Are We Alone?
From ancient times,/people have wondered/if there are other planets like Earth.// That is still an open question today.// Richard Dawkins tells us/that we are getting closer to an answer.//
　　Is there life on other planets?// Nobody knows for sure,/but I’d say yes.// One day/we may have clear evidence/of life on other planets,/but for now,/the best we can do/is to reduce the uncertainty.//
　　The first thing we might ask/is how many planets there are.// People used to believe/that the ones orbiting our sun/were the only ones.// But now we know/that in our galaxy/most stars have planets.// So,/we can probably conclude/that most of the stars in the universe/have planets.//
　　The number of stars in our galaxy/is about 100 billion,/and the number of galaxies in the universe/is about the same.// That means/10,000 billion billion stars/in total.//
　　According to astronomers,/only about 10 percent of stars/are like the sun.// Stars that are very different from the sun/are unlikely to support life on their planets/for various reasons:/for example,/stars that are much bigger than the sun/will not last long enough before exploding.// But even if we are talking/only about the planets orbiting sun-like stars,/we are dealing in billions of billions.//

　　There being so many planets in the universe,/the chances are great/that there is life somewhere out there.// But not all planets support life.// Most extra-solar planets are “gas giants,”/which are unsuitable for life/as we know it.// Of course,/that doesn’t mean/that life as we know it/is the only possible kind of life.// There might be life on gas giants,/although I doubt it.// We don’t know/what proportion of planets are rocky like Earth.// But even if the proportion is quite low,/the number will still be high/because the total is so huge.//
　　Scientists searching for extraterrestrial life/regard water as essential.// If there is no water in liquid form,/life can probably not exist.// Ice won’t do,/nor will steam.// Mars shows evidence of liquid water,/in the past if not today.// Europa,/one of the moons of Jupiter,/is covered with ice/and under the ice/there could be a sea of liquid water.//
　　People once thought/Mars was the most likely planet/to have extraterrestrial life/within our solar system.// Now Europa has taken the place of Mars,/but most scientists think/we must keep looking.// Evidence suggests/that water is not particularly rare/on extra-solar planets.//

　　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.//

　　To sum up,/in order for a planet to support life,/it has to satisfy certain conditions.// First,/it must have water in liquid form.// Second,/the temperature must be just right/—not too hot or not too cold—/so that water may remain liquid.// Third,/the mass,/which determines the gravity of the planet,/must be just right/—not too big or not too small.// In short,/a habitable planet must be/in a so-called “Goldilocks Zone”:/“just right” between two extremes.// Even though the conditions which sustain life/are very special,/it is likely/that life exists elsewhere/because there are so many planets.//
　　Although we have not discovered life on other planets,/I hope to have shown/how much science can tell us.// Our search for life is not random;/our scientific knowledge equips us/to seek out meaningful information/and to identify habitable planets.// We need much more information/than we have now,/but we can at least ask sensible questions/and get sensible answers.// We don’t have to invent implausible stories;/we have the joy/of real scientific investigation and discovery.// In the end,/that is more exciting than science fiction.//