re what planets are habitable i would appreciate any thoughts on what makes a planet habitable for humans i am making asumptions that life and a similar atmosphere evolve given a range of physical aspects of the planet the question is what physical aspects simply disallow earth like conditions this is a good question there are major blind spots in our understanding of what makes the earth habitable for example why does the earth s atmosphere have the concentration of oxygen it does the naive answer is photosynthesis but this is clearly incomplete photosynthesis by itself can t make the atmosphere oxygenated as the oxygen produced is consumed when the plants decay or are eaten what is needed is photosynthesis plus some mechanism to sequester some fraction of the resulting reduced material on earth this mechanism is burial in seafloor sediments of organic matter mostly from oceanic sources however this burial requires continental sediments in the deep ocean the burial rate is so slow that most material is consumed before it can be sequestered this suggests that a planet without large oceans or a planet without continents undergoing weathering will have a hard time accumulating an oxygen atmosphere in particular an all ocean planet may have a hard time supporting an oxygen atmosphere there is also the problem of why the oxygen in the earth s atmosphere has been relatively stable over geological time for a period at least orders of magnitude longer than the decay time of atmospheric o to weathering in the absence of replenishment no convincing feedback mechanism has been identified perhaps the reason is the weak anthropic principle if during the last myr or so the oxygen level had dropped too low we wouldn t be here to be wondering about it paul f dietz dietz cs rochester edu
