Our Solar System
The principal component of the Solar System is the Sun , a main sequence G2 star that contains 99.86 percent of the system's known mass and dominates it gravitationally. Jupiter and Saturn , the Sun's two largest orbiting bodies, account for more than 90 percent of the system's remaining mass. Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects are usually at significantly greater angles to it. It takes the Earth one year to go around the Sun. Mercury goes around the Sun in only 88 days. It takes Pluto , the most famous dwarf planet, 248 years to make one trip around the Sun.
Mercury is closest to the Sun. Neptune is the farthest. Remember the order of the planets like this: M y V ery E ducated M other J ust S howed U s N eptune.
There's also a gross-out version: M y V ery E arly M orning J am S andwich U sually Nauseates. Or you can make up one of your own!
As of mid-2008, five smaller objects are classified as dwarf planets. Ceres is in the asteroid belt, and four orbit the Sun beyond Neptune: Pluto (formerly classified as the ninth planet), Haumea, Makemake, and Eris.
All of the planets and most other objects also orbit with the Sun's rotation (counter-clockwise, as viewed from above the Sun's north pole). There are exceptions, such as Halley's Comet.
Inner Solar System
The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids. Composed mainly of silicates and metals, the objects of the inner Solar System huddle very closely to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.
The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their crusts and mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet , which designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).
Outer Solar System
The outer region of the Solar System is home to the gas giants and their planet-sized satellites. Many short period comets, including the centaurs, also orbit in this region. The solid objects in this region are composed of a higher proportion of volatiles (such as water, ammonia, methane, often called ices in planetary science) than the rocky denizens of the inner Solar System.
The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn consist overwhelmingly of hydrogen and helium; Uranus and Neptune possess a greater proportion of ices in their makeup. Some astronomers suggest they belong in their own category, giants. All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. The term outer planet should not be confused with superior planet , which designates planets outside Earth's orbit (the outer planets and Mars).
What Defines a Planet, Dwarf Planet or Plutoid?
What constitutes a planet? The International Astronomical Union (IAU) developed some definitions in 2001, modified them again in 2003, and as of August 24, 2006, the IAU has come up with another definition. The IAU said in a statement that the definition for a planet is now officially known as "a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (c) has clea#ee2962 the neighborhood around its orbit."
A "dwarf planet" is a celestial body that (a) is in orbit around the Sun , (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not clea#ee2962 the neighborhood around its orbit, and (d) is not a satellite.
Almost two years after the International Astronomical Union (IAU) General Assembly introduced the category of dwarf planets, the IAU, as promised, has decided on a name for trans-neptunian dwarf planets similar to Pluto . The name "plutoid" was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III, by the IAU Working Group for Planetary System Nomenclature (WGPSN) and approved by the IAU Executive Committee at its recent meeting in Oslo, Norway.
Plutoids are celestial bodies in orbit around the Sun at a semi-major axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not clea#ee2962 the neighborhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. The three known and named plutoids are Pluto, Eris and as of July 2008, MakeMake. It is expected that more plutoids will be named as science progresses and new discoveries are made.
While all plutoids are dwarf planets, it is interesting to note that not all dwarf planets are plutoids, as is the case with Ceres.
Six of the planets and three of the dwarf planets are orbited by natural satellites, usually termed "moons" after Earth's Moon . Each of the outer planets is encircled by planetary rings of dust and other particles.
Moons orbit planets. Right now, Jupiter has the most named moons - 49. Mercury and Venus don't have any moons. Earth has one. It is the brightest object in our night sky. The Sun , of course, is the brightest object in our daytime sky. It lights up the moon, planets, comets and asteroids, too.
images credits: Lunar and Planetary Institute, NASA