This the fourth and final article in a series of posts by me at Things We Don’t Know about the many unknowns involved in the study of planets in the orbit of other stars across the galaxy. It started off their coverage for World Space Week 2013.
As the catalogue of planets orbiting other stars (called exoplanets) known to us continues to grow, increasing discoveries of potentially ‘habitable’ planets are likely to follow. The ‘habitable zone’ (HZ) concept, which was introduced in a previous post, is becoming increasingly important to our interpretation of these announcements. However, when used unilaterally as it often is, the HZ metric may be misleading – and should rather be considered as a good initial indicator of possible habitable conditions, interpreted relative to other available planetary characteristics.
The habitable zone 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 existence of life based on our understanding of the evolution of the biosphere on Earth. It is often referred to as “the Goldilocks Zone“, since it looks for the region “not too hot, and not too cold”. The concept is based on terrestrial (rocky, as opposed to gaseous or icy) planets that exhibit dynamic tectonic activity (volcanism and/or possible plate tectonics) and that have active magnetic fields to protect their atmospheres from high energy stellar particles that could strip it away. The composition of atmosphere is assumed to consist of water vapour, carbon dioxide and nitrogen with liquid water available at the surface, as on the Earth. Liquid water is the key; the giver of life and the fundamental factor in defining the habitable zone in any planetary system.
It should be relatively easy to spot a number of limitations of the habitable zone concept already; we are still unsure of the atmospheric composition of many of the planets we have already discovered, which would significantly affect any habitability analysis.
Also, we assume that any potential exobiology (the biology of life on other worlds) would have the same requirements as Earth-based life, which may not necessarily be so. The wide variety of extremophile organisms (those able to tolerate extremes of temperature, pressure, salinity, radiation etc.) on Earth might mean we should extend the parameters of the habitable zone beyond those originally considered. All in all, the idea of a habitable zone is a great thought experiment, but it may not necessarily translate into a warm, clement planet in reality. Planetary processes, such as tectonics and atmospheric greenhouse effects, warp the boundaries of the habitable zone. Furthermore, astrobiologists are now considering the very real possibility of salty liquid waterexisting in massive sub-surface oceans of Jupiter’s icy moon Europa, a body well outside of the defined habitable zone of our solar system1.
Another good example of the limitations of using the habitable zone concept in isolation was the furore that resulted from the discovery of the first planet definitively found to be within the habitable zone of its star, Kepler 22b, in late 20112. The popular science media and news outlets were awash with articles and posts describing Kepler 22b 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 the Sun-like star Kepler 22. The media circus surrounding this announcement was an unusual situation, and one that had not been afforded to many other exoplanet announcements before or since. It’s clear that the possibility that this distant world may be suitable for life had spurred the imagination of scientists and the public alike. However, what was usually skipped over, or not mentioned at all, is that Kepler 22b has a radius 2.1 times that of the Earth, and estimates of its mass range from 10 to 34 times that of our planet. The large uncertainty in these figures are due to the method used in its detection, more about which can be found in this previous post in my TWDK series. The unknowns inherent in the discovery of Kepler 22b meant that it could be either a warm, ocean covered rocky planet with a greenhouse atmosphere similar in composition to that of the Earth, or a gaseous planet with crushing gravity and surface temperatures closer to a lead-melting 460 °C, depending on its mass and composition. These are attributes we cannot yet determine effectively.
More data and better detection technology will provide the answer in time, but until then it remains important not to over-hype planets that are only borderline habitable in the very best case scenario as this will most likely be damaging to the public perception of this exciting field in the long term.
Recently, there has been revitalised interest in the habitable zone concept itself, which was first proposed in 1953, with updated estimates based on new climate models published in the scientific literature, as well as increased use of integrated habitability metrics which take other planetary factors into account. However, our understanding of the factors that control the habitability of extrasolar planets is at a very early stage, as is our grasp on the limits that life can endure, and it remains too early to say with much confidence that we have discovered another world suitable for life.
 Tyler, Robert H. “Strong ocean tidal flow and heating on moons of the outer planets.” Nature 456, 770-772 DOI: doi:10.1038/nature07571
 Borucki, William J et al. “Kepler-22b: a 2.4 Earth-radius planet in the habitable zone of a Sun-like star.” The Astrophysical Journal 745.2 (2012): 120. (PDF)