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The Habitability of Our Earth and Other Earths: Astrophysical, Geochemical, Geophysical, and Biological Limits on Planet Habitability
Charles H. Lineweaver and Aditya Chopra
Planetary Science Institute, Research School of Astronomy and Astrophysics, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia; email: charley@mso.anu.edu.au

Annu. Rev. Earth Planet. Sci. 2012. 40:597­623 First published online as a Review in Advance on March 8, 2012 The Annual Review of Earth and Planetary Sciences is online at earth.annualreviews.org This article's doi: 10.1146/annurev-earth-042711-105531 Copyright c 2012 by Annual Reviews. All rights reserved 0084-6597/12/0530-0597$20.00

Keywords
habitable zones, circumstellar habitable zones, terrestrial planets, life, abiogenesis

Abstract
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 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.

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1. INTRODUCTION
The Sun with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do. --Galileo

The universe is filled with stars like our Sun (Robles et al. 2008), rocky planets like our Earth (Howard et al. 2011), water like in our oceans (Mottl et al. 2007), amino acids like those that make up our proteins, and all the other ingredients for life (Pizzarello 2007). But is the universe filled with anything we would recognize as life (Lineweaver 2006)? de Duve (1995) has argued that the initial deterministic nature of proto-biochemistry makes life a "cosmic imperative" built into the chemistry of the universe, and we should therefore expect life to be common in the universe. Terrestrial life emerged from nonlife approximately four billion years ago (Battistuzzi et al. 2004, Sleep & Bird 2008) (Figure 1). Descriptions of where this happened include warm little ponds (Darwin 1871), hot hydrothermal vents (Wachtershauser 2006, Martin et al. 2008), and ¨ ¨ cold little ponds (Bada et al. 1994). Scenarios for how life emerged include a prebiotic soup under a reducing atmosphere (Oparin 1924, Miller 1953) and some form of semi-deterministic molecular chemistry (Dyson 1999, Segre & Lancet 2000) that evolved into auto-catalytic reactions (Eigen 1971) and produced replicating molecules (Cairns-Smith 1982, Gesteland et al. 1999), proto-metabolisms (Pascal et al. 2006), and cell membranes (Deamer & Szostak 2010). Despite the variety of these scenarios, some common threads represent our best estimates of what we might expect to share with extraterrestrial life. We can expect extraterrestrial life to be based on Darwinian evolution and the most fundamental features of terrestrial biochemistry (Feinberg & Shapiro 1978, Pace 2001, Conway-Morris 2003, Benner et al. 2004, de Duve 2007, Lineweaver & Chopra 2012). Noncoincidentally, these are the same features that are often used to define life (e.g., Schrodinger 1944, Sagan 1970, Joyce 1994, and the contributions in Leach et al. 2006). During the past few decades, the exploration of some extreme environments on Earth has uncovered extremophile microbial life in conditions previously thought to be too hostile for life. These discoveries have expanded the variety of terrestrial environments known to harbor life (Rothchild & Mancinelli 2001, Stetter 2006, Shock & Holland 2007, Baross et al. 2007, Madigan et al. 2010, Pedersen 2010). During the same few decades, progress in the characterization of the planets and moons of our Solar System, and progress in the detection of a wide variety of exoplanets in orbit around an increasingly large fraction of stars, has broadened the range of known extraterrestrial environments (Sections 3 and 4). The growing overlap of these two sets of environments suggests that habitable planets are abundant (Figure 2). This increases the probability of finding some kind of extraterrestrial life. The number of papers and conferences reporting new exoplanet and extremophile discoveries, and those grappling with the issue of planetary habitability, has enormously increased. Recent reviews of habitability include Kasting & Catling (2003), Gaidos et al. (2005), Hoehler et al. (2007), and Fishbaugh et al. (2007). This review provides an overview of habitability and focuses on what we know about the habitability of Earth, the habitability of planets orbiting other stars, and the habitability of our galaxy. It synthesizes facts and current ideas about the microbiology of the earliest terrestrial life and the latest findings of planet hunters. It is organized as follows: Section 2 reviews the limits of terrestrial life and illustrates where life is found, and is not found, on the habitable planet that we know best--Earth. We discuss energy constraints on life and how the conditions for life's emergence may be different and more specific than the broader conditions to which life can adapt. Section 3 presents the increasingly compelling evidence that planets in general, and rocky planets
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Botany otan
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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.

