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In August, 2006, at a meeting of the International Astronomy Union, it was agreed that for a celestial body to be classified as a planet, it must meet three basic criteria: 1) It must be in orbit around a star, while not being a star itself. (2) It must be large enough that the pull of gravity has given it a spherical shape. (3) The body must dominate its orbit, having cleared away other objects from its path.

With the adoption of this definition, there are now only eight planets in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.



Dwarf Planets

A dwarf planet is defined as a celestial body that orbits the Sun; is not a satellite of a planet; has sufficient mass to assume a nearly spherical shape; but is not large enough to clear its orbital path gravationally of debris and other objects.

Based on these qualifactions, the Solar System now includes 5 dwarf planets: Ceres, located in the Asteroid Belt, between the orbits of Mars and Jupiter; and Pluto, Haumea, Makemake, and Eris, all of which are in the Kuiper Belt, beyond the orbit of Neptune.

Sand dunes on Mars

Sand dunes on Mars


An extrasolar planet, or exoplanet, is one that orbits a star in a solar system other than that of Earth. The first confirmed discovery of an exoplanet orbiting a main sequence star was in 1995. As of November, 2012, the total number of exoplanets detected is 846, all of them within the Milky Way. The closest exoplanet to Earth is in the Alpha Centauri star system and is designated as Alpha Centauri Bb.

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Solar System

Solar System Facts


This planet's slow rotation and fast solar orbit results in a Mercurial year of 88 Earth days, and a day of about two Earth months.


Unlike other planets in the Solar System, Venus spins not from west to east, but from east to west; and the planet takes longer to spin on its axis than it does to orbit the Sun, so that a Venusian day is longer than a Venusian year. One Venusian day = 243 earth days. A Venusian year = 225 Earth days.

Venus does not tilt as it goes around the Sun, and consequently it has no seasons.


Earth is the densest planet in the Solar System and the only one which is not named after a mythical god.

More than one million Earths could fit inside the Sun.


The largest volcano presently known is on Mars: Olympus Mons, 370 miles wide and 79,000 feet high, is nearly three times higher than Mount Everest, and has a 40 mile wide crater.

Mars also has the longest known rift valley, Valles Marineris. It's between 50 and 300 miles wide, up to 4 miles deep, and stretches 2,500 miles, or roughly the breadth of the United States, from Los Angeles to New York.


The fifth planet out from the sun, Jupiter is so huge that it could contain all of the other planets in the Solar System with room to spare.


Perhaps the most fascinating planet within the Solar System for its beautiful rings, it is 75,000 miles in diametre and 886 million miles from the Sun. Rotates once in about 17 Earth hours; orbits the Sun once in 29.46 Earth years.


Named after Ouranos, the Greek sky god, Uranus is tilted on its axis more than any other planet. While Earth, for example, is tilted by only 23 degrees, Uranus is tilted at about 98 degrees - more or less on its side. This drastic tilt creates the Solar System's longest seasons: winters and summers that are 21 Earth years long!

Only one spacecraft in the history of spaceflight has ever made a close approach to Uranus: NASA's Voyager II in January, 1986, coming within 81,000 km.


After Uranus was discovered in 1781, astronomers noticed that the planet was not keeping to its predicted course in the sky. One possible explanation was that it was being tugged by another planet, as yet unknown, which would be the eighth in the Solar System. In 1846, the French astronomer Urbain Leverrier calculated where this eighth planet might lie and wrote to the Berlin Observatory to ask astronomers there to look for it. The letter arrived on September 23, the birthday of the observatory's director, Johann Encke. Since Encke was taking the evening off, he gave the letter to his assistant, Johann Galle, who, together with the 24-year old student Heinrich d'Arrest (later to win fame through his studies of nebulas) sighted the new planet that evening.

Neptune has the strongest recorded winds in the Solar System, up to 2,100 km/hour.


Discovered in 1930 by the American astronomer, Clyde Tombaugh, Pluto was named after the Roman god of the Underworld by Venetia Phair née Burney. Long considered the nineth planet of the Solar System, Pluto is now officially classified as a dwarf planet by the International Astronomical Union.

On January 19th, 2006, NASA launched the New Horizons spacecraft which flew past Pluto in July 2015 and will explore a Kuiper Belt Object in 2019.

With a diameter of 2,370 km (or 1,470 miles), Pluto could fit between Washington DC and Denver, Colorado.

Pluto's surface temperature is estimated to range between -228oC to -238oC, or -378oF to -396oF.

The distance from Earth to Pluto is roughly 5 billion kilometres (3 billion miles), or 32 times the Earth-Sun distance. Travelling at lightspeed, radio signals from Earth to Pluto take about 4 hours to complete the journey.

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