In my last post, I talked about why I think KOI-172.02 is uninhabitable. But in the words of The Dude,
And… in a way, it is just my opinion. KOI-172.02 sits in a bit of a “grey area” where we can’t *absolutely* rule out habitability, but it’s habitability depends on the planet being different from Earth in important ways.
The habitable zone calculations I was basing my analysis on assume the planet absorbs incoming radiation about as efficiently as the Earth does. But it’s possible that a planet would absorb *less* energy than Earth. That could happen if the planet has more continent coverage, as continents absorb less incoming energy than oceans do. Or the planet could have significant cloud coverage. Or it could have an alien society that is flying mirrors in orbit around the planet to offset the effects of its star getting ever brighter (stars do this over time).
The problem with all of these (especially the last one) is they all take a leap of faith. And that’s not a criticism of the Kepler team – their claims of potential habitability lean on published habitable zone boundaries. The issue is with the published literature not exploring these effects completely. That falls on planetary climate researchers (such as yours truly), who need to take a more comprehensive look at planetary albedo.
What happens if we move Titan, which has a strong anti-greenhouse effect, to a place where it gets as much energy as KOI-172.02? I don’t know. What happens if a planet similar to early Earth (which may also have had an anti-greenhouse effect) gets that much energy? I don’t know. Are there ways for the global cloud coverage of KOI-172.02 to be high enough to allow the planet to be habitable? I don’t know. The bottom line is we haven’t done the work needed to properly demonstrate any of these processes with our models, and we don’t have a planet that has these properties that we can observe. The one exception to this is some outstanding work on the habitable zone limits for dry planets like Dune/Arrakis/Rakis.
If you’re a talented researcher that works in this area, these ideas are all worth pursuing. (Full disclosure: it’s in my future research plans.) And it would be timely research, because the radiative effects of clouds and other aerosols is a hot topic right now. Most of the uncertainty in projections of climate change results from the difficulty in predicting our future emissions; but most of the remaining uncertainty comes from the effects of clouds and aerosols. So this is a project that would have both “cool, alien-hunter” implications as well as “important, saving the planet” implications. Get to it!