On a vast scale maybe millions of years the Earth ' s ability to sustain human life is limited . Now , odds are our species ' life expectancy is far far more limited , but if we are to have any chance to outlive our planet , we must colonize other planets eventually planets orbiting other stars , because our sun also has a limited lifespan . The ability of the Earth to sustain human life might also be compromised a lot sooner . Not just because of things we do climate change , nuclear war , etc . the greatest damage we can do won ' t measure up to things that have already happened to the Earth ice ages , meteor impacts , etc . . Life on Earth has undergone multiple extinction events , and one might happen in the future that would wipe us out . Or we might wipe ourselves out . So , for the indefinite continuation of the human species , leaving Earth is a necessity . If we don ' t leave Earth , we absolutely will not go on indefinitely . If we do , we might not for ever , but for some arbitrarily long time , maybe no guarantees . These aren ' t necessarily imminent concerns if we correct course hard enough , the Earth might sustain us for a long time , barring unexpected catastrophies but they ' re eventual , unavoidable concerns . If postSol humanity happens , it ' ll probably bear little resemblance to us now , but the urge to go on forever is a pretty understandable one .