• Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars

    Last week, my first research paper was published in the journal Astrobiology. The paper outlines our method for estimating how long ‘habitable’ conditions may exist for on planets that have been discovered in the ‘habitable zone’ – a concept I regularly discuss on this blog and elsewhere. The run-up to its publication has been surprisingly hectic, and it has received a lot of media attention. Whilst this is great for getting the science out there, I want to make sure that there is something available on the internet where I discuss the paper in my own words in case there are any misconceptions about our results.

    The habitable zone describes an area around a star where a planet, if it was discovered to be orbiting within this area, could have liquid water on its surface. Stars of different masses and classifications have different habitable zone distances, and not all planets in the habitable zone are habitable: some may be too massive, others too small, many wouldn’t have the correct mix of atmospheric constituents, others may have no atmosphere at all. In fact, there are more reasons to think that planets, whether inside or outside the habitable zone, are more likely to be completely unsuitable for (Earth-like) life than there are to consider the opposite.

    However, whilst habitability is variable in space, it is almost certainly variable in time as well. The habitable zone isn’t a fixed distance: its boundaries move outwards as the star undergoes main-sequence evolution, growing larger and hotter over time. More massive stars (classifications F, G and K) have the shortest main sequence lifetimes and therefore the habitable zone boundaries around these stars migrate outwards at a proportionally more rapid rate. Low mass stars, M-stars for example, have extensive lifetimes on the order of tens or hundreds of billions of (Earth) years, and therefore their habitable zones are relatively more static in time. I should stress that the planet itself is not moving, but rather the boundaries of the possible habitable zone that extends around the star are changing, and planets may be left in the heat, or brought in from the cold, as the star ages.

    Building on this idea, if it is possible to determine the extent of the habitable zone at the beginning and end of the star’s main sequence lifetime using modelling techniques, and estimate the approximate age of the star, then a rate of outward migration of the boundaries of the habitable zone can be derived. The time that a planet spends within the habitable zone can be considered its ‘habitable zone lifetime‘ (HZL). The HZL of a planet is an important factor when considering the possibility of life on these worlds. A planet with a long habitable period is perhaps more likely to host complex organisms that require more time to evolve, if we make the assumption that evolution by natural selection is a universal constant, operating in a similar way in potential exobiological systems as it does on Earth.

    We coupled a stellar evolution model, with the classic habitable zone and applied it to planets that had already been discovered in the habitable zone by workers at the Planetary Habitability Laboratory. (Bio)geochemical feedback mechanisms operating on individual planet to buffer the climate will affect the boundaries of the habitable zone, but because these processes are complex and likely planet-dependent, we left them out and assumed constant conditions. We made the same assumptions about the planets’ atmospheres that the original authors of the habitable zone model (Kasting et al. 1993) did: a nitrogen rich atmosphere, with about 300ppm carbon dioxide and no clouds or other complex atmospheric physics or chemistry.

    Nevertheless, this produced some interesting results. The Earth seems to be habitable for perhaps 6.29 billion years (Gyr), but this is excluding the influence of humans and our pesky habit of pumping extra CO2 into the atmosphere. This obviously can’t account for other random events (asteroids etc.), and it’s important to remember that we’re making no allowances for the natural biogeochemical cycles of the planet to buffer climate – this is a very simplified picture. Luckily for us, these estimates are similar to those produced by other more complex and Earth-centric models, so we were happy to continue to try to apply the simple model on other planets with reasonable confidence.

    Other potentially habitable exoplanets do pretty well too. Kepler 22b may be habitable for 4.3 Gyr, Gliese 581g (if it exists!) will be in a habitable position for 11.2 Gyr, whilst its neighbour Gliese 581d might be clement for 42 Gyr! A huge amount of time. This star system is already approximately 8 Gyr old, so both these planets would be very interesting candidates for further study.

    Our intention was to supply these figures so that they could be incorporated in habitability metrics in the future to capture the temporal aspect of the planetary habitability. Also, we hope that this framework can be used with other habitable zone formulations (several updated versions already exist) that focus on different aspects of the planetary system. Further, we hope that we can identify interesting planets for further study by future space telescopes or SETI campaigns. These would be planets that have been habitable for a similar or greater amount of time to the Earth, because we think that the evolution of intelligence will require a very, very long time, so pinpointing worlds with long HZLs would make sense.

    I’ve noticed from the very many interviews I did that the press machine is a rapid, yet inefficient beast. I worry now that the purpose of the paper (to find habitable exoplanets like the Earth) has been eclipsed by the fact that we tested it on Earth first, and that we are making some definitive statement about how long we can comfortably live here. We are not. Earth is or test, our standard, our control. The press releases I have seen have put all the emphasis on this small part of the project, (and the fact that we should move to Mars!), whilst in reality we only validated the model against other more complex models for the Earth, and came to similar conclusion.

    Further to this, it now appears that some people seem to have taken my work to show that human induced climate change will have little effect in the long term and that it undermines climate research. This really was not my intention. If I could have, I would have avoided all discussion of anthropogenic climate change in the first place, because we were investigating a different question (long term, solar-forced exoplanetary habitability) using a different tool. However, my institution (the University of East Anglia) has a strong reputation for climate science, and I fully support the findings of my colleagues at the UEA and elsewhere that illustrate the warming effects of increased levels of atmospheric CO2 over human timescales, and I did what I could to mention this in my interviews. When we state that the Earth will be in the habitable zone for 1.7 billion years longer, we have left anything that humans could do to the atmosphere in the interim out of the equation out of necessity.