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.

2. 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
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HZ: habitable zone

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A few decades ago

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Figure 2 Emergence of habitable planets. Habitable planets are emerging from the increasing overlap of two sets of environments: the increasingly large set of terrestrial environments known to harbor life and the increasingly large set of extraterrestrial environments on the newly detected rocky exoplanets. The overlap of these research fields bolsters expectations that the universe may be filled with habitable planets.

and the life on it have coevolved. However, life is not infinitely adaptive. Some parts of Earth are habitable and some, even after approximately four billion years of evolution, are not. Life as we know it has limits, and we can explore these limits most easily on Earth. In a specific region, when some requirement for life is outside the optimal range for life, the total biomass in that region is low (Figure 3 presents these regions). We generically call such regions deserts. Low rainfall makes water deserts. Low temperatures make low-temperature deserts at Earth's poles. Far from land, in mid-oceanic gyres [where windblown dust and aerosols are at a minimum (Duce & Tindale 1991)], low nitrate levels produce nitrate deserts. Chlorophyll maps of the ocean (McClain et al. 2006) show regions where, despite ample water, photons, and nitrates, there are low concentrations of chlorophyll. Iron fertilization experiments in these enigmatic high-nitrate-low-chlorophyll regions found that the biomass was iron limited, rather than phosphate limited or limited by some other nutrient (Falkowski et al. 1998, Smetacek & Naqvi 2008). Such iron deserts have been identified in the Southern Ocean, the northwest Pacific, and the eastern equatorial Pacific. Low-phosphate regions overlap significantly with nitrate deserts (Garcia et al. 2006). Because the elements H, O, C, N, P, and S make up 98% (by mass) of life (Lineweaver & Chopra 2012), one might expect analogous C and S deserts. The subtle variations of biomass over the horizontal surface (Figure 3) are dwarfed by the nonsubtle variations of biomass in the vertical direction. The terrestrial biosphere is a thin bioshell whose thickness (10 km) is 1/600 of Earth's 6,400 km radius. Figure 4 is a vertical profile of terrestrial biomass. Low temperatures and low densities prevent life from living permanently in air or on the highest mountains (low-temperature deserts). High temperatures prevent life from existing more than 5 kilometers underground, because the average continental geothermal gradient of 20­30 per km reaches the upper temperature limit of life [122 C, Takai et al. (2008)] at approximately that depth. (Shield gradients and those above subduction zones can be as low as 10 C per km.) Thus, the interior of Earth is a spherical high-temperature desert. The bioshell is thin because life is kept squeezed into a narrow HZ between a high-temperature desert below and a low-temperature desert above (Figures 4 and 5).
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Water deserts
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Low-temperature deserts

Nitrate deserts

Iron deserts

Four deserts on Earth's surface. Life is not evenly distributed deserts (sandy brown), low-temperature deserts (white), nitrate green), where the abundance of life is significantly lower than constructed with data from Stockli et al. (2005) and McClain

over the surface of Earth. There are water deserts (dark blue), and iron deserts (light in surrounding regions. This map was et al. (2006).

% of solid surface per elevation
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Vertical profile of biomass in the thin terrestrial bioshell ( ± 10 km). The hypsographic curve on the right shows the fraction of Earth's solid surface above a given elevation. For example, 30% of the solid surface is above sea level, whereas the remaining 70% is below sea level. The hypsometric curve on the left (blue line) (Perotti & Rinaldi 2011) shows the fraction of Earth's solid surface at any given elevation. The histogram on the left shows our estimate of the vertical profile of terrestrial biomass (total carbon in terrestrial life forms) derived from combining data from Whitman et al. (1998) and Houghton (2003).

