In the past when a country's population became overcrowded or drought, flood or other natural disasters caused the inhabitants to look beyond their country's borders, they explored other parts of their own continent and eventually the continents around the world.
But what happens when Planet Earth becomes overpopulated? What would happen if a global disaster were to take place?
That is precisely what NASA has been working to answer. And the answer is to find another planet now capable of supporting life.
The Kepler Mission has recently discovered two new planetary systems in what is termed the "habitable zone" - an area the perfect distance from the sun, in which the planet is neither too hot nor too cold for human life and there is an abundance of water to support life.
The first planetary system discovered is called Kepler-62 and it contains five planets orbiting a star that is about two thirds the size of our sun and only about one fifth as bright. Of the five planets, one is 40% larger than Earth and the other is about 60% larger than Earth. They are around 1,200 light-years away from Planet Earth.
The second planetary system discovered is called Kepler-69 and consists of two planets. They both orbit a sun that is about 93% of the size of our own sun and about 80% as bright. One planet is about 70% larger than Earth. It is around 2,700 light-years from Earth.
The Kepler space telescope is the first of its kind that can detect planets revolving around stars much like our sun. So far, it has detected over 2,740 candidates - planets that are in a habitable zone much like our own.
Perhaps one day our descendants will explore new planets in much the same way our ancestors explored new continents.