单行星物种和多行星物种之间存在着巨大的差异。
如果有一天我们有能力到访其他恒星星系，我们可能会发现许多早已消亡的单行星文明。

There is a profound difference between single-planet & multiplanet species.
If we are able to visit other stars one day, we may discover many long-dead single-planet civilizations.