However exciting this may sound, Walker is quick to remind the optimistic observer we still have zero evidence that life actually exists on any of these far-off planets.

这是 However 引导的让步状语从句。意为：不管这听起来多么令人兴奋。

在英语中，wh-word + ever 引导让步从句很常见。wh-word+ever 这类词既可用作疑问代词或疑问副词，用来引导特殊疑问句，也可引导名词性从句，还可引导让步状语从句。这类词在引导让步状语从句，可换成“no matter+wh-word”。

    Whatever (=No matter what) happened, I would believe in you.

    不管发生了什么，我都相信你。

    Whenever (=No matter when) you come, you are welcome.

    不管你什么时候来，都欢迎你。

    Whoever (=No matter who) you are, you must obey the rules.

    不管你是谁，你都得遵守规定。

    I'll wait for you however (=no matter how) late it is.

    不管多么晚我都会等着你。