Question

一个简单的英语语法问题

MKII, a planet that is billions of kilometers away, is the origin of life in many religions. Physicists proved the theory that its gentle climate and the pull of its gravity are similar to the earth’s. And astronomer have found that oxygen toms and carton dioxide, which are fundamental to life
请问那个found that中，that 的作用

Answer 1

that是连接从句的连词，found后面that引导的句子是宾语从句，that在这里可以省略。
所有句子语法分析如下：
MKII（主语）, a planet that is billions of kilometers away（同位语，补充说明主语）, is（连系动词） the origin of life in many religions（表语）. Physicists（主语） proved（谓语） the theory（宾语） that its gentle climate and the pull of its gravity are similar to the earth’s（同位语，theory的内容）. And astronomers（主语） have found（谓语） that oxygen toms and carton dioxide, which are fundamental to life（宾语从句）
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Answer 2

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。