We know that the atmosphere of Venus is 96,5% CO2. Everything else in it is trace amounts. We know that from the Venera probes. We also know that not only is the Venus atmosphere much hotter than Earth’s, it’s also a whole lot denser. Its weight on anything on the surface of the planet would be crushing. This means a lot more gas in it. But where does all this CO2 come from? And where have all the other gases gone? Let me speculate a bit. Earth’s and Venus’s atmosphere may have the same origin. Volcanic outgassing. This is mainly CO2. But on Earth we had life come in and transform much of it into stuff that goes under Earth and produced a breathable atmosphere. Venus is much closer to the sun and its atmosphere is much denser so pressure and heat prevented the processes that allowed life on Earth. Runaway greenhouse is crap. If it would be valid – why has the heating stopped at one point. Why is it not an inferno of molten rock?
Earth wasn’t the only potentially habitable planet in the early Solar System. What happened to Mars?