Originally Posted by DuckieHo
Had... but was it long enough or massive enough?
The ROI on a habitable planet is quite high even though there is a very high startup costs.
We still would care because how many planet could we terraform? Mars and Mercury.
Well first, by that logic mars didn't have long enough geological activity since it cooled off after a billion years from it's formation. Protoplanets could have had a similar amount of time, or at least a substantial fraction, needed to generate certain geologically based compounds, such as diamonds for example, which are abundant in the asteroid belt .
The real issue is when exactly the ROI becomes equal to the startup costs. That is probably going to take about 500 years, but that is complete speculation for a variety of reasons, it will probably be longer. However it is based on the fact that producing the right atmospheric composition for humans would be extremely challenging, expensive, and time consuming once the planet had been warmed up from 100 years of greenhouse gas production. Transporting large quantities of people needed to generate that ROI and also build the infrastructure would also be extremely expensive (you would need about a billion people to generate a large enough roi) unless you want to wait for population growth.
Mercury is actually much harder to terraform than venus. Since the planet's rotational period is quite slow, one half of the planet is baked, while the other is frozen. The pressure differential would create massive hurricanes assuming you could actually get mercury to hold an atmosphere. The low gravity combined with high solar winds would blast the atmosphere off as fast as you could create it as well.
The best way to terraform venus would be to heavily nuke one side of the planet, causing lots of the dense atmosphere to blow off and creating a nuclear winter. You could also dump highly reflective particles into the atmosphere too reduce solar radiation. I think venus' potential for terraforming is actually underestimated.