Climate sensitivity

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Frequency distribution of climate sensitivity, based on model simulations.^[1] Few of the simulations result in less than 2 °C of warming—near the low end of estimates by the Intergovernmental Panel on Climate Change (IPCC).^[1] Some simulations result in significantly more than the 4 °C, which is at the high end of the IPCC estimates.^[1] This pattern (statisticians call it a "right-skewed distribution") suggests that if carbon dioxide concentrations double, the probability of very large increases in temperature is greater than the probability of very small increases.^[1]

Climate sensitivity is the equilibrium temperature change in response to changes of the radiative forcing.^[2] Therefore climate sensitivity depends on the initial climate state, but potentially can be accurately inferred from precise palaeoclimate data. Slow climate feedbacks, especially changes of ice sheet size and atmospheric CO₂, amplify the total Earth system sensitivity by an amount that depends on the time scale considered.^[3]

Although climate sensitivity is usually used in the context of radiative forcing by carbon dioxide (CO₂), it is thought of as a general property of the climate system: the change in surface air temperature (ΔT_s) following a unit change in radiative forcing (RF), and thus is expressed in units of °C/(W/m²). For this to be useful, the measure must be independent of the nature of the forcing (e.g. from greenhouse gases or solar variation); to first order this is indeed found to be so^{[citation needed]}.
The climate sensitivity specifically due to CO₂ is often expressed as the temperature change in °C associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.
For coupled atmosphere-ocean global climate models (e.g. CMIP5) the climate sensitivity is an emergent property: it is not a model parameter, but rather a result of a combination of model physics and parameters. By contrast, simpler energy-balance models may have climate sensitivity as an explicit parameter.

The terms represented in the equation relate radiative forcing (RF) to linear changes in global surface temperature change (ΔT_s) via the climate sensitivity λ.

It is also possible to estimate climate sensitivity from observations; however, this is difficult due to uncertainties in the forcing and temperature histories.

Equilibrium and transient climate sensitivity

The equilibrium climate sensitivity (ECS) refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent) carbon dioxide concentration (ΔT_x2). As estimated by the IPCC Fifth Assessment Report (AR5) "there is high confidence that ECS is extremely unlikely less than 1°C and medium confidence that the ECS is likely between 1.5°C and 4.5°C and very unlikely greater than 6°C."^[4] This is a change from the IPCC Fourth Assessment Report (AR4), which said it was likely to be in the range 2 to 4.5 °C with a best estimate of about 3 °C, and is very unlikely to be less than 1.5 °C. Values substantially higher than 4.5 °C cannot be excluded, but agreement of models with observations is not as good for those values.^[5] The IPCC Third Assessment Report (TAR) said it was "likely to be in the range of 1.5 to 4.5 °C".^[6] Other estimates of climate sensitivity are discussed later on.
A model estimate of equilibrium sensitivity thus requires a very long model integration; fully equilibrating ocean temperatures requires integrations of thousands of model years. A measure requiring shorter integrations is the transient climate response (TCR) which is defined as the average temperature response over a twenty-year period centered at CO₂ doubling in a transient simulation with CO₂ increasing at 1% per year.^[7] The transient response is lower than the equilibrium sensitivity, due to the "inertia" of ocean heat uptake.

Over the 50–100 year timescale, the climate response to forcing is likely to follow the TCR; for considerations of climate stabilization, the ECS is more useful.

An estimate of the equilibrium climate sensitivity may be made from combining the transient climate sensitivity with the known properties of the ocean reservoirs and the surface heat fluxes; this is the effective climate sensitivity. This "may vary with forcing history and climate state".^{[8] [9]}

A less commonly used concept, the Earth system sensitivity (ESS), can be defined which includes the effects of slower feedbacks, such as the albedo change from melting the large ice sheets that covered much of the northern hemisphere during the last glacial maximum. These extra feedbacks make the ESS larger than the ECS — possibly twice as large — but also mean that it may well not apply to current conditions.^[10]

Sensitivity to carbon dioxide forcing

Climate sensitivity is often evaluated in terms of the change in equilibrium temperature due to radiative forcing due to the greenhouse effect. According to the Arrhenius relation,^[11] the radiative forcing (and hence the change in temperature) is proportional to the logarithm of the concentration of infrared-absorbing gasses in the atmosphere. Thus, the sensitivity of temperature to gasses in the atmosphere (most notably carbon dioxide) is often expressed in terms of the change in temperature per doubling of the concentration of the gas.

