你句子没复制完啊,全句一起看就明白了.
And astronomers（主语） have found （谓语）that oxygen atoms and carbon dioxide,which are fundamental to life,exist in its atmosphere（宾语从句） through satellites（状语）.
that是从句的引导词,引导宾语从句,从句的主干是：
(that) oxygen atoms and carbon dioxide exist in its atmosphere.
which are fundamental to life 是oxygen toms and carbon dioxide的定语从句.