The Planets
The solar system is the only part of the universe that currently is practical to explore with space probes. The solar system is made up of one star, the Sun, nine planets, 66 moons, and countless asteroids, comets, and meteoroids. The sun is considered a normal star, although some astronomers regard it as a yellow dwarf. But it is the supreme controller of our solar system, and all its members are flooded with thermal energy from it. The solar system is believed to have been formed by accretion from a material cloud, which surrounded the new born sun. The evidence indicates that this happened "only" 5 to 6 billion years ago.
The solar system is divided into two parts; the inner solar system and the outer
solar system. The inner comprises the small solid planets,
Mercury, Venus, the Earth, and Mars. The vast gap between Mars and Jupiter is full of
debris and is called the Asteroid belt. The number of asteroids is in the
millions, although were they all put together they wouldn´t be bigger than the moon.
Beyond the asteroid belt is the outer solar system. That is where the bigger gaseous
planets, Jupiter, Saturn, Uranus, and Neptune reside. Even farther out is
Pluto. Pluto is not a gas giant like the other four, maybe it was a part of the so-called
Kuiper belt, but Pluto is too small and lightweight to be classified as a bona-fide planet.
There are 66 satellites in our solar system. The Earth has one,
the Moon, which is the closest natural body in the sky (excluding occasional wandering
asteroids). Of the other planets, Mars has two small satellites, Phobos and Deimos. Jupiter
has sixteen, the largest is Ganymede; the largest satellite in our solar system. As a matter of
fact, it is larger than the planet Mercury. Saturn has eighteen, Titan being the largest. Uranus has
fifteen, the largest Miranda. Neptune has eight, the largest Triton, and Pluto has one,
Charon.
Comets may be spectacular to view, but their mass is very low
when compared to the planets. Their only solid part is the nucleus. An apt description is
"a dirty ice ball".
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto