The surface conditions on the planet Mars are more like the Earth's than are those of any other planet in the solar system.
than are those 中间的are 不能省略 如果省略 则意思 不一样
因为 这是定语从句 不是 比较状语从句
than是关系代词 引导 定从 并作 定从中的主语
当然 也 有人 把 than 看作介词 后面是省略what的宾语从句
下面是 than 引导定从 的 例子
8．The problem may be more difficult in nature than would appear．
9．He got more pocket money from his parents than was demanded．
10．The project requires more labor than has been put in because it is extremely difficult．