Education and well balanced people.

Education is well known to cause a population to tend to have fewer children. As people become more educated, more capable and more well cared for ... they tend to not want to have as many children. Conversely, when people are poorer and less educated, they tend to survive by having more children ... it's our survival instinct because not all the children may survive but enough offspring survive long enough to have lots more children again.

So if you have a planet full of well educated and well off people everywhere, they will have less children and everyone that is alive will all work towards making things better for everyone else. It's probably a fantasy or a magical utopia that we can never achieve ... but we can hope that we can move in that direction.