Wednesday, January 14, 2009

Let’s Take a Stroll

Our solar system is centered on a single star, the sun. We have recently come to appreciate that about 50 percent of all stars form in binary systems (containing two stars), so our sun is a bit lonely as stars go. In orbit around the sun are nine planets (in order of distance from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Around some of these planets orbit moons—more than 70, at latest count. By the 1990s, astronomers had observed more than 6,000 large asteroids, of which approximately 5,000 have been assigned catalog numbers.
(Such an assignment is made as soon as accurate orbital data is recorded.) Most asteroids are rather small; it is estimated that there are 1 million with diameters greater than 1 km (or about 3⁄5 of a mile). Some, perhaps 250, have diameters of at least 62 miles (100 km). About 30 have diameters of more than 124 miles (200 km). All of these planets and asteroids are the debris from the formation of the sun. They coalesced through the mutual attraction of gravity.
In addition, the solar system contains a great many comets and billions of smaller, rock-size meteoroids.

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