The solar system is made up of the sun, nine planets, asteroids, coments and meteoroids.
The planets are divided into two groups: terrestrial (Earth like) and jovian (Jupiter like)
Jovian planets are more massive than terestrial planets. They are: Jupiter, Saturn, Uranus and Neptune.
Terrestrial planets are Mercury, Venus, Earth and Mars. They are denser than Jovian planets.
Planets are composed of gases, rocks and ices - divisions of matter based on melting points.
Terrestrial planets are mostly composed of rocks.
Jovian planets are predominately composed of gases with significant quantities of ices.
Jovian planets have very thick atmospheres compared with terrestrial planets.
The escape velocity of the Jovian planets ranges two to six times greater than Earth's which is 7 miles per second.
Nebular hypothesis - planets and the sun formed at the same time from the same material.
Pluto Not a Planet, Astronomers Rule
Mason Inman for National Geographic News
August 24, 2006
Pluto has been voted off the island.
The distant, ice-covered world is no longer a true planet, according to a new definition of the term voted on by scientists today.
"Whoa! Pluto's dead," said astronomer Mike Brown, of the California Institute of Technology in Pasadena, as he watched a Webcast of the vote. "There are finally, officially, eight planets in the solar system."
In a move that's already generating controversy and will force textbooks to be rewritten, Pluto will now be dubbed a dwarf planet.
But it's no longer part of an exclusive club, since there are more than 40 of these dwarfs, including the large asteroid Ceres and 2003 UB313, nicknamed Xena—a distant object slightly larger than Pluto discovered by Brown last year.
"We know of 44" dwarf planets so far, Brown said. "We will find hundreds. It's a very huge category."
A clear majority of researchers voted for the new definition at a meeting of the International Astronomical Union (IAU) in Prague, in the Czech Republic. The IAU decides the official names of all celestial bodies.
The tough decision comes after a multiyear search for a scientific definition of the word "planet." The term never had an official meaning before.
What Is a Planet Today?
According to the new definition, a full-fledged planet is an object that orbits the sun and is large enough to have become round due to the force of its own gravity. In addition, a planet has to dominate the neighborhood around its orbit.
Pluto has been demoted because it does not dominate its neighborhood. Charon, its large "moon," is only about half the size of Pluto, while all the true planets are far larger than their moons.
In addition, bodies that dominate their neighborhoods, "sweep up" asteroids, comets, and other debris, clearing a path along their orbits. By contrast, Pluto's orbit is somewhat untidy.
A meteroid is a small solid particle. When it enters the Earth's atmosphere it is called a meteor or a shooting star. When some of the meteoroid lands on Earth the remains are called meteorites.
Meteorites are classified as irons (nickel and iron), stony (silicate minerals) and stony-irons (mixtures).