A、The planet must be as big and heavy as the earth. B、Proper conditions are essential to the existence of life. C、Double stars c

Many scientists today are convinced that life exists elsewhere in the universe. Life probably much like that on our own planet. As far as astronomers can determine, the entire universe is built of the same matter. They have no reason to doubt that matter obeys the same laws in every part of the universe. Therefore, it is reasonable to guess that other stars, with their own planets, were born in the same way as our own solar system. What we know of life on earth suggests that life will arise wherever the proper conditions exist.
   If we suppose that every star in the universe has a family of planets, then how many planets might support life? First, eliminate those stars that are not like our sun. Next. eliminate most of their planets; they are either too far from or too close to their suns. Then eliminate all those planets which are not the same size and weight as the earth. Finally, remember that the proper conditions do not necessarily mean that life actually does exist on a planet. It may not have begun yet, or it may have already died out.
   This process of elimination seems to leave very few planets on which earthlike life might be found. However. even if life could exist on only one planet in a million, there are so many billions of planets that this would still leave a vast number on which life could exist.