Radiative forcing due to doubled CO₂

CO₂ climate sensitivity has a component directly due to radiative forcing by CO₂, and a further contribution arising from climate feedbacks, both positive and negative. "Without any feedbacks, a doubling of CO₂ (which amounts to a forcing of 3.7 W/m²) would result in 1 °C global warming, which is easy to calculate and is undisputed. The remaining uncertainty is due entirely to feedbacks in the system, namely, the water vapor feedback, the ice-albedo feedback, the cloud feedback, and the lapse rate feedback";^[12] addition of these feedbacks leads to a value of the sensitivity to CO₂ doubling of approximately 3 °C ± 1.5 °C, which corresponds to a value of λ of 0.8 K/(W/m²).

In the earlier 1979 NAS report^[13] (p. 7), the radiative forcing due to doubled CO₂ is estimated to be 4 W/m², as calculated (for example) in Ramanathan et al. (1979).^[14] In 2001 the IPCC adopted the revised value of 3.7 W/m², the difference attributed to a "stratospheric temperature adjustment".^[15] More recently an intercomparison of radiative transfer codes (Collins et al., 2006)^[16] showed discrepancies among climate models and between climate models and more exact radiation codes in the forcing attributed to doubled CO₂ even in cloud-free sky; presumably the differences would be even greater if forcing were evaluated in the presence of clouds because of differences in the treatment of clouds in different models. Undoubtedly the difference in forcing attributed to doubled CO₂ in different climate models contributes to differences in apparent sensitivities of the models, although this effect is thought to be small relative to the intrinsic differences in sensitivities of the models themselves.^[17]

Frequency distribution of climate sensitivity, based on model simulations.^[1] Few of the simulations result in less than 2 °C of warming—near the low end of estimates by the Intergovernmental Panel on Climate Change (IPCC).^[1] Some simulations result in significantly more than the 4 °C, which is at the high end of the IPCC estimates.^[1] This pattern (statisticians call it a "right-skewed distribution") suggests that if carbon dioxide concentrations double, the probability of very large increases in temperature is greater than the probability of very small increases.^[1]

Consensus estimates

A committee on anthropogenic global warming convened in 1979 by the National Academy of Sciences and chaired by Jule Charney^[13] estimated climate sensitivity to be 3 °C, plus or minus 1.5 °C. Only two sets of models were available; one, due to Syukuro Manabe, exhibited a climate sensitivity of 2 °C, the other, due to James E. Hansen, exhibited a climate sensitivity of 4 °C. "According to Manabe, Charney chose 0.5 °C as a not-unreasonable margin of error, subtracted it from Manabe’s number, and added it to Hansen’s. Thus was born the 1.5 °C-to-4.5 °C range of likely climate sensitivity that has appeared in every greenhouse assessment since..."^[18]

Chapter 4 of the "Charney report" compares the predictions of the models: "We conclude that the predictions ... are basically consistent and mutually supporting. The differences in model results are relatively small and may be accounted for by differences in model characteristics and simplifying assumptions."^[13]

In 2008 climatologist Stefan Rahmstorf wrote, regarding the Charney report's original range of uncertainty: "At that time, this range was on very shaky ground. Since then, many vastly improved models have been developed by a number of climate research centers around the world. Current state-of-the-art climate models span a range of 2.6–4.1 °C, most clustering around 3 °C."^[12]

Intergovernmental Panel on Climate Change

The 1990 IPCC First Assessment Report estimated that equilibrium climate sensitivity to CO₂ doubling lay between 1.5 and 4.5 °C, with a "best guess in the light of current knowledge" of 2.5 °C.^[19] This used models with strongly simplified representations of the ocean dynamics. The IPCC supplementary report, 1992 which used full ocean GCMs nonetheless saw "no compelling reason to warrant changing" from this estimate ^[20] and the IPCC Second Assessment Report found that "No strong reasons have emerged to change" these estimates,^[21] with much of the uncertainty attributed to cloud processes. As noted above, the IPCC TAR retained the likely range 1.5 to 4.5 °C.^[6]

