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How Many Exoplanets Have Been Found in a Habitable Zone?

A habitable zone around a star is not a uniquely defined quantity. Complex calculations are required that involve assumptions and guesses. When talking about habitable zones we must specify precise which calculations and assumptions we are referring to.

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For confirmed exoplanets, the habitable zone gallery calculates and enables visualization of their habitable zones as desribed in a paper by Kane and Gelino (2012). Spefically, the tool displays a column that gives the times spent by an exoplanet in its calculated habitable zone as a percentage of the total orbital time. If we sort the column in descending order of this percentage, we can find out how many exoplanets correspond to given range in the percentage. For example, on 9 August, 2012, 19 exoplanets spend more than 90% of their orbital period in the calculcated habitable zone. (However, note that not all confirmed exoplanets are included in the table.) On that date there were 777 confirmed exoplanets, so this means that more than 2.4% of confirmed exoplanets spend more than 90% of their time in the adopted habitable zone boundaries.

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For the Kepler mission exoplanet candidates, Borucki et al. (2011, ApJ, 736, 19) found 54 out of 1202 (about 4.5%) candidates to be in the habitable zone (using their adopted definition of the zone). However, this estimate does not properly take into account observational biases and selection effects. In a paper by Catanzarite and Shao (2011), the authors found that the occurrence rate of habitable planets around sun-like stars in the Kepler sample is 1.1(+0.6,-0.3)% using a particular set of assumptions about the habitable zone. (In other words, 1.1(+0.6,-0.3)% of sun-like stars are estimated to host a habitable planet). The same study found that using a habitable zone based on different (more optimistic) assumptions gave an answer of 2.8(+1.9,-0.9)%. (Note that the authors assumed a restricted set of parameters for the host stars in order to consider them to be favorable for hosting habitable planets.)

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).

In July 2012 there was quite a commotion about the announcement of the top 5 potentially habitable exoplanets based on, amongst other things, a quantity called the Earth Similarity Index (ESI). The problem is that the criteria exoplanets have to satisfy for qualification for the list involve a lot of guesses and assumptions. Please read the articles on the exoplanet surface temperature estimates and on the Earth Similarity Index. In addition to these caveats, the exoplanet at the top of the top 5 list (i.e., the number-one candidate), Gliese 581 g, has had its very existence doubted and this is still being debated. The planet is not detected directly: its signal has to be pulled out the noise and the result does not have a high statistical significance. So we are left with the curious situation that the top exoplanet in the top 5 may not actually exist.

Advanced Readers: The existence of Gliese 581 g has been challenged, for example, by Tadeu dos Santos et al. (2012), who say:

“We stress the fact that the planet g is intimately related with the orbital elements of the planet d; more precisely, we conclude that it is not possible to disconnect its existence from the determination of the eccentricity of the planet d. Concerning the planet f, we have found one solution with period $\approx 450$ days, but we are judicious about any affirmation concernig this body because its signal is in the threshold of detection and the high period is in a spectral region where the occurrence of aliases is very common.”

However, even this result has been challenged by Vogt et al. (2012).

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File under: Habitable zone occupation rates; Number and frequency of occurrence of exoplanets in a habitable zone.

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