And some scientists say the odds of success are worse than winning a lottery. He argued that although simple life forms may be scattered throughout the galaxy, the odds against the evolution of intelligent life are overwhelming. We can't set a number. Still, it suggests that planetary systems could be commonplace in our galaxy. Planets, however, are just the first step.
Life also requires a hospitable climate and atmosphere.
And even if living organisms appear, there's no guarantee that intelligence will evolve. DeVore pointed out that more than 99 percent of species that have ever lived on Earth have gone extinct. There is absolutely nothing inevitable about the evolution of higher intelligence. Another question of course, is whether alien beings would be friendly or hostile. Therefore, any kind of encounter would profoundly change human society, whether that was the aliens' intention or not.