Selected Kuiper Belt And Oort Cloud Objects In Approximate Size Order


Eris (2003 UBJ|3)

Pluto Sedna (2003 VB12) 2004 DW 50000 Quaoar

Charon 20000 Varuna



28978 Ixion (2001 KX )

1999 CL

15760 (1992 QB)

Approximate diameter (miles [km])

1,430 (2,302) ~1,000 (~1,600) ~1,000 (~1,600) 780 (1,250)

Fraction of Pluto's diameter


Discovery and comments

Mike Brown, Chad Trujillo, and David

Rabinowitz; though there is uncertainty in the measurement of the diameter, the body is certainly larger than Pluto

Clyde Tombaugh, 1930

Mike Brown; may be the first object found in the Oort cloud

Mike Brown and Chad Trujillo; plutino Chad Trujillo and Mike Brown; cubewano

James Christy and Robert Harrington, 1978 Robert S. MacMillan, Spacewatch project; cubewano

Chad Trujillo and Mike Brown; cubewano

Jet Propulsion Laboratory NEAT program; cubewano

Lawrence Wasserman and colleagues at the Deep Ecliptic Survey; plutino Most distant object in the Kuiper belt (perihelion 46.6 AU, no other perihelion farther); cubewano

David Jewitt and Jane Luu; the definitive cubewano and first discovered Kuiper belt object

Note: Diameters marked as approximate (~) may have errors as large as 125 miles (200 km).

Varuna is thought to be about 620 miles (1,000 km) in diameter. Its diameter is therefore less than half of Pluto's. It is still larger than 1 Ceres, the largest asteroid, which is 577 by 596 miles (930 by 960 km). Until Varuna was found, researchers thought that all Kuiper belt objects might have albedos of about 4 percent (with the exception of brilliantly bright Pluto). Varuna seems to have an albedo of about 7 percent. Its brightness energized the science community, since the brighter the objects are, the easier they are to find, and so more searches for Kuiper belt objects might be successful. Initially, Varuna was thought to be as large as Charon, but with more refined calculations of its albedo the discovery team announced that it was in fact significantly smaller. Still, it was and is one of the larger objects yet discovered. Since Varuna, there have been at least four Kuiper belt objects discovered that are larger.

In July 2001 Robert Millis and his colleagues at the Massachusetts Institute ofTechnology, Lowell Observatory, and the Large Binocular Telescope Observatory in Arizona discovered 28978 Ixion (originally 2001 KX ), a large Kuiper belt object (this was largely the same team that discovered Uranus's rings in 1977). Along with Varuna, Ixion is larger than 1 Ceres (see figure on page 139). Ixion's size was highly uncertain when it was discovered because the telescope through which it was discovered had neither the high resolution required to measure size directly (though the Hubble Space Telescope can do this) nor the ability to measure infrared radiation, which is related to size. At the time of discovery, the team thought Ixion was at least 750 miles (1,200 km) in diameter.This estimated size made a big media splash, since at the time it would have been the largest Kuiper belt object after Pluto and in fact larger than Charon itself. Further study shows Ixion to be only about 580 miles (930 km) in diameter, and in the intervening years a number of larger Kuiper belt bodies have been discovered, including at least two that really may be larger than Charon.

In June 2002 California Institute of Technology scientists Chad Trujillo (now at the Gemini Observatory) and Mike Brown saw for the first time a Kuiper belt body with the preliminary name 2002 LM , later named 50000 Quaoar (pronounced "KWAH-o-wahr"). Quaoar is named for the god found in the creation stories of the Tongva tribe, early inhabitants of what is now southern California. Quaoar lies at about 42 AU from the Sun. Its orbit takes about 285

Comparison of Pluto, Charon, Sedna, Etc.

Pluto radius: 715 miles (1.151 km)


Varuna radius: -310 miles (-500 km)

Mercury radius: 1,515 miles (2,440 km)

Earth's Moon radius: 1,079.6 miles (1,737.4 km)

Sedna First observed Oort cloud object?

Ceres Largest asteroid in the main belt

Kuiper belt objects are labeled at the top; other objects at the bottom. Bodies in italic may have errors in their diameters as large as 125 miles (200 km)

Earth years and is almost circular, with an eccentricity of only 0.04 and an inclination of about eight degrees. Pluto's orbital eccentricity is about six times larger than that of Quaoar's, and Pluto's inclination is about twice Quaoar's. Because Quaoar is so bright, within a month of discovery they were able to trace its position back two decades in previously taken telescope images. Quaoar has a diameter of 780 miles (1,250 km), about half the size of Pluto. Quaoar was at the time of its discovery the largest solar system body found since Pluto itself.

