Pluto removed from the list . . .

Eight Major Planets in our Solar System

Image of the planet Saturn by the Cassini spacecraft
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.

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.

Planets: How Big, How Far?
in miles
Sun Distance
in miles
Mercury3,03236 million
Venus7,54367 million
Earth7,92693 million
Mars4,217142 million
Jupiter88,732483 million
Saturn74,975870 million
Uranus31,7631.8 billion
Neptune30,7752.8 billion
Pluto1,4293.7 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.

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.

The Kuiper Belt
  • There are objects orbiting the Sun beyond the planet Neptune in a formation that astronomers call the Kuiper Belt.
  • Together, the numerous objects seem to form a disc or belt around the Sun.
  • More than 400 of these icy objects are known, and there may be many more.
  • They seem to be remnants of materials near the Sun as our Solar System formed.
The Oort Cloud
  • The large planetoids, such as Sedna and Quaoar, as well as comets and other smaller bodies, may be part of something astronomers call the Oort Cloud.
  • The Oort Cloud may have been formed by gravity from a rogue star near the Sun in the early days of the Solar System.
  • Our star may have been born into a cluster of stars, which means there would have been many stars close to the Sun back then.
  • The Sun and Earth are estimated to be 4.6 billion years old.
  • Sedna, Quaoar and the other large planetoids might represent the first actual detection of the previously suspected collection of icy bodies.
  • The belt of objects seems to be circling the Sun in a spherical orbit that extends as far as 3 lightyears out from our star.
  • That would be at the extreme edge of the Sun's gravitational influence.
  • The Oort Cloud may extend out from our star as far as half the distance to the nearest star.
  • Sedna is ten times closer to the Sun than the distance that had been predicted for the Oort Cloud, which raises a question of whether there is an inner and an outer Oort Cloud.
  • Sometimes comets from the Oort Cloud may be pushed by passing stars into the inner area of the Solar System near Earth.
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:

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:

How Do Planets Get Names?
  • 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.


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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