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What is a Goldilocks Planet?

Goldilocks is the name of the little girl in the famous fable Goldilocks and the Three Bears, in which there was some porridge and a bed for her that were “just right”. A Goldilocks planet is supposed to be one that is just right for life (as we know it) and the term is used synonymously with habitable planet or a planet in the habitable zone (of its host star). The big qualifier here is the “as we know it” part, which inevitable involves the existence of liquid water. Another problem, propagated by popular articles as well as Wikipedia is that there is not simply one way to calculate the parameters of a habitable zone (resulting in different zones for the same planet), and more details are provided in the What is a Habitable Zone? article on this website.

The term Goldilocks planet is not actually used much in the scientific literature. On 7 August, 2012, for example, a search in the ADS astrophysics literature database simply on the word Goldilocks revealed only 14 results since 1995, and only 8 of these had anything to do with planets and their host stars.

When you search the internet for “What is a Goldilocks Planet?” you are shown a top result from Wikipedia, which redirects you to a the Wikipedia Habitable Zone page, at the top of which you are told to go to check out the page forthe exoplanet 70 Virginis b. This planet has been shown not to fit into any conceivable definition of a “Goldilocks Planet.” It is a hot Jupiter, a massive gas giant that is extremely close to its host star. It is not going to satisfy the “life as we know it,” is it? If the Wikipedia article is going to mention any planet by name in this context, the search of the ADS database I decribed above would seem more appropriate. Of the 14 search results, only 1 paper mentions a specific planet, and that is, Habitability of the Goldilocks Planet Gliese 581g: Results from Geodynamic Models, by Bloh et al. (2011). In this paper we find the following sentence in the abstract: “The existence of habitability is found to critically depend on the relative planetary continental area, lending a considerable advantage to the possibility of life if Gl 581g's ocean coverage is relatively high.” Notice that even here, the wording is carefully constructed to point out the possibility of ocean coverage, not of life, although that is of course the implication and end goal.

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