Authors of the IPCC Fourth Assessment Report (Meehl et al., 2007)^[22] stated that confidence in estimates of equilibrium climate sensitivity had increased substantially since the TAR. AR4's assessment was based on a combination of several independent lines of evidence, including observed climate change and the strength of known "feedbacks" simulated in general circulation models.^[23] IPCC authors concluded that the global mean equilibrium warming for doubling CO₂ (to a concentration of 560 ppmv), or equilibrium climate sensitivity, very likely is greater than 1.5 °C (2.7 °F) and likely to lie in the range 2 to 4.5 °C (4 to 8.1 °F), with a most likely value of about 3 °C (5 °F). For fundamental physical reasons, as well as data limitations, the IPCC states a climate sensitivity higher than 4.5 °C (8.1 °F) cannot be ruled out, but that agreement for these values with observations and "proxy" climate data is generally worse compared to values in the 2 to 4.5 °C (4 to 8.1 °F) range.^[23]

The TAR uses the word "likely" in a qualitative sense to describe the likelihood of the 1.5 to 4.5 °C range being correct.^[22] AR4, however, quantifies the probable range of climate sensitivity estimates:^[24]

• 2-4.5 °C is "likely", = greater than 66% chance of being correct
• less than 1.5 °C is "very unlikely" = less than 10%

The IPCC Fifth Assessment Report stated: Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence).

These are Bayesian probabilities, which are based on an expert assessment of the available evidence.^[24]

Calculations of CO₂ sensitivity from observational data

Sample calculation using industrial-age data

Rahmstorf (2008)^[12] provides an informal example of how climate sensitivity might be estimated empirically, from which the following is modified. Denote the sensitivity, i.e. the equilibrium increase in global mean temperature including the effects of feedbacks due to a sustained forcing by doubled CO₂ (taken as 3.7 W/m²), as x °C. If Earth were to experience an equilibrium temperature change of ΔT (°C) due to a sustained forcing of ΔF (W/m²), then one might say that x/(ΔT) = (3.7 W/m²)/(ΔF), i.e. that x = ΔT * (3.7 W/m²)/ΔF. The global temperature increase since the beginning of the industrial period (taken as 1750) is about 0.8 °C, and the radiative forcing due to CO₂ and other long-lived greenhouse gases (mainly methane, nitrous oxide, and chlorofluorocarbons) emitted since that time is about 2.6 W/m². Neglecting other forcings and considering the temperature increase to be an equilibrium increase would lead to a sensitivity of about 1.1 °C. However, ΔF also contains contributions due to solar activity (+0.3 W/m²), aerosols (-1 W/m²), ozone (0.3 W/m²) and other lesser influences, bringing the total forcing over the industrial period to 1.6 W/m² according to best estimate of the IPCC AR4, albeit with substantial uncertainty. Additionally the fact that the climate system is not at equilibrium must be accounted for; this is done by subtracting the planetary heat uptake rate H from the forcing; i.e., x = ΔT * (3.7 W/m²)/(ΔF-H). Taking planetary heat uptake rate as the rate of ocean heat uptake, estimated by the IPCC AR4 as 0.2 W/m², yields a value for x of 2.1 °C. (All numbers are approximate and quite uncertain.)

Sample calculation using ice-age data

In 2008, Farley wrote: "... examine the change in temperature and solar forcing between glaciation (ice age) and interglacial (no ice age) periods. The change in temperature, revealed in ice core samples, is 5 °C, while the change in solar forcing is 7.1 W/m². The computed climate sensitivity is therefore 5/7.1 = 0.7 K(W/m²)⁻¹. We can use this empirically derived climate sensitivity to predict the temperature rise from a forcing of 4 W/m², arising from a doubling of the atmospheric CO₂ from pre-industrial levels. The result is a predicted temperature increase of 3 °C."^[25]

