If you're taking about the European explorers - they didn't - they went looking for new trade routes to lands they already knew about. It was about commerce, not exploration. Only after someone "hit it big" (Columbus) did others follow because they realized the commercial opportunities of finding "new" places.

The Polynesians sailing into the unknown Pacific is a different case, but that was most likely (based on what I've read) a situation of "the poor" venturing out to find new lands for themselves, not well funded expeditions paid for by existing kings.

They had _real_ water to follow. Proof was right there, just go a little further.

Aliens is 100% pure fantasy.