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Astronomy : Planets - Facts I

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Nine Planets

The nine planets in our solar system may be classifed into two groups:

  • Mercury, Venus, Earth and Mars are the Inner Planets. They are dense, rocky, relatively small bodies close to the Sun. They are also referred to as terrestrial (Earth-like) planets.
  • Jupiter, Saturn, Uranus, Neptune and Pluto are the Outer Planets. They are gaseous or icy bodies away from the Sun. The giant planets (Jupiter, Saturn, Uranus and Neptune) are also referred to as Jovian (Jupiter-like) planets because all four planets are made up of gases and encircled by rings. Pluto is the only outer planet made of solid material.

Asteroids are small planet-like objects that orbit the Sun mainly between the orbits of Mars and Jupiter. Thus, the asteroid belt separates the inner and outer planets.

All planets, with the exception of Venus and Uranus, rotate from west to east on their axes. In other words, Venus and Uranus rotate clockwise (retrograde rotation), whereas all the other planets rotate counterclockwise (prograde rotation).

Mercury has the most cratered surface. It greatly resembles the Moon in appearance and size.

Venus is known as "Earth's twin" or "Earth's sister" because it is similar to the Earth in size and mass. Venus is the planet closest to Earth. At times, it is about 25 million miles (40 million km) away from the Earth.

Earth is known as the "watery planet" or "blue planet". It is the largest of the inner planets.

Mars, known as the Red Planet, is at a distance of about 34 million miles (55 million km). It is the planet most likely to support life after Earth. It seems to have the necessary conditions that may make life possible on the planet; however, observations from recent space probes show no life on Mars. Mars was the destination of the 1997 Pathfinder mission. White polar caps, whose size changes with the seasons, are visible on Mars through telescopes. The surface of Mars shows high mountains, enormous volcanoes, deep craters and huge canyons. Olympus Mons (Olympic Mountain) is the highest mountain on Mars as well as the highest mountain and volcano in the entire solar system. Olympus Mons (about 16 miles or 26 km in height) is about 3 times higher than Mt. Everest.

Jupiter is the 'King of the Planets'. The planet is named after the King of Roman Gods because it is the largest and the most massive of the planets. The Great Red Spot of Jupiter, which is equal to two Earth-sized circles, is the biggest hurricane in the solar system.

Saturn is the second largest planet after Jupiter. Saturn's spectacular rings consist of ice-covered rock and dust particles. Saturn and Titan (which is Saturn's moon) were the destination of the Cassini-Huygens mission. After traveling about 2.2 billion miles (3.55 billion km) over 7 years, Cassini successfully flew by Titan in March-April 2005.

Uranus appears greenish in color due to the presence of methane gas (which absorbs the reddish color from sunlight). Uranus (and Venus) rotate clockwise, whereas the rest of the planets rotate anti-clockwise.

Neptune is the last giant planet. It is very similar to Uranus (although slightly smaller and colder than Uranus) and is considered its twin. Neptune is thought to have storms as revealed through the Hubble space telescope in 1994 by the dark spots in its southern hemisphere. It has winds of upto 1250 miles per hour (2000 km/h).

Pluto has an unusual orbit, which is tilted at a steeper angle (17o) compared to the orbits of other planets. Also, Pluto's orbit is so unusually oval that it gets closer to the Sun than Neptune at times.

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