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Because it is so far from the Sun, astronomers had a hard time measuring Pluto's size. They finally got it right in the 1980s, after James Christy discovered a companion object. By watching Pluto and the companion, named Charon, eclipse each other, they measured Pluto's diameter at about 1,400 miles -- about one-third less than the diameter of Earth's Moon.

Pluto is basically a ball of frozen nitrogen, methane, and carbon dioxide wrapped around a small core of rock. On average, it's farther from the Sun than any other of the major planets, so its surface is bitterly cold.


For astronomers, though, a comet's beauty is not just in its appearance, but in its content: A comet is an icy time capsule that contains some of the original material from the cloud of gas and dust that gave birth to Earth and the other planets.

Current theory says that the planets formed as small grains of ice and rock stuck together to form larger bodies, called planetesimals, which then merged to form the planets.


But the next year, astronomers found a second object in a similar orbit, then another, and another. Instead of a planet, they had found the first of the asteroids -- large chunks of rock that were left over from the formation of the solar system. Ceres is the largest, with a diameter of almost 600 miles (1,000 km).

Astronomers have discovered hundreds of thousands of these boulders, and with automated searches they are discovering thousands more each year. Most orbit the Sun in a broad region known as the asteroid belt, which is between the orbits of Mars and Jupiter.


After the discovery of Uranus in 1781, astronomers had a difficult time accurately describing the planet's orbit. Uranus seemed to be pulled by the gravity of an unseen planet. Working independently, Leverrier and Adams calculated the position of such a planet and passed it on to astronomers. When Galle aimed his telescope at that position, he quickly detected the new world.


Like its larger sibling, Jupiter, Saturn is a ball of hydrogen and helium gas wrapped around a heavy core. Instead of an abrupt boundary between them, the core and the surrounding layers probably flow together.

Saturn spins so fast that it bulges outward at the equator, so the planet is much thicker at the equator than through the poles.


The poles of most planets aim away from the plane of the solar system; like Earth's poles, they point roughly north and south. But the poles of Uranus point east and west, close to the plane of its orbit around the Sun. As a result, its north pole aims at the Sun at the beginning of northern summer, and away from the Sun at the start of southern summer. That gives the planet's northern and southern hemispheres 42 years of sunlight followed by 42 years of darkness.


As the Great Red Spot demonstrates, Jupiter is a world of superlatives. It is the largest planet in the solar system (big enough to swallow more than 1,300 Earths, and bigger than some types of stars), and more massive than the solar system's other known planets and moons combined. It rotates faster than any other planet, it produces the most powerful magnetic field, and it offers the largest and most interesting assortment of moons.


Scientists, too, have speculated about the possibility of life on Mars. During the 1800s, some leading astronomers thought that an advanced civilization inhabited the planet -- a civilization that built vast "canals" to sustain a dying world. But investigations with increasingly sophisticated ground-based telescopes, followed by the Mariner and Viking missions of the 1960s and '70s, crushed the dreams of grand Martian cities. Mars is too cold and too dry, and its air is too thin, to sustain large organisms.


But it was only in the sixteenth century, around the time of Copernicus, that people began to see Earth as a planet at all -- akin to the "wanderers" that were seen crossing the night sky against the stable background of stars. Since then, telescopic and probe-based studies of other bodies in the solar system have put our remarkable home into a broader context.

We know that Earth is the largest of the "terrestrial" planets in our solar system -- the four rocky planets close to the Sun.