We would rejoice at the disvovery of extraterrestrial (intelligent) life, wouldn’t we? Wait a minute, the circumspect antinatalist will tell us. Rather than jubilate we should feel sorrow. Why? Well, because other life forms, however alien they may be, will be in need of some kind of conscious alert system to prevent the single living beings from self destruction. Alien living beings, strange though they may be, will be able to feel what we call pain. This is the reason why the full grown antinatalist will not jubilate at incoming radio signals from Alpha Centauri or other regions of space and time.
Ever more inhabitable planets that are being discovered are a strong indicator that there will be much more suffering out there in space than people were generally inclined to assume only a few decades ago.
For more see “Should Extraterrestrials Exist?” (Sollen Außerirdische sein?, chapter 15 of my book ALIEN INTRODUCTION INTO PHILOSOPHY / AUSSERIRDISCHE EINLEITUNG IN DIE PHILOSOPHIE, 2002).