3
　　What other conditions are necessary/to support life?// Temperature is important.// It must not be/too hot or too cold.// The orbit of Earth is “just right”:/not too close to the sun,/where water would boil,/and not too far from the sun,/where water would freeze.//
　　In 2011,/a planet was found/orbiting a star called Gliese 581.// This planet is rocky/and appears to have the right temperature.// Nobody is suggesting that it has life.// But/since it was discovered/soon after we started looking,/we assume/that there are lots of life-supporting planets/out there.//
　　Size is also important.// A planet’s size/—more strictly its mass—/has an impact upon life/because of gravity.// If Earth were made of gold,/the gravity would be over three times as great/as it is now.// The gravity would be so strong/that a mouse would need thick bones/and walk like a tiny rhinoceros,/which is unlikely to happen.//
　　Just as gold is heavier/than iron, nickel and the other things that Earth is made of,/coal is much lighter.// If Earth were made of coal,/the gravity would be about one-fifth as strong/as it is now.// A rhinoceros could jump around/on thin legs like a spider,/which is, again, unlikely to happen.//