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【课文】
Recent developments in astronomy have made it possible to detect planets in
our won Milky Way and in other galaxies. This is a major achievement because, in
relative terms, planets are very small and old not emit light. Finding planets
is proving hard enough, but finding life on them will prove infinitely more
difficult. The first question to answer is whether a planet can actually support
life. In our won solar system, for example, Venus is far too hot and Mars is far
too cold to support life. Only the Earth provides ideal conditions, and even
here it has taken more than four billion years for plant and animal life to
evolve.