10 March 2008

A New View of the Solar System

From: http://www.statesman.com/news/content/news/stories/nation/03/09/0309solarsystem.html
First, until recently, people thought that there were two parts to the solar system: four small, rocky inner planets — Mercury, Venus, Earth and Mars — and four gas giant outer planets — Jupiter, Saturn, Neptune and Uranus..."That was the old view,"...Under the new definition, the International Astronomical Union has officially recognized 11 planets: eight traditional ones plus three "dwarf planets." The dwarfs are Pluto; Ceres, which was thought to be an asteroid between Mars and Jupiter; and Eris, an object that's slightly larger than Pluto and farther from the sun. At least 40 more dwarf planets have been found even farther out and are awaiting official recognition. They bear names such as Quaoar, Sedna, Orcus, Varuna and Ixion. Dozens of others are known only by code numbers.

Stern said the solar system now was thought to be composed of three zones instead of two.
The four rocky planets make up the inner zone. The gas giants form a "middle solar system." Beyond them lies an enormous third zone composed of the Kuiper Belt and the Oort Cloud, both named for the astronomers who predicted their existence.
"This third class of planets vastly outnumbers the terrestrial planets and gas giants," Stern said.
The Kuiper Belt, which was discovered in the 1990s, is a ring of dwarf planets, including Pluto, and smaller icy objects that range from 3 billion to 5 billion miles beyond the sun.
More than 1,000 Kuiper Belt objects have been detected, and astronomers say there might be 50,000 to 100,000 more. Most are small, but some rival Pluto in size. Some have atmospheres and moons of their own, and some may have warm, wet interiors.
Far outside the Kuiper Belt looms the Oort Cloud, which Stern calls "the solar system's attic." The cloud is a gigantic sphere with an outer edge almost 5 trillion miles from the sun and is nearly a quarter of the way to the next nearest star, Proxima Centauri.
"If the Oort Cloud were the diameter of a football stadium, the inner solar system would be the size of a washer one-eighth of an inch in diameter," said David Aguilar, a spokesman for the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
The Oort Cloud is thought to contain at least 1,000 planetary bodies, some as large as Earth or larger, and as many as a trillion comets. Periodically, a passing star knocks an Oort comet loose and sends it diving toward the sun.
A second upheaval in planetary science is the realization that most members of the solar system weren't born where they are now. Instead, gravitational forces forced them to migrate from their birthplaces to their present homes.
"This is a true revolution," Stern said.
"The planets didn't necessarily form where we see them today," said Douglas Lin, an astronomer at the University of California-Santa Cruz. "They move all over the place."
For example, the giant planets Neptune and Uranus formed where Saturn now lives and drifted outward to their present orbits.
The third revolution in planetary science is the realization that making planets is a common process in the universe. In the past dozen years, 276 planets — some of them forming miniature solar systems — have been detected orbiting other stars. One such system has at least five planets. Astronomers discover an average of 25 planets each year.
NASA will launch a new spaceship, named Kepler, next year hoping to find hundreds more of such faraway planets.

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