153. Life in outer space
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For years, the idea of intelligent life on other planets has been very popular.
Many books and movies tell stories of what those forms of life might be like.
Many scientists believe that very simple forms of life are likely to exist on many other planets.
Under the right conditions, simple life forms might arise.
Those conditions, which include moisture and warmth, might occur in many parts of the universe.
Fewer scientists believe that very intelligent forms of life are likely to exist on many other planets.
For intelligent life to evolve, a very long period of time is needed.
During that time, the conditions on a planet must not be too harsh.
Otherwise, the evolving life forms will die.
The amount of water, heat, and various chemicals must be "just right".
If not, then complex life might never evolve.
The conditions needed for intelligent life to evolve are very unlikely to occur on any one planet.
However, some scientists believe that intelligent life might be common in the universe.
Because there are so many stars and planets in the universe,
there might be a few places that have intelligent life.
However, those places are probably very, very far away.
Other scientists disagree.
They think that the conditions needed for intelligent life are extremely rare.
Because of this, our planet might be the only place that has intelligent life!
So far, it is impossible to know whether or not there are intelligent beings on other planets.
But even if those beings do exist,
it seems very unlikely that we will ever meet them.