Based on analysis of uncertainties in total forcing, in Antarctic cooling, and in the ratio of global to Antarctic cooling of the last glacial maximum relative to the present, Ganopolski and Schneider von Deimling (2008) infer a range of 1.3 to 6.8 °C for climate sensitivity determined by this approach.^[26]

A lower figure was calculated in a 2011 Science paper by Schmittner et al., who combined temperature reconstructions of the Last Glacial Maximum with climate model simulations to suggest a rate of global warming from doubling of atmospheric carbon dioxide of a median of 2.3 °C and uncertainty 1.7–2.6 °C (66% probability range), less than the earlier estimates of 2 to 4.5 °C as the 66% probability range. Schmittner et al. said their "results imply less probability of extreme climatic change than previously thought." Their work suggests that climate sensitivities >6 °C "cannot be reconciled with paleoclimatic and geologic evidence, and hence should be assigned near-zero probability."^[27][28]

Other experimental estimates

Idso (1998)^[29] calculated based on eight natural experiments a λ of 0.1 °C/(Wm⁻²) resulting in a climate sensitivity of only 0.4 °C for a doubling of the concentration of CO₂ in the atmosphere.
Andronova and Schlesinger (2001) found that the climate sensitivity could lie between 1 and 10 °C, with a 54 percent likelihood that it lies outside the IPCC range.^[30] The exact range depends on which factors are most important during the instrumental period: "At present, the most likely scenario is one that includes anthropogenic sulfate aerosol forcing but not solar variation. Although the value of the climate sensitivity in that case is most uncertain, there is a 70 percent chance that it exceeds the maximum IPCC value. This is not good news," said Schlesinger.

Forest, et al. (2002)^[31] using patterns of change and the MIT EMIC estimated a 95% confidence interval of 1.4–7.7 °C for the climate sensitivity, and a 30% probability that sensitivity was outside the 1.5 to 4.5 °C range.

Gregory, et al. (2002)^[32] estimated a lower bound of 1.6 °C by estimating the change in Earth's radiation budget and comparing it to the global warming observed over the 20th century.

Shaviv (2005)^[33] carried out a similar analysis for 6 different time scales, ranging from the 11-yr solar cycle to the climate variations over geological time scales. He found a typical sensitivity of 0.54±0.12 K/(W m⁻²) or 2.1 °C (ranging between 1.6 °C and 2.5 °C at 99% confidence) if there is no cosmic-ray climate connection, or a typical sensitivity of 0.35±0.09 K/(W m⁻²) or 1.3 °C (between 1.0 °C and 1.7 °C at 99% confidence), if the cosmic-ray climate link is real. (Note Shaviv quotes a radiative forcing equivalent of 3.8 Wm⁻². [ΔT_x2=3.8 Wm⁻² λ].)

Frame, et al. (2005)^[34] noted that the range of the confidence limits is dependent on the nature of the prior assumptions made.

Annan and Hargreaves (2006)^[35] presented an estimate that resulted from combining prior estimates based on analyses of paleoclimate, responses to volcanic eruptions, and the temperature change in response to forcings over the twentieth century. They also introduced a triad notation (L, C, H) to convey the probability distribution function (pdf) of the sensitivity, where the central value C indicates the maximum likelihood estimate in degrees Celsius and the outer values L and H represent the limits of the 95% confidence interval for a pdf, or 95% of the area under the curve for a likelihood function. In this notation their estimate of sensitivity was (1.7, 2.9, 4.9) °C.

Forster and Gregory (2006)^[36] presented a new independent estimate based on the slope of a plot of calculated greenhouse gas forcing minus top-of-atmosphere energy imbalance, as measured by satellite borne radiometers, versus global mean surface temperature. In the triad notation of Annan and Hargreaves their estimate of sensitivity was (1.0, 1.6, 4.1) °C.

