Science fiction writers may invent planets other than our own as wonderful places to live but as far as we know there is nothing like Earth in this solar system or in any other. Our Sun is a long-lived stable star with a reliable energy output over billions of years, and our planet is just the right distance away to avoid getting overheated like Venus or being out in the cold like Mars. Earth is big enough (that is, its gravity is strong enough) to retain an atmosphere (which would escape on a small planet like the Moon). Also, the atmosphere is dense enough and has the right composition to form a pleasant greenhouse blanket. Seventy percent of Earth is covered by water, which ensures there is plenty of water vapor available for the atmosphere, and this, in turn, ensures that the greenhouse effect is strong enough for producing an average balmy 60°F on the surface of Earth.