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Temperature (K)
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Figure 5 Uninhabited water on Earth and the potential biosphere of Mars. This pressure-temperature phase diagram is a superposition of the region where H2 O is liquid (blue), all terrestrial environments (brown), inhabited terrestrial environments ( green), and all Martian environments (hatched red ). Notice the large regions of uninhabited terrestrial liquid water that is either too cold (-80 C < T < -20 C) or too hot (122 C < T < 400 C) for life. The ± 10 km vertical extent of the green area corresponds to the elevation range of the biomass profile in Figure 4. The shape of the green area shows that the presence of liquid water and the -20 C to +122 C temperature range are the dominant variables determining habitability. These inhabited and uninhabited terrestrial environments, set by the need for liquid water and a specific range of temperature, are our best guides to exoplanet habitability. Figure from Jones et al. (2011). See also Mottl et al. (2007), Jones & Lineweaver (2010, 2012), and Cockell (2011).

Terrestrial biomass is approximately evenly divided between eukaryotes (55%; Figure 4, light purple) and prokaryotes (45%; Figure 4, orange). Biomass is also roughly evenly divided between above sea level (56%) and below sea level (44%). Of Earth's eukaryotic biomass, 99.5% is on land (Sundquist & Visser 2003). About 96% of prokaryotic biomass is below sea level, mostly in seafloor sediments (Whitman et al. 1998). The oxygenic photosynthesis that powers most of the eukaryotic life on land was a relatively late adaptation (Kiang et al. 2007, Sleep & Bird 2007), as was the ability to colonize the land (Battistuzzi et al. 2004). Five hundred million years ago, there was land but little eukaryotic life on it. Thus, for approximately the first three billion years, the profile of terrestrial biomass may have resembled the current prokaryotic profile (Figure 4, orange).

2.1. 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
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~4 Gya

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Figure 6 Abiogenesis habitable zone (AHZ). The conditions needed for the origin of life (before life could adapt) are narrower than the broader conditions to which life can adapt.

manage the transition from the nonliving to the living--to qualify as an AHZ--juxtapositions and flows of specific combinations of molecules are needed to produce auto-catalytic reactions and proto-metabolisms that can tap into either the flow of redox pairs or photons (de Duve 1995, Lane et al. 2010). Once life begins, organisms do not just passively adapt to preexisting environments. They actively change and construct the world they live in (Odling-Smee et al. 2003). The evolutionary history of life on Earth can be written in terms of how organisms have modified their environments. From the oxygenation of the atmosphere to the creation of beaver dams, life modifies its environment. But life also modifies itself and adapts to fit the environment--evolving, for example, spores to survive dry conditions, antifreeze for survival at low temperatures, and salt pumps to survive at high salinity. Whether life adapts to fit an environment or modifies an environment to be able to survive, the result is the same: The initial specific AHZ is widened (Figure 6). Through its management of the greenhouse and its partitioning of reductants and oxidants, the activity of life increases the range of inhabited environments (Nisbet et al. 2007, Hazen et al. 2008). Environments change life-forms, and life-forms change environments. This feedback between life and environment may be so strong that, for a planet to be habitable, it might have to be inhabited. Thus, planetary habitability becomes a dynamic concept with different stages. Habitability begins as a relatively narrow AHZ, completely dependent on the chemistry and other physical characteristics of the planet. Once life emerges and passes the "Darwinian threshold" (Woese 2002) habitability shifts to a dependence on both the characteristics of the planet and the adaptability of its life-forms. Finally, habitability becomes predominantly dependent on the ability of the inhabitants to regulate their environment. The thermoregulation of Earth over the past four billion years, despite a 30% increase in solar luminosity, is a possible example of such Gaian regulation (Lovelock 1965, 1979; Lovelock & Margulis 1974; Schneider & Boston 1991; Schneider et al. 2004). It may be that natural negative-feedback processes, such as the carbonsilicate cycle (Walker et al. 1981), without Gaian regulation, are not conducive to life any more
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than a nonevolving body would be (Schwartzman & Volk 1989). Thus, eventually, habitability becomes a property of life, as much as, or perhaps more than, it is a property of a planet. The persistence of life requires liquid water, an appropriate temperature range, nutrients, and an energy source. Self-assembly is an example of an additional requirement for abiogenesis that would be relaxed once life got started (making the AHZ narrower than the HZ). For example, origin-of-life chemists are trying to understand how RNA could self-assemble in the presence of water (e.g., Szostak et al. 2001). To self-assemble, dehydration reactions are needed. Cyclic evaporative dehydration could happen near continental hot springs or in warm tidal pools as the kilometer-high tides came in and out every few hours (e.g., Lathe 2004). On ocean planets (Section 4), there could be no dehydration reactions because there would be no evaporation from solid surfaces. Thus RNA would not self-assemble and life might not be able to get started on ocean planets. If the self-assembly of RNA required cyclic evaporation, and this assembly was critical to the emergence of life, ocean planets would be lifeless. They would be habitable but uninhabited planets.

