Solar systems & galaxies

The actual definition of a solar system is the sun together with the group of celestial bodies that are held by its attraction and revolve around it, according to Merriam-Webster’s Online Dictionary. The solar system consists of the Sun, the eight official planets, at least three dwarf planets, more than 130 satellites of the planets, a large number of small bodies, the comets and asteroids, and the interplanetary medium. There are probably also many more planetary satellites that have not yet been discovered.

The inner solar system contains the Sun, Mercury, Venus, Earth and Mars.
The main asteroid belt lies between the orbits of Mars and Jupiter. The planets of the outer solar system are Jupiter, Saturn, Uranus, and Neptune; Pluto is now classified as a dwarf planet.

Traditionally, the solar system has been divided into:

1. Planets,
   
2. Their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun)
   
3. And comets (small icy objects with highly eccentric orbits).

However, the solar system has been found to be more complicated than this would suggest:

• There are several moons larger than Pluto and two larger than Mercury.

• There are many small moons that are probably started out as asteroids and were only later captured by a planet; Comets sometimes fizzle out and become different from asteroids.

• The Kuiper Belt objects (including Pluto) and others like Chiron don't fit this scheme well.

• The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets".

**The eight bodies officially categorized as planets are often further classified in several ways:
By composition:

• Terrestrial or rocky planets: Mercury, Venus, Earth, and Mars:

1. The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.

• Jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune:

1. The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.

By size:

• Small planets: Mercury, Venus, Earth, Mars.

1. The small planets have diameters less than 13000 km.

• Giant planets: Jupiter, Saturn, Uranus and Neptune.

1. The giant planets have diameters greater than 48000 km.
2. The giant planets are sometimes also referred to as gas giants.

By position relative to the Sun:

• Inner planets: Mercury, Venus, Earth and Mars.

• Outer planets: Jupiter, Saturn, Uranus, Neptune.

By position relative to Earth:

• Inferior planets: Mercury and Venus.

• Closer to the Sun than Earth.

1. The inferior planets show phases like the Moon's when viewed from Earth.
2. Superior planets = Mars throughNeptune.
3. Farther from the Sun than Earth.
4. The superior planets always appear full or nearly so.

By history:

• Classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.

1. Known since prehistorical times
2. Visible to the unaided eye

• Modern planets: Uranus, Neptune.

1. Discovered in modern times
2. Visible only with optical aid

A galaxy is, by definition, any of numerous large-scale aggregates of stars, gas, and dust that constitute the universe, containing an average of 100 billion (1011) solar masses and ranging in diameter from 1,500 to 300,000 light-years. Also called nebula.

Galaxies come in different shapes and sizes. Usually they look like a flat circle with a bulge in the middle. If we were able to look at a galaxy from above, it would appear as a bright ball with arms spiraling out of it, spinning. Some different types include:

• THE ANDROMEDA GALAXY: The Andromeda galaxy is the closest galaxy to our own, the Milky Way. It is also a similar shape to the Milky Way, although it is four times bigger. It can be visible from Earth on a clear night provided there are no lights nearby illuminating the sky and the Moon is a New Moon and therefore not visible.
• TWO GALAXIES COLLIDING: The smaller galaxy would be similar to The Milky Way in size if the bigger galaxy was the Andromeda Galaxy. It is likely that the smaller galaxy is being attracted to the larger galaxy by the combined gravitational pull of the many billions of stars in its center.

• THE PINWHEEL GALAXY: This galaxy is another spiral galaxy. It spins in an anticlockwise direction, showing its spiral tails spinning around like a Catherine Wheel. These tails are very long and will take millions of years to return to the same spot that they are in now.

• ELLIPTICAL GALAXIES: Elliptical galaxies contain older stars and very little gas and dust. They can be different shapes ranging from round, to flattened, elongated spheres.

***Is there life beyond the solar system? Intelligent life?

• I think there is life...but i really don't like to think about it. It's creepy to think that aliens or a completely different species exists. What do you think???