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Circumstellar Habitable Zone (CHZ) or Goldilocks Zone

The “circumstellar habitable zone,” or CHZ, is a concept that was invented before exoplanets were actually discovered, in order to quantify the properties of a star-planet system that were amenable to supporting life. It is also sometimes referred to as the “Goldilocks zone” after the famous fable in which the little girl called Goldilocks had to find some porridge and a bed that was “just right” for her. Note that the word circumstellar is used to distinguish the fact that we are talking about habitable zones around stars as opposed the habtiable regions in our Galaxy (Galactic habitable zone).

extrasolar planets circumstellar habitable zone artists impression

Now, let me say first that there is a problem with the manner in which the terminology is commonly used. In the popular media and even in some scientific literature you will see references to the habitable zone. But here's the problem: as we shall see below there is not one single clearly-defined habitable zone that can be calculated so we should refer to a habitable zone, not the habitable zone. If we use the latter phrase, we must specify whose calculations and assumptions we are talking about.

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Okay, so the concept of a habitable zone is based on the premise that life requires liquid water and the CHZ is a spatial region corresponding to distances from the host star at which a planet can support liquid water. Although water has some remarkable properties, including the wide temperature range over which it is liquid and its ability to act as a solvent in a wide-range of circumstances, we do not ultimately know if the premise is true. In fact the continuing discoveries of extremophiles right here on Earth do not support the premise. The necessity of liquid water (and therefore the restrictiveness of the CHZ) has begun to be questioned in the literature. Bacteria have been discovered in the South Pole that can reproduce at −10 degrees Celsius (14 degrees Fahrenheit) and that can exist at −85 degrees Celsius (about −120 degrees Fahrenheit). Strains of archaea have been cultured that remain viable at temperatures higher than the boiling point of water, up to about 130 degrees Celsius (or about 270 degrees Fahrenheit). Still, if we are talking about “higher,” intelligent life-forms, you might argue that liquid water is definitely needed in that case. Ultimately, we don't know.

Even if we accept the liquid water criterion, it should be clear from the preceding discussions that calculation of the habitable zone is going to be riddled with complications. The list of issues is very long and I will only mention a few here. The freezing and boiling points of water depend on the pressure, so there is going to be some uncertainty from this. The properties and compositions of Exoplanet atmospheres for terrestrial planets are currently largely unknown. Even if the physical make-up of an atmosphere is known, calculations of the planetary climate are extremely complex and involve many uncertainties, forcing a large number of assumptions and adjustable parameters. Whatever the detailed assumptions are, two more complications are that the CHZ depends on the properties of the host star as well as the planet, and that the CHZ varies with time as the host star evolves, and as the planet's properties change. Moreover, highly eccentric orbits may take a planet in and out of a calculated habitable zone.

Remember also that currently exoplanet temperature estimates could be wrong by hundreds of degrees because they are based on assumptions about unknown physical quantities. Remember the case of Venus: an example in which the “predicted” (according to how most exoplanet temperatures are estimated) and actual temperature disagree by about 500 degrees Celsius (900 degrees Fahrenheit). Venus does not fit any conceivable definition of a habitable zone but it is not very far from Earth. To an observer outside our solar system, Venus the exoplanet could be mistaken for a habitable planet.

With the above caveats in mind, the reader may find the web-based tool Habitable Zone Gallery useful. Details of the assumptions and calculations are given in a paper by the authors of the tool (Kane and Gelino 2012). The tool offers visualization of the calculated habitable zones around confirmed exoplanets and the time spent by each listed exoplanet in the habitable zone, as a perecentage of the orbital time.

A recent study ( by Spiegel et al. 2008) that criticized the standard definition of the CHZ evaluated the so-called “fractional habitability” of Earth as a function of seasonal effects, planet rotation rates, and land to ocean ratios. The fractional habitability quantifies the fraction of a planet that is habitable (under a given set of assumptions). The study found that the predicted fractional habitability for Earth is different to its actual fractional habitability that we know. Another study by the same researchers investigated the effects of the obliquity of the orbit and eccentricity of the orbit on a planet's habitability and found the effects to be important.

Although in principle the inner and outer edges of the CHZ are straightforward to define, calculations in the literature differ in the assumptions and approximations involved in the complex physics with the outcome that different researchers can come to different conclusions about the habitability of the same planet. The inner edge of the CHZ (closest to the host star) is determined by the breakup of water into its constituents due to intense radiation. The outer edge of the CHZ (farthest from the host star) is affected by the condensation of carbon dioxide out of the atmosphere, which affects the reflection (scattering) of light, and therefore the energy balance (and therefore the temperature). The so-called carbon-silicate cycle affects the outer edge of the CHZ. This cycle is complex and has a very long timescale (millions of years) and involves the reaction of carbon dioxide (via rain, making carbonic acid) with silicates in rock to make carbonates which are incorporated into living creatures. The carbonates are eventually returned to Earth, after which volcanism then returns carbon dioxide to the atmosphere. This is of course a grossly oversimplified picture, but the main point is that there are a large number of uncertainties in implementing the physics to calculate the CHZ. On top of that, the climate model used for an exoplanet may be one-dimensional, unable to faithfully capture the full effects of a three-dimensional model.

A study of the multiplanet system Gliese 581 in 2011 (Bloh et al. 2011) summarized this sentiment nicely: “The precise inner and outer limits of the climatic habitable zone are still unknown owing to the limitations of the existing climate models.” The bottom line is that whenever you read about an exoplanet being in the habitable zone, you should be aware that at the moment the conclusion is likely not to be definitive and that the claim is probably open to debate. Also note that for the Exoplanets App, which nicely shows visualizations of some attributes of cataloged exoplanets, the habitable zone boundaries are based on just one particular formulation (and therefore one particular set of assumptions, specifically those of Selsis et al. 2007). On the other hand, the assumptions for a habitable zone adopted by papers reporting on the Kepler mission data on exoplanet candidates are those of Kasting et al. 1993.

Read more about habitable zones and exoplanets with Exoplanets and Alien Solar Systems, which includes comprehensive references to the scientific literature.

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