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Introduction
The attributes of the planet Earth are of critical importance to the existence and survival of life upon it. In fact, it may be so finely tuned that few planets in the Galaxy share its life-friendly characteristics. From this premise, it is almost inevitable to reach the conclusion that intelligent life (at least any that is predicated on evolution from complex metazoans) is also rare - perhaps unique to the planet Earth.
These ideas have been encapsulated in what is known as the Rare Earth Hypothesis (Ward & Brownlee 2000). It can be summarized as follows.
1.
Simple life may be commonplace in the Universe. The existence of extremophilic organisms in what were originally considered to be inhospitable regions (hydrothermal vents, acidic pools, toxic waste, deep in the Earth's crust) has shown the hardiness of simple life (Cavicchioli 2002; Diaz & Schulze-Makuch 2006). Indeed, these habitats are believed to be duplicated elsewhere in the Solar System, e.g. Mars (Formisano et al. 2004; Krasnopolsky et al. 2004), Europa (Carr et al. 1998), Titan (Stofan et al. 2007) and Enceladus (Parkinson et al. 2007; Spencer & Grinspoon 2007), so it is still possible that 'alien' life may be closer to home than once thought.
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However, although simple life is resilient and adaptable, the evolution of complex animal life is extremely difficult. For this to be achieved, there are certain criteria (hereafter referred to as the Earth Criteria) that must be satisfied, in order for animals to thrive.
The following is a (non-exhaustive) list of the Earth Criteria.
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A planet within a critical range of orbital radii - the 'stellar habitable zone' (Hart 1979; Kasting et al. 1993).
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A star within a critical mass range (large enough to push the habitable zone outside the planet tidal locking radius, and small enough to provide sufficient energy while avoiding UV exposure).
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A star located in a critical region of the Galaxy - the 'galactic habitable zone' (GHZ) (Lineweaver et al. 2004).
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A planet within a critical mass range to maintain a suitable atmosphere.
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A planet with a stable low eccentricity orbit (to avoid extreme temperature changes). This also requires a relatively large moon to provide axial...