In February 2004 Brown, Trujillo, and their colleague David Rabinowitz from Yale University had a new announcement about the then-largest-known Kuiper belt object, having found a new object designated 2004 DW that is still larger than Quaoar. Based on its current distance of about 48 AU from the Sun, its brightness, and its presumed albedo, 2004 DW has been estimated to be around 870 to 990 miles (1,400 to 1,600 km) in diameter, or more than half the size of Pluto. As with many other found objects, once they are identified they can then be found in photographs from sky surveys in the past. The object 2004 DW has been found in a First Palomar Sky Survey photo-

A comparison of the sizes of Mercury, the Moon, Pluto, and a series of minor planets shows that all known Kuiper and asteroid belt objects are smaller than Pluto, but even Pluto is smaller than Earth's Moon.

graph of November 23, 1954, and in a November 8, 1951, photograph from the Siding Spring Observatory in Australia. It appears to be a plutino with an orbit that carries it from 30.9 and 48.1 AU with an orbital inclination of about 20.6 degrees. It requires 248 years to complete its orbit. It reached aphelion in 1989 and will reach perihelion in 2113.

In July 2005 the hardworking team of Brown, Trujillo, and Rabinowitz made their historic announcement:They had confirmed the existence of a Kuiper belt body larger than Pluto. The scientists had searched for outer solar-system objects using the Oschin Telescope at Palomar, California. It has a mirror diameter of 3.9 feet (1.2 m), which is large compared to amateur telescopes (typically ranging from 0.3 to one foot [0.1 to 0.3 m] in diameter), but small compared to most professional telescopes (3.28 to 32.8 feet [1 to 10 m] in diameter). The scientists examined a sequence of high-resolution telescope images of the same region in space, looking for objects that moved relative to the starry background (the stars are moving, also, but they are so far away relative to Kuiper belt objects that their movement is undetectable over small times). This new body, temporarily named 2003 UB313, was first seen in 2003, but only recognized to be moving relative to background stars in January 2005. Now it is known to orbit at 44 degrees to the ecliptic, with a perihelion of 36 AU and an aphelion of 97 AU. It is thought to have a diameter of about 1,500 miles (2,400 km), making it the largest solar system body discovered since the discovery of Neptune in 1846. Discovery of this object, now known as Eris, caused the International Astronomical Union to redefine solar system bodies and reclassify Pluto as a dwarf planet.Trujillo and Brown have said they expect to find five to 10 more large objects, and perhaps some larger than Pluto.

Though Jewitt and others think Kuiper belt objects as large as Mars may remain undiscovered in the outer Kuiper belt, it is unlikely that any Kuiper belt object exists of a size comparable to the larger planets. Any large body in the outer solar system would have perturbed the paths of the spacecraft Voyager 1,Voyager 2, and Pioneer 10 (see the lower color insert on page C-7) as they passed the solar system, and should further influence the orbit of comet Halley. To have avoided changing the orbits of these objects, any remaining large Kuiper belt objects are thought to be less than five Earth masses, which in its turn is far larger than the estimated total mass of the Kuiper belt. According to Kuiper's original calculations as well as computer simulations of losses to the inner solar system and gravitational expulsions into outer space, the Kuiper belt's original mass in the beginning of the solar system was the equivalent of about 30 Earth masses. Now it is thought to be 0.2 Earth masses, or about 100 times the mass of the asteroid belt.

The figure below demonstrates the loss of mass from the inner solar system, particularly from the asteroid belt. The mass of matter at a given orbit divided by the area of the orbit is plotted on the verti- The density of material in the cal axis as a measure of density of material existing at that distance solar system decreases with from the Sun. The horizontal axis measures distance from the Sun in distance from the Sun.

Density of Solar System with Distance from the Sun

Density of Solar System with Distance from the Sun

AU from the Sun

AU from the Sun

AU.Theories and models of the solar nebula clearly indicate that densities in the solar nebula should smoothly decrease from the Sun outward, but this graph clearly shows that the decrease in density is not smooth. Could the excess mass needed to smooth this graph exist in the outer solar system as more moderately large bodies, like Pluto? There may be bodies even as large as Mars remaining undiscovered in the outer solar system.

Part Three: Beyond the Kuiper Belt

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