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Pluto removed from the list . . .
Eight Major Planets in our Solar System
For decades, we have counted nine of the largest bodies of the Solar System as planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
Saturn
Now, the International Astronomical Union, for the first time, created in 2006 a scientific definition for the word planet, demoting the former major planet Pluto to the lesser status of dwarf planet.
At the same time, it promoted the distant, deep-space object Eris and the big asteroid Ceres to dwarf planet.
Eris has been referred to by astronomers as 2003 UB313 and sometimes was called Xena and the tenth planet. The IAU's action made Eris the largest known dwarf planet. It's larger than Pluto.
Two other very distant Solar System bodies are being considered for the category of dwarf planet.
Eris is in the Kuiper belt, a disk-shaped ring of ice that circles the Sun at distances far beyond the major planet Neptune. It holds at least 70,000 small objects. The Kuiper belt is named after Dutch-American astronomer Gerard P. Kuiper, who predicted its existence in 1951. It finally was discovered in 1992. Comets such as Halley's may come from the Kuiper belt.
Planet
NameDiameter
in milesSun Distance
in milesMercury 3,032 36 million Venus 7,543 67 million Earth 7,926 93 million Mars 4,217 142 million Jupiter 88,732 483 million Saturn 74,975 870 million Uranus 31,763 1.8 billion Neptune 30,775 2.8 billion Pluto 1,429 3.7 billion
How big are they? As you might expect, dwarf planets are smaller than major planets. Even Earth's Moon is bigger than the dwarf planets.
- Earth is 7,926 miles in diameter.
- Earth's Moon is about 2,100 miles in diameter.
- Neptune's diameter is about 30,775 miles.
- Eris is about 1,500 miles in diameter.
- Eris has a moon named Dysnomia and also known as Eris 1. It is about 155 miles in diameter.
- Pluto is about 1,413 miles in diameter.
- Pluto's moon Charon is about 727 miles in diameter.
- Quaoar and Orcus are about the same size as each other at about 800 miles in diameter, and just a bit more than half the size of Pluto. They are larger than Pluto's moon, Charon, which is about 728 miles.
How far away are they? Earth is third outward from the Sun. Neptune is eighth. Pluto is ninth. Now, Pluto is downgraded in its classification to dwarf leaving only eight planets of the Solar System described as major.
Earth is only 93 million miles from the Sun, while Pluto's average distance from the Sun is 3.6 billion miles. At 9.7 billion miles, Eris is three times farther from the Sun than Pluto.
Eris is more distant than the planetoid Sedna discovered in 2003. Sedna, also designated 2003 VB12, is 2.5 billion miles beyond Pluto.
Eris, Sedna, Quaoar and Orcus are large dark objects orbiting the Sun way beyond Neptune along the far distant reaches at the edge of our Solar System in the so-called Kuiper Belt swarm of icy objects. Astronomers think they are remnants of ancient materials that formed the Solar System.
Quaoar is pronounced "kwa-whar." It also is known as 2002 LM60.
Sedna is 2.5 billion miles beyond Pluto, while Quaoar is more than a billion miles farther away than Pluto. Quaoar is so far away, it takes light from the Sun five hours to reach it. The object Orcus is 4.4 billion miles from Earth.
How the planets differ. The remaining eight major planets of the Solar System differ in some ways from Pluto and the dwarf planets:Pluto and the other dwarf planets are different:
- The so-called terrestrial planets – Mercury, Venus, Earth, Mars – are medium-sized rocky bodies.
- The so-called gas giants – Jupiter, Saturn, Uranus, Neptune – are much larger.
- The orbits of the eight major planets are nearly circular and in a thin disk around the sun.
- They are made of ice and rock.
- They travel very eccentric orbits.
- Their orbits tilt away from the rest of the planets.
How long have we known? Some of these Solar System bodies have been recognized for a long time and some have been discovered only recently:
- 1801 – Ceres, the largest of the asteroids, was the first asteroid ever discovered.
- 1846 – The planet Neptune and its moon Triton were discovered.
- 1930 – Pluto was discovered and called a planet.
- 2000 – The far-distant planetoid Varuna, also known as 2000 WR, was discovered.
- 2001 – The far-distant planetoid Ixion, also known as 2001 KX, was discovered.
- 2002 – The possible dwarf planet Quaoar, also known as 2002 LM60, was discovered.
- 2003 – The potential dwarf planet Sedna, also known 2003 VB12, was discovered.
- 2003 – The dwarf planet Eris, also known as 2003 UB313, was spotted.
- 2004 – The planetoid Orcus, also known as 2004 DW, was found
- When a large object is found, the International Astronomical Union decides what it will be named officially.
- Planets, moons and minor planets are given names from Greek and Roman mythology.
- For instance, Venus was named for the Roman god of love because it was said to be the most beautiful planet. Mars was named for the Roman god of war because of its blood red color.
- If the IAU were to name 2003 VB12 something other than Sedna, it wouldn't be the first time names proposed by astronomers have been changed.
- For instance, astronomer William Herschel was court astronomer for the English king George III. In 1781, when Herschel found the planet we now know as Uranus, he tried to name it Georgian Sidus – the Georgian Star – after the king. Astronomers didn't like the name and the planet eventually was named after the Greek god of the sky, Uranus. The Greek name for the god was Ouranos. Mars, Jupiter, Saturn, and Uranus were a sequence of generations in mythology.
- Of course, there are only so many names in classical mythology, so another source of names will be needed as more minor planets, stars, asteroids and comets are found.
- There is an interest in making astronomy inclusive of more of Earth's races and cultures by using non-traditional names.
- Sedna's official number, 2003 VB12, was composed from the discovery year, month and date.
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Read more about the Solar System . . . Star: The Sun Inner Planets: Mercury Venus Earth Mars Outer Planets: Jupiter Saturn Uranus Neptune Pluto Other Bodies: Moons Asteroids Comets Kuiper Belt Beyond: Pioneers Voyagers
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