Astronomers have a keen interest in finding earth-like planets orbiting other stars. Their hope is that earth-like planets might harbor life. This hope is based upon the belief that life arose naturally on earth, that is, without a Creator. This worldview assumes that there is nothing special about the earth, so life probably develops wherever the conditions are right. Therefore, life ought to be common in the universe, if the conditions are right in enough places.
The Right Conditions for Life
But just where are the conditions for life right? Until about twenty years ago, we didn't know if other stars even had orbiting planets, but we now know that many stars have planets orbiting them. However, most of those planets are not the proper distance from their stars for life to exist. If a planet is too close to the star it orbits, the planet is too hot to have liquid water; if a planet is too far away from its star, it is too cold for liquid water.
Since liquid water appears to be an essential ingredient for life, any planet outside of a narrow habitable zone around its star where liquid water could exist would seem to be eliminated as a possible home for life. But there is far more required for life to exist on a planet. Planets in a star's habitable zone must have the proper composition and size. Of about 2,000 planets so far discovered around other stars, none of them are suitable for life.
The Right Type of Stars
Even the type of star that a planet orbits is important. Massive stars are very hot and bright. Such stars emit large amounts of ultraviolet radiation. Ultraviolet radiation is harmful to life, but the earth’s upper atmosphere shields the earth's surface from most of the sun's modest ultraviolet radiation. However, it is doubtful that any atmosphere otherwise conducive for life could effectively shield a planetary surface from the large amount of ultraviolet radiation from a massive star. Furthermore, massive stars have relatively short lifetimes, far shorter than the billions of years usually assumed for life to develop in the evolutionary worldview. Therefore, evolutionary scientists assume that lower mass stars are the most likely candidates for having planets where life might exist. Of prime interest are lower mass stars similar to the sun. Astronomers call such stars solar analogues.