“Too hot, too cold, and just right.” This not only characterizes bowls of porridge in the classic fairy tale, but it also describes Venus, Mars, and Earth, respectively. When considering the habitability of a planet (where habitability requires the presence of surface liquid water), Earth is at a nice happy medium between the other terrestrial planets. However, just like bowls of porridge, planets do not remain the same temperature throughout time.

The Sun (and all other main sequence stars) brightens about 30% over its lifetime. This means that earlier in the history of the Solar System, all of the planets received less energy from the Sun. For Earth and Mars, this has presented some problems to be solved because above-freezing temperatures are recorded in the geologic record. There must have been some atmospheric warming and/or insulating device that allowed such conditions. As for Venus, it is entirely possible that the faint young Sun did not immediately trigger the runaway greenhouse seen on the second planet today. Venus may have been very Earth-like in it’s early history, complete with oceans and plate tectonics–and who knows, maybe even life. But as the Sun increased in luminosity and positive feedback in the Venusian system continued to intensify, the planet eventually reaching something similar to its present state.

Unlike porridge, however, this hot planet will stay warm. Maybe Mars will have a revival as the Sun continues to brighten. Geocentrically, though, the Earth’s climate will certainly change as a result of this gradual brightening. A temperature of “just right” is only good for a limited time.