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Following the excitement surrounding the discovery of potentially Earth-like exoplanet Kepler 22-b (K22-b) earlier this week, the ‘habitable zone’ (HZ) concept is becoming increasingly important to our interpretation of these announcements. Used unilaterally, the HZ metric may be misleading and it should be considered as a rough guide, interpreted relative to other available planetary characteristics.
The HZ describes the theoretical distance (with both upper and lower limits) at which a given planet must orbit a star to support the basic fundamental requirements for the emergence of life based on our understanding of the evolution of the biosphere on Earth. The HZ theory is based on terrestrial (rocky, as opposed to gaseous or icy) planets that exhibit dynamic tectonic activity, that have active magnetospheres and atmospheres of water, carbon dioxide and nitrogen with liquid water available at the surface. Liquid water is the key; the giver of life and the fundamental factor in defining the HZ in any planetary system.
It should be relatively easy to spot a number of limitations of the HZ concept already; we are still unsure of the composition of K22-b which would significantly affect any habitability analysis. Also, we assume that exobiology would have the same requisites for life which may not necessarily be so; this alludes to our inability to detach ourselves from our anthropic bias. The ubiquity of extremophilic organisms (those able to tolerate extremes of temperature, pressure, salinity, radiation etc.) on Earth has also lent some credence to the extension of the parameters of the habitable zone beyond those originally included. All in all, the idea of a habitable zone is a great thought experiment, but it may not necessarily translate into reality. Planetary processes, such as tectonics and atmospheric greenhouse effects, warp the boundaries of the HZ. Astronomers are now considering the very real possibility of liquid water existing in sub-surface oceans of Jupiter’s moon Europa, a body well outside of the habitable zone of our solar system.
I think that the hype surrounding the discovery of K22-b has highlighted the limitations of using the habitable zone concept in isolation when considering the potential for life on other planets. It certainly provides an excellent starting point to narrow down the enormous Kepler catalogue, and it will play an important role in identifying habitable planet candidates in the future. Clearly however, other factors need to be taken into consideration. I’ve seen numerous articles and posts describing Kepler 22-b as “Earth’s twin” and “Earth 2.0” based solely on the fact that it has been discovered to be orbiting within the habitable zone of Kepler 22. What is usually skipped over, or not mentioned at all, is that K22-b has a radius 2.4 times that of the Earth, and estimates of its mass range from 10 to 34 times that of our planet. With a surface gravity possibly nearing 6 g, an ‘Earth-twin’ this is not. My favourite resource for planetary habitability, the Habitable Exoplanet Catalog run by UPR Arecibo has released an updated best and worst case scenario analysis for K22-b:
I covered the worst case scenario in my last post, but here it is again (updated):
A gaseous planet with huge mass and crushing gravity, and still a very hot 42 °C for those that can stand being squished to a pulp. With a greenhouse atmosphere similar in composition to that of the Earth, the temperature would be roughly -15 °C; with Venus’ super-greenhouse, the surface would be closer to a lead-melting 460 °C.
This estimation is based on an ocean planet with a rocky core, no continents and thick cloud cover. It’s mass has been downgraded slightly, but its still an impressive ~10 times that of Earth. Its surface gravity isn’t quite as oppressive as first thought, but it would still be very uncomfortable especially in the heat. The conclusion is that it would be habitable, but only at a push, and you certainly wouldn’t choose to spend your summer holidays on Kepler 22-b.
I can only reiterate the conclusions of my last post; it’s important not to over-hype planets that are only borderline habitable in the very best case scenario. It’s unscientific and will be damaging to the public perception of exoplantology in the long run. There are only so many times that the exoplanet community can cry wolf and expect the public to be interested in these kind of Earth analogue stories. When an excellent candidate comes along, which I firmly believe that it will, there is a risk that this monumental announcement will be made to an empty room.