2.2. Habitable Energies
On Earth today, 60% of the biomass is phototrophic and 40% is chemotrophic (Figure 4). Thus, the dominant energy source for life is the solar photon flux. However, when life emerged approximately four billion years ago there may have been much less dry land (Taylor & McLennan 2009) and no eukaryotes. The terrestrial biomass distribution thus may have more closely resembled the current prokaryotic distribution in which the majority of the biomass is not necessarily in the photic zone but at hydrothermal vents at various depths, which were more common approximately four billion years ago (Southam et al. 2007, Sleep 2010). Studying the earliest and most fundamental metabolisms of terrestrial life is our best hope for understanding how likely such energy-transducing metabolisms (and thus life) are to emerge elsewhere. Candidates for the earliest metabolisms include two broad categories of primary producers: anoxygenic phototrophs and chemolithotrophs. Chemolithotrophs live off inorganic redox pairs supplied by chemical disequilibria at hydrothermal vents (Nisbet & Sleep 2001, Kelly & Wood 2006). Hyperthermophiles are the deepest- and shortest-branched organisms on the phylogenetic tree of life (e.g., Pace 1997, Lineweaver & Schwartzman 2003). Hyperthermophiles gain energy by inorganic redox reactions employing compounds such as H2 ,CO2 ,S0 ,Fe2+ ,and Fe3+ (Stetter 2006). Redox reactions in hydrothermal vents and hot springs probably played a dominant role in early metabolism. The earliest life forms were probably chemoheterotrophs that evolved in hightemperature, low-pH, and high-salinity environments resembling hydrothermal vents (Martin et al. 2008) or hot springs. The transition of life from a redox-only energy source to a redox and photon energy source is suggested by comparing the energies of different metabolic reactions in Figure 7 (Sleep & Bird 2008). The earliest redox reactions--pyrite formation, sulfur reduction, methanogenesis, and acetogenesis (Wachtershauser 1998, Martin & Russell 2007, Blank 2009, ¨ ¨ Ljungdahl, 1986)--provide less energy than photosynthesis. However, these early reactions do provide enough energy to charge transmembrane potentials in a chemiosmotic coupling and convert low-energy molecules such as ADP, NAD+ , and NADP+ to higher-energy molecules such as ATP, NADH, and NADPH (Mitchell 1961). These molecules are universal energy currencies and likely to have been adopted by the earliest organisms. Energy sources based on a redox gradient may have been ubiquitous on the early Earth, particularly because hydrothermal activity may have been more widespread than it is today (Sleep 2007). The similar energies of the earliest metabolic pathways and the availability of the reactants in environments such as hydrothermal vents bolster the case that life began by using energy
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Figure 7 Comparison of the photon and redox energy sources of early life with the dominant energy currencies of life. The two sources of energy available to early life were solar photons [represented by the solar spectrum as seen from beneath 5 cm of water (orange)] and inorganic redox pairs available at hydrothermal vents (black squares). The energy obtained from both these sources was converted into the currencies life uses to store or circulate energy (transmembrane potential, ATP, and NADH). Uncertainties indicate the common range of energies associated with these currencies under physiological conditions. Five redox reactions are shown that are candidates for the sources of energy for the first chemolithotrophs [see Supplemental Table 1 (follow the Supplemental Materials link from the Annual Reviews home page at http://www.annualreviews.org) and Blank 2009]. Blackbody curves are shown for Sun-like G stars and for the most common star in the universe, low-mass M stars such as Gl581 (Section 4). On the left, using a different y-axis, the blackbody spectra of a 430 C hydrothermal vent (or hot spring) fluid and a 100 C fluid represent the native environment of hyperthermophiles. Absorption spectra are shown for two candidates for the earliest anoxygenic photosynthesis: bacteriochlorophyll (Frigaard et al. 1996, Madigan 2006) and the spectra of intact cells of an obligately photosynthetic bacterial anaerobe (green sulfur bacterium, GSB1) isolated from a deep-sea hydrothermal vent environment (Beatty et al. 2005). The absorption spectrum of eukaryotic chlorophyll a (Cinque et al. 2000) and the maximum energy available from oxic glucose respiration are shown for comparison.

