If you're ever studied astronomy, you've probably been exposed to something called the Drake equation.
원문출처 : https://www.cnn.com/2021/06/14/world/ufos-search-for-alien-life-scn/index.html
원문출처 : https://www.cnn.com/2021/06/14/world/ufos-search-for-alien-life-scn/index.html