The Habitability of Our Earth and Other Earths: Astrophysical, Geochemical, Geophysical, and Biological Limits on Planet Habitability
Annual Review of Earth and Planetary Science
For life-forms like us, the most important feature of Earth is its habitability. Understanding habitability and using that knowledge to locate the nearest habitable planet may be crucial for our survival as a species. During the past decade, expectations that the universe could be filled with habitable planets have been bolstered by the increasingly large overlap between terrestrial environments known to harbor life and the variety of environments on newly detected rocky exoplanets. The
... lanets. The inhabited and uninhabited regions on Earth tell us that temperature and the presence of water are the main constraints that can be used in a habitability classification scheme for rocky planets. Our compilation and review of recent exoplanet detections suggests that the fraction of stars with planets is ∼100%, and that the fraction with rocky planets may be comparably large. We review extensions to the circumstellar habitable zone (HZ), including an abiogenesis habitable zone and the galactic habitable zone. 597 Annu. Rev. Earth Planet. Sci. 2012.40:597-623. Downloaded from www.annualreviews.org by Australian National University on 05/02/12. For personal use only. E u b a c t e r ia A r c h a e a E u k a r y o t e s Figure 1 The emergence of biologists from astronomy. Starting from the big bang at the bottom, deterministic physical sciences set the context for the emergence of life. The resulting biologists (animals) at the top of the tree (e.g., Pace 1997, Hedges & Kumar 2009) have constructed the brown phylogenetic tree based on the molecular fossils inside the DNA of the inhabitants of the biosphere. The terrestrial tree of life took root approximately four billion years ago. We review the features of rocky planets that are implicated in the ability to give root to, and maintain, a tree of life. HZ: habitable zone in particular, are a common product of star formation. Section 4 discusses the habitability of the most Earth-like exoplanets and the traditional circumstellar habitable zone (HZ). Section 5 reviews the supply of water to terrestrial planets. Finally, Section 6 reviews work on the galactic HZ and discusses a variety of habitability issues. A list of Summary Points precedes the references. THE HABITABLE ZONES ON EARTH Because habitability is about the complex relationship between life and environment, we start close to home with a discussion of the relationship between terrestrial life and terrestrial environments. The close fit between our requirements and what Earth can provide is not coincidental. Earth www.annualreviews.org • Habitability of Earth and Other Earths 599 Annu. Rev. Earth Planet. Sci. 2012.40:597-623. Downloaded from www.annualreviews.org by Australian National University on 05/02/12. For personal use only. www.annualreviews.org • Habitability of Earth and Other Earths 601 Annu. Rev. Earth Planet. Sci. 2012.40:597-623. Downloaded from www.annualreviews.org by Australian National University on 05/02/12. For personal use only. The Abiogenesis Habitable Zone and Habitable but Uninhabited Planets As we learn more about the origin of life, we can start to define an abiogenesis habitable zone (AHZ) where the requirements for life's emergence are met. The habitability requirements for the origin of life may be substantially different from, and more specific than, the requirements to maintain life on a planet-think of the difference between a spark plug to start an engine and a carburetor to supply it with fuel. If you shine light onto a vat of HOCNPS, or bubble molecular hydrogen through a flask of amino acids, life does not spontaneously emerge. For a planet to 602 Lineweaver · Chopra Annu. Rev. Earth Planet. Sci. 2012.40:597-623. Downloaded from www.annualreviews.org by Australian National University on 05/02/12. For personal use only.