时间:2013-01-13 09:18:33
Life in the Universe Many scientists today are convinced that life exists elsewhere in the universe —— life probably much like that on our own planet. They reason in the following way. As far as astronomers （天文学家） 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. Life requires the right amount and kind of atmosphere. This eliminates all those planets in the universe that are not about the same size and weight as the earth. A smaller planet would lose its atmosphere; a larger one would hold too much of it.转自学易网 www.studyez.com Life also requires a steady supply of heat and light. This eliminates double stars, or stars that flare up （闪耀） suddenly. Only single stars that are steady sources of heat and light like our sun would qualify. Finally, life could evolve only if the planet is just the right distance from its sun. With a weaker sun than our own, the planet would have to be closer to it. With a stronger sun, it would have to be farther away. 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.
A.obeys different laws.
B.obeys one common law.
C.obeys the same laws.
D.obeys no common law.
A.the right amount of atmosphere.
B.a steady supply of water.
C.steady heat and light.
D. the right distance from the sun.
A.Astronomers believe that the universe is built of the same matter.
B.Proper conditions are essential to the existence of life.
C.Double stars can provide steady light and heat.
D.Astronomers believe that other solar systems are similar in structure to ours.
A.Those double stars'' planets.
B.Those planets much hotter than our earth.
C.Those planets similar to the earth.
D.Those planets very close to their suns.
A.it is impossible for life to exist on other planets.
B.earthlike life could only exist on a few planets.
C. life could exist on only one planet in a million.
D.life could exist on a great number of planets.