• More on the prospects for life on KOI-172.02

    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!

    • Shawn Domagal-Goldman

      I do have one quibble with the Kepler team’s habitability metrics, specifically the use of equilibrium temperature to estimate habitability. The equilibrium temperatures published by the Kepler team assume an “Earth-like” reflectivity. But in order to be habitable, KOI-172.02 planet must have a reflectivity higher than Earth. So, the planet is either outside the habitable zone, contrary to the team’s claims… OR the planet has an equilibrium temperature that is different from the value published by the team. The team has put themselves in a scenario where they can’t be right in both respects.

    • randy

      Assuming same albedo and scaled greenhouse as earth (same co2 and water vapor % with no other greenhouse gases), I get 320. It heavily depends on atmosphere, assumptions of co2 sequestration from plate tectonics (likely for this large a planet) and water vapor %. If you take albedo at .5 (between earth and venus) you get same raw temp as earth 254 (no greenhouse taken into account). Also, there’s habitable to archeobacteria, and habitable to us. Earth may not have been habitable to us until about .6 billion ya. Do you really think we have enough info to declare this planet out of the habitable zone?

      • Shawn Domagal-Goldman


        Your math is correct! If the planet has the same albedo and same greenhouse effect as Earth it would have a surface temperature around 320 K (47 C/116 F). That’s sounds pretty hot to you and me, but it’s not uninhabitable. And as you point out, many microbes are quite happy in such temperatures. Heck, my folks have been through a few days that hot in the sun of Arizona. Some researchers (not me) even think Earth’s average temperature was that high for much of its history.

        However, there’s an inherent assumption there – that the planet has the same greenhouse effect the Earth does. But if KOI-172.02 has an albedo of 0.3, it will absorb a LOT more energy than Earth does. And if that happens, it will have a lot more water vapor in the atmosphere, in turn leading to a higher greenhouse effect and a higher surface temperature. This probably still wouldn’t “kill” the planet, but it would make it warmer than 320K.

        What’s worse, with that much water in the atmosphere, lots of it would get up into the upper atmosphere where it can be blown up by radiation and have its hydrogen atoms escape to space. That would lead to loss of the oceans — and a planet that is uninhabitable, at least in the classic sense of the word.

        You’re also correct that if the albedo of the planet is higher, the planet could be habitable. That’s the idea behind it having a haze layer similar to Titan or tremendously high amounts of cloud cover (something like 90% or more). That kind of planet could keep its oceans.