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sources based on redox gradients and over time evolved to perform higher-energy reactions such as oxygenic photosynthesis and oxic respiration. Canfield et al. (2006) claim that the most-active, earliest ecosystems were probably driven by the cycling of H2 and Fe2+ through primary production conducted by anoxygenic phototrophs. The absorption spectra of bacteriochlorophylls, which are considered to be more ancient than chlorophylls, and the photosynthetic pigments in green sulfur bacterium (GSB1) peak outside the visible region of the spectrum in the near-IR, where photons can penetrate to some degree through murky water. Although the blackbody emission of hot hydrothermal fluids has far fewer photons in the near-IR than in the mid-IR, they are in sufficient numbers and of high enough energy that GSB1 can photosynthesize and live in a dark environment. Photons of even lower energies (1,000 nm) are used by purple anoxygenic bacteria (Kiang et al. 2007). Note that all redox reactions in Figure 7 are above the peak energy of the 100 C ambient temperature of a hyperthermophile environment. Any redox reaction that is used by life must satisfy the constraint that the activation energies must be higher than what is available as background energy in the environment (Shock & Holland 2007). An upper limit for the temperature at which metabolic activity can take place is set by the temperature at which the molecular dissociation of proteins and membranes takes place. If the conditions that permitted terrestrial abiogenesis based on energy-yielding metabolic reactions, such as those plotted in Figure 7, are not available on other planets, then the search for life elsewhere in the universe may yield the discovery of numerous habitable planets that have remained uninhabited. This is one possible outcome of Mars exploration. The search for life elsewhere in our Solar System has focused on Mars because it is relatively close and because Mars probably contains a lot of subsurface water ( Jones et al. 2011). In Figure 5, the substantial overlap of the green and red areas represents extensive water at habitable temperatures on Mars. This, combined with much other evidence for water on Mars, suggests that we will find liquid water in the Martian subsurface at temperatures compatible with terrestrial life. If appropriate redox pairs exist, psychrophilic terrestrial life should be able to live between tens of centimeters and ten kilometers beneath the Martian surface ( Jones et al. 2011). However, Mars exploration has not yet found any life. In 1976, two Viking spacecraft landed on Mars with life-detection instruments, which returned ambiguous results that have been interpreted as offering no evidence for life (Klein 1979, Klein 1999, Navarro-Gonzalez et al. 2010; see, however, Levin & Straat 1981). Although the Viking mission only attempted to search for active life on the surface (and not in the region below 10 cm where liquid water might be present), there are other potential biosignatures of subsurface life that may be detectable on the surface. One such potential biosignature was recently detected as transitory faint traces of methane (Formisano et al. 2004, Krasnopolsky et al. 2004, Mumma et al. 2009, Lefevre & Forget 2009). The debate about whether this methane could be biotic or abiotic seems to be leaning toward an abiotic explanation: Hot olivine exposed to water and CO2 undergoes serpentinizatio