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J. Parker, P. Thomas,
L. McFadden, M. Mutchler,
Z. Levay, STScI, ESA, NASA
Larger visible and uv image.
The IAU definition of a dwarf planet
also includes Ceres, the largest
Main-Belt asteroid (more).
The Dwarf Planet
In 1801, Giuseppe Piazzi (1746-1826) discovered the first as well as largest Main-Belt asteroid ever found, now commonly designated as "minor planet (1) Ceres." Subsequent observations determined that the planetary body has an orbital period of 4.60 years, with a semi-major axis of around 2.77 AUs (slightly farther than Vesta's 2.36 AUs) and an eccentricity of 0.079 and an inclination of 10.58 degrees from the plane of the ecliptic (NASA factsheet). According to one NASA estimate, Ceres may have over one-third of the total mass of the Main Asteroid Belt. Although relatively round, its diameter actually varies between 579 to 597 miles (or around 932 to 960 kilometers) across. Although rocky and icy protoplanets formed in the Main Asteroid Belt (within and beyond the Solar System's "ice line" that is currently located around 2.7 AUs) , the early development of Jupiter prevented protoplanets like Ceres from agglomerating into larger planetary bodies, by sweeping many into pulverizing collisions as well as out into the Oort Cloud or beyond Sol's gravitational reach altogether. On August 24, 2006, the International Astronomical Union voted at the end of its 26th General Assembly to establish a new class of celestial objects in Solar System called a "dwarf planet," which appears to include Ceres.
By the IAU's definition, dwarf planets can be found anywhere in the Solar system. Like other objects in the Main Asteroid Belt, Ceres moves around the Sun between the orbits of Mars and Jupiter. In addition, the next three largest asteroids in the Main Belt (Vesta, Pallas, and Hygeia) may also be "round enough" to be eventually classified as dwarf planets.
Ceres completes a rotation within 9.1 hours. Based on its relatively round shape, density, and rotation rate, astronomers believe that Ceres may have a layered (or "differentiated") interior like Vesta and the four rocky inner planets (which include Mercury, Venus, Mars, and Earth). The dwarf planet appears to have a thin and dusty dark crust that covers a thicker mantle of water ice and a rocky core. Based on its low density (~2.2 grams per cubic centimeters) and the detection of water-bearing minerals on its surface, Ceres may be made of 25 to 30 percent water ice (Hubble news release; and update from: David Shiga, New Scientist, July 17, 2008; and William B. McKinnon, 2008).
Although classified as carbonaceous, Ceres reflects roughly 10 percent of the sunlight that strikes it ("albedo") and so is not as dark as other low-density asteroids called carbonaceous chondrites, which have albedos of around three to five percent. While exposed water ice would not be stable at the surface of Ceres at its relatively close distance to the Sun, water and ammonia do appear to be bound up in crustal minerals on the asteroid's surface. That Ceres has not lost these volatile components from destructive impacts suggests that the asteroid may be a surviving protoplanet from the formation of the Solar System. (More discussion is available from the Planetary Society.)
During July 14-18, 2008, astronomer William B. McKinnon presented his hypothesis that the "largest asteroid" (Ceres) may be a wayward member of the outer Solar System (i.e., Edgeworth-Kuiper Belt) at Asteroids, Comets, Meteors 2008. In addition, Ceres' light spectrum suggests that it may have ammonium-rich clay at the surface, which would fit the expected ammonia-rich composition of large, differentiated substellar objects in the outer Solar System, including the "plutoids" defined on June 11, 2008 by the International Astronomical Union (IAU) as a new class of bright "dwarf planets beyond the orbit of Neptune (IAU press release). [As of September 17, 2008, the four known and named plutoids are Pluto, Eris, Haumea, and Make-make.] In McKinnon's scenario, Ceres formed in Pluto's neighborhood but later migrated inward under the gravitational pull of Uranus and Neptune during a "Nice model" reorganization of the giant planets around 3.9 billion years ago (David Shiga, New Scientist, July 17, 2008; and William B. McKinnon, 2008).
More images of asteroids are available at NASA's Planetary Photojournal. A fact sheet on many asteroids is also available from NASA's National Space Science Data Center, and the IAU Minor Planet Center has historical observational data for minor planet (1) Ceres.
David Seal (a mission planner and engineer at NASA's Jet Propulsion Laboratory at CalTech) has a web site that generates simulated images of the Sun, planets, and major moons from different perspectives and at different times of the year. Try his Solar System Simulator.
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