Countless Small Objects Are Part of Our Solar System's Extended Family
Asteroids are rocky remnants from our early solar system. Many of them reside in the asteroid belt that lies between the inner and outer planets. They range in size from 620 miles (1000 kilometers) down to dozens of meters. Jupiter's powerful gravitational field prevented them from accreting into a planet. Asteroids occasionally reach Earth's surface as meteorites, providing scientists with information about when our solar system formed and the processes that occurred.
Beyond the orbit of Neptune, there is a collection of small, icy planetary bodies that were left over from the formation of our early solar system. This region is called the Kuiper belt. Only a few Kuiper belt objects — such as the dwarf planet Pluto — have been imaged. Occasionally a Kuiper belt object may have a close encounter with Neptune that either flings the object out of the solar system or pushes it into a closer orbit where we may observe it periodically as a comet. Scientists believe that short-period comets, those with orbits less than 200 years, such as Comet Halley, originate in the Kuiper belt.
Beyond the Kuiper belt is the Oort cloud, which also contains icy remnants of our solar system's formation. The Oort cloud is a sphere that envelops our solar system and may extend 30 trillion kilometers (about 20 trillion miles) away from its center. Long-period comets, those that take more than 200 years to orbit our Sun, such as Comet Hale-Bopp or Comet Hyakutake, are believed to come from the Oort cloud. Objects in the Oort cloud are too far and small to be seen.
Don't Forget Pluto!
Pluto didn't fall off the map when the International Astronomical Union (IAU) created a standard definition for the word "planet" in 2006. On the contrary, Pluto joined a whole new class of objects called "dwarf planets." Newly discovered Haumea, Makemake, and Eris are simply too different from massive Jupiter or even steady Mercury to be called by the same name. They belong with the planet-like asteroid, Ceres, in a new group, the dwarf planets.
The search for these bodies is one of the hottest topics in astronomy. Dwarf planets often have enough gravitational clout to hold on to a moon, but not enough to clear their orbits of debris. Unlike comets and many asteroids, dwarf planets collected enough mass in their infancy to form a fairly spherical shape.
Even the Hubble Space Telescope sees the dwarf planet Pluto (and its moons, Charon, Nix, and Hydra) and Eris (and its moon, Dysnomia) as bright "stars" shining with light reflected from the distant Sun. At present, Eris' oval orbit has taken it three times farther from the Sun than Pluto.
Scientists have an active interest in divulging the secrets of Pluto. NASA's New Horizons mission to the dwarf planet will fly by in 2015.