The inner planets are Mercury, Venus, Earth and Mars.
Mercury is the planet closest to the Sun and the smallest. It rotates on its own axis in 58.65 Earth days, while it takes 88 days to complete one revolution round the Sun.
According to Gerard Kupier, Mercury was probably twice as massive originally but the Sun evaporated away half its substances.
When it is closest to the Sun, temperatures reach 345°C (650°F) on the equator though they probably drop during the long night to minus 300°F (-150°C).
From the Earth, it can sometimes be seen for a short time as an Evening Star just after sunset or as a Morning Star just before dawn.
Venus, the closest planet to the Earth, is also the brightest planet in the Solar System. Venus is slightly smaller than the Earth, being some 480 km less in diameter. Venus, unlike most other planets, rotates backward.
It takes the planet 225 days to make one orbit round the Sun. Venus has no satellites like Mercury, no rings like Saturn and no ice caps like Mars.
The earth is the third nearest planet to the sun. In size, it is the fifth largest planet. It is slightly flattened at the poles. That is why, its shape is described as a Geoid. Geoid means an earth-like shape.
Conditions favourable to support life are probably found only on the earth. The earth is neither too hot nor too cold. It has water and air, which are very essential for the survival. The air has life-supporting gases like oxygen. Because of these reasons, the earth is a unique planet in the solar system.
From the outer space, the earth appears blue because its two-thirds surface is covered by water. Therefore, it is called blue planet.
Mars is the fourth planet from the Sun. When Mars is favorably situated, it is brighter than most of the stars and is definitely red (Red Planet).
Mars polar caps are similar to those of the Earth and because of Martian axis is tilted at almost the same angle as the Earth's, its polar regions are exposed to sunlight in alternation, giving each hemisphere summer and winter.
The relative orbits of Mars and Earth bring them very close, a little more than 50 million kilometres, on two occasions about 2 years apart and then remove them far apart for another 15 years.
Mars is a dusty planet, with the dust often in motion, which sometimes envelop the major portion of the planet in a dusty haze.
Mars has two small satellites.