You may have seen planet Venus, Mars, Jupiter, or Saturn but it is difficult to see Mercury, Uranus, Neptune, and Pluto. Some planets can be seen because they reflect sunlight.
Distances
Astronomers use astronomical unit, or AU, to measure the average distance of Earth from the Sun. An AU is about 150million kilometers.
Mercury is less than 0.5 AU from the Sun
Jupiter is about 5 AU from the Sun
Pluto gets nearly 50AU from the Sun at times.
The planets are spaced unevenly. They move around the Sun and the first four planets that are close to the Sun define a region called the inner solar system.
The solar system formed from a swirling cloud of gas and dust.
The planets all orbit the Sun counterclockwise and most of them rotate on their axes in this direction.
According to the best scientific model, the solar system formed out of a huge cloud of different gases and specks of dust.
The most of the mass fell to the center and became the Sun.
At the same time, tiny bits of dust and frozen gases in the disk stuck together into clumps and large clumps became planets.
Some of the objects close to the Sun are like rocks or mountains in space and are called asteroids. Other objects, farther from the Sun, are more like enormous snowballs or icebergs. (comets)
depending of its shape, you can understand the size of an object in space.
* Lumpy objects->smaller than round objects
The gravity of each part affects every other part. The pieces pull each other closer and when an object has enough mass, pulling becomes strong enough to make the object round. Any parts that would stick far out are pulled in toward the center until the object becomes a sphere.
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