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Beyond Neptune: the Kuiper Belt


Starting from the upper left the four diagrams above pan out in a clockwise direction; upper left: our solar system out to Jupiter’s orbit; upper right: our solar system out to Pluto’s orbit and the Kuiper belt with Sedna at its present location; Lower right: Sedna’s entire orbit; Lower left: Sedna’s orbit within the Oort cloud (the blue circle is actually a slice through the Oort cloud sphere). Compare this with the Oort cloud drawing the Oort Cloud page.


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 Earth’s orbit around the Sun), Pluto’s orbit is the most inclined as well. Because of these anomalies and its extremely small size—2/3 the size of our Moon—astronomers came to believe that Pluto, its moon Charon (named after the river Styx ferryman who carried the dead to Hades/Pluto), and Triton (the largest moon of Neptune, named after Poseidon's son with Amphitrite) were merely the largest members of what is known as the Kuiper Belt.

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 (named for the Inuit goddess who created the arctic sea creatures) was announced. About 1000 miles in diameter and with an amazing 10,500-year orbit 8 to 84 billion miles from the Sun, Sedna was the largest KBO discovered. 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 has a very sharp edge to it, just beyond the orbit of Pluto, and Sedna wanders in a no-man’s land between the Kuiper belt and the Oort cloud, many astronomers believe Sedna was pulled from a more circular orbit by a passing star about 4 billion years ago.

Then in August 2005 a KBO even larger than Pluto was discovered. About 1800 miles in diameter (25% larger than Pluto), 2003 UB313, now known as Eris, is the ninth largest body directly circling the Sun. And it has a moon, Dysnomia, as do 10 - 20% of all KBOs. Its 557-year orbit ranges from 38 to 98 AU (the Earth-Sun distance is 1 AU), whereas Pluto’s 250-year orbit varies from 30 to 50 AU. As of September 2011 the eight largest KBOs—in decreasing size—are Eris, Pluto, Makemake, Haumea, Sedna, Orcus, 2007 OR10 and Quaoar. As larger and larger KBOs turned up the case for Pluto as a planet weakened, and in 2006 the IAU, having redefined a planet as a body that orbits the Sun, is large enough for its own gravity to make it round, and has "cleared the neighborhood around its orbit" of other bodies of significant size except its own satellites, demoted Pluto to a "dwarf planet" (Although this is true astronomically, Pluto still holds a very significant place in astrology as a symbol of humanity’s shadow and of personal evolution). 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.

Beyond Neptune and Pluto we pass through the rest of the Kuiper Belt: the frozen remains of a disk of primordial matter that condensed to our present solar system five billion years ago. As many as ten billion icy bodies ten miles or more across orbit the Sun here, from just beyond Neptune’s orbit out to about 4.5 billion miles (30 to 55 AU from the Sun). The Kuiper Belt was initially believed to be the source of short-period comets like Halley’s that take less than 200 years to orbit the Sun. Recent studies, however, have shown it to be a dynamically stable region, and that short-period comets actually originate in an area now called the "scattered disk" that consists of objects perturbed by Neptune with high orbital eccentricities and inclinations over 30 AU from the Sun.

The Kuiper Belt is now believed to contain over 100,000 objects more than 60 miles wide, several hundred times the number of similar sized objects in the asteroid belt. Beyond the Kuiper Belt we are about 9 billion miles from the Sun, now merely the brightest star in the sky. Here we enter a realm that many believe holds the answer to the ancient question "Where do [long-period] comets come from?"

Short-period comets reside in the scattered disk; but comets that take over 200 years to circle the Sun, "long period" comets, must come from beyond the scattered disk. The true origin of these objects hasn’t been well known or hypothesized—until recently (continue to the Oort Cloud).

This Kuiper Belt-Beyond Neptune page and much of this 600-page website are excerpted from the personalized Fine Art Book You and the Universe.

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© Carl Woebcke, Beyond Neptune: the Kuiper Belt, 1991-2016. All rights reserved.