Only the thinnest of atmospheres exist on Mercury and Mars. As a result there is a great difference between day and night-time temperatures. On Mercury the change can be as high as 400 °C. Earth and Venus, however, have shielding atmospheres. Their temperatures are fairly constant. On Earth at the equator, this is about 15 °C while most of Venus roasts at nearly 500 °C— hot enough to melt lead!
Orbiting in the 550 million km gap between Mars and Jupiter are tens of thousands of small rocky objects called asteroids. Ceres (recently named a dwarf planet) is the biggest, with only 950 km across—most are only house or boulder-sized.
Astronomers think that the asteroids are the building blocks of a planet which never formed.
