天文学家开玩笑说，银河系中可能有几个萨根单位的行星。

Astronomers joke that there might be a few ___Sagans of planets in the Milky Way.

科学家们讨论宇宙中可能存在数以亿计的行星——这就是所谓的萨根数量级。

Scientists discuss the possibility of billions of planets in the universe—that's what we call a ___Sagan scale.