minds set out to discover this planet X. In 1845 the astronomers John Couch Adams in England and Urbain Le Verrier in Paris used Newton's laws to calculate exactly where this unknown planet must be, and the next year in 1846 Neptune was discovered. In a sensational moment for 19th century science it was near the position predicted from their calculations based on the perturbed positions of Jupiter, Saturn and Uranus. And finally in 1930, 24-year-old Clyde Tombaugh discovered the planet Pluto that for 76 years was to be the ninth planet of our solar system.

Pluto’s 2.6 to 4.6 billion-mile orbit from the Sun is so elliptical that it was actually closer to the Sun than Neptune from 1979 to 1999. With a 17° inclination to the ecliptic (the plane of the Earth's orbit around the Sun), Pluto’s orbit is also the most inclined. Because of these anomalies and its extremely small size—2/3 the size of our Moon—astronomers began to believe that Pluto, its moon Charon, and Triton (the largest moon of Neptune) were merely the largest members of the Kuiper Belt. (Charon takes its name from the river Styx ferryman who carried the dead to Hades/Pluto. The dead were buried with a coin in their mouth or on their eyelids to pay for the crossing.)

The Kuiper Belt is the frozen remains of a disk of primordial material that condensed into our present solar system five billion years ago. As many as 10 billion icy bodies 10 miles or more across orbit the Sun in this region stretching from just beyond Neptune’s orbit to about 9 billion miles out. Many also believed it to be the source of "short-period" comets—comets like Halley’s that take less than 200 years to orbit the Sun (The Kuiper Belt's dynamic stability however, has shifted the source of short-period comets to a region known as the "scattered disk".) The Kuiper Belt is estimated to contain more than 70,000 objects greater than 60 miles in diameter, several hundred times the number and mass of similar sized objects in the asteroid belt. There may be 100 million comets 12 or so miles across in the scattered disk still detectable by Hubble.

In June 2002 a KBO (Kuiper Belt Object) 750 miles across was discovered, the biggest solar system find since Pluto in 1930. Called Quaoar, it orbits the Sun once every 288 years from four billion miles out. Then in March 2004 Sedna (the Inuit goddess who created the arctic sea creatures) was announced. About 1000 miles in diameter with an amazing 10,500-year orbit 8-84 billion miles from the Sun, Sedna was the largest KBO yet. It may even be the first seen member of the Oort Cloud, though that would place it 10 times closer than the Oort Cloud’s theoretical inner limit. Since the Kuiper belt’s sharp edge is just beyond the orbit of Pluto, and Sedna wanders in a no-man’s land between the Kuiper belt and the Oort cloud, astronomers think Sedna was pulled from a more circular orbit by a passing star about 4 billion years ago.

Then in January 2005 a KBO larger than Pluto was discovered. About 1490 miles in diameter (Pluto’s diameter is 1433 miles), 2003 UB313, nicknamed Xena and now known as Eris, appeared to be the tenth planet of our solar system. And it had a moon, as do 10–20% of all KBOs. Its 557-year orbit ranges from 38–97 times the Earth-Sun distance (1 AU), whereas Pluto’s 248-year orbit varies from 30–49 AU. On August 24, 2006, however, the International Astronomical Union (IAU) redefined a planet as an object that:

1) is in orbit around the Sun;

2) has sufficient mass to overcome its compressive strength and to achieve hydrostatic equilibrium; i.e., a nearly round shape;

3) has cleared the neighborhood around its orbit of significantly sized bodies other than its own satellites;

At the same time, the IAU defined a "dwarf planet" as an object that:

1) is in orbit around the Sun;

2) assumes a round shape due to sufficient mass;

3) has NOT cleared its neighborhood of significantly sized bodies other than its own satellites;

4) is not a satellite of a planet or non-stellar body.

Because other objects, one (Eris) even larger than Pluto, were being discovered in Pluto's neighborhood, the Kuiper Belt, Pluto lost its astronomical status as a planet and was demoted to a dwarf planet. As of September 2011 there are five dwarf planets in our solar system: Eris (a TNO [trans-Neptunian object] larger than Pluto); Pluto a TNO in the Kuiper belt; Haumea (originally 2003 EL61, an elongated KBO longer than Pluto and with a moon); Makemake (2005 FY9); and the asteroid Ceres. The four TNOs Sedna, Orcus, 2007 OR10, and Quaoar are virtually certain to eventually be included as dwarf planets. It is estimated that at least 50 already known solar system objects are dwarf planets, and that up to 200 may reside in the Kuiper Belt. If the region outside the Kuiper belt known as the scattered disk is included, that estimate rises to 2000 dwarf planets in our solar system.

Astronomically speaking, Pluto is now no longer a planet; however, that in no way diminishes its place in astrology as a powerful symbol of mankind's shadow and of an individual's personal growth and evolution.

This Planet-Pluto-in-astronomy page and much of this 600-page website are excerpted from the Fine Art Book You and the Universe.

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