Royer, et al. (2007)^[37] determined climate sensitivity within a major part of the Phanerozoic. The range of values—1.5 °C minimum, 2.8 °C best estimate, and 6.2 °C maximum—is, given various uncertainties, consistent with sensitivities of current climate models and with other determinations.^[38]

Lindzen and Choi (2011) find the equilibrium climate sensitivity to be 0.7 C, implying a negative feedback of clouds.^[39]

Ring et all (2012) find the equilibrium climate sensitivity to be in the range 1.45 C- 2.01 C, depending on the data set used as an input in model simulations.^[40]

Skeie et al (2013) use the Bayesian analysis of the OHC data and conclude that the equilibrium climate sensitivity is 1.8 C, far lower than previous best estimate relied upon by the IPCC.^[41]

Aldrin et al (2012)use simple deterministic climate model, modelling yearly hemispheric surface temperature and global ocean heat content as a function of historical radiative forcing and combine it with an empirical, stochastic model. By using a Bayesian framework they estimate the equilibrium climate sensitivity to be 1.98 C.^[42]

Lewis (2013) estimates by using the Bayesian framework that the equilibrium climate sensitivity is 1.6 K, with the likely range (90% confidence level) 1.2-2.2 K.^[43]

ScienceDaily reported on a study by Fasullo and Trenberth (2012),^[44] who tested model estimates of climate sensitivity based on their ability to reproduce observed relative humidity in the tropics and subtropics. The best performing models tended to project relatively high climate sensitivities, of around 4 °C.^[44]

Previdi et al. 2013 reviewed the 2×CO₂ Earth system sensitivity, and concluded it is higher if the ice sheet and the vegetation albedo feedback is included in addition to the fast feedbacks, being ∼4–6 °C, and higher still if climate–GHG feedbacks are also included.^[45]

Lewis and Curry (2014) estimated that equilibrium climate sensitivity was 1.64  °C, based on the 1750-2011 time series and "the uncertainty ranges for forcing components" in the IPCC's Fifth Assessment Report.^[46]

Literature reviews

A literature review by Knutti and Hegerl (2008)^[47] concluded that "various observations favour a climate sensitivity value of about 3 °C, with a likely range of about 2-4.5 °C. However, the physics of the response and uncertainties in forcing lead to difficulties in ruling out higher values."

Radiative forcing functions

A number of different inputs can give rise to radiative forcing. In addition to the downwelling radiation due to the greenhouse effect, the IPCC First Scientific Assessment Report listed solar radiation variability due to orbital changes, variability due to changes in solar irradiance, direct aerosol effects (e.g., changes in albedo due to cloud cover), indirect aerosol effects, and surface characteristics.^[48]

Sensitivity to solar forcing

Solar irradiance is about 0.9 W/m² brighter during solar maximum than during solar minimum. Analysis by Camp and Tung shows that this correlates with a variation of ±0.1°C in measured average global temperature between the peak and minimum of the 11-year solar cycle.^[49] From this data (incorporating the Earth's albedo and the fact that the solar absorption cross-section is 1/4 of the surface area of the Earth), Tung, Zhou and Camp (2008) derive a transient sensitivity value of 0.69 to 0.97 °C/(W/m²).^[50] This would correspond to a transient climate sensitivity to carbon dioxide doubling of 2.5 to 3.6 K, similar to the range of the current scientific consensus. However, they note that this is the transient response to a forcing with an 11 year cycle; due to lag effects, they estimate the equilibrium response to forcing would be about 1.5 times as high.

Circumstellar habitable zone

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An example of a system based on stellar luminosity for predicting the location of the habitable zone around various types of stars. Planet sizes, star sizes, orbit lengths, and habitable zone sizes are not to scale.

In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the region around a star within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.^[1][2] The bounds of the CHZ are calculated using the known requirements of Earth's biosphere, its position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to life as it exists on Earth, the nature of the CHZ and the objects within is believed to be instrumental in determining the scope and distribution of Earth-like extraterrestrial life and intelligence.

The habitable zone is also called the Goldilocks zone, a metaphor of the children's fairy tale of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".

Since the concept was first presented in 1953,^[3] stars have been confirmed to possess a CHZ planet, including some systems that consist of multiple CHZ planets.^[4] Most such planets, being super-Earths or gas giants, are more massive than Earth, because such planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way.^[5][6] 11 billion of these may be orbiting Sun-like stars.^[7] The nearest such planet may be 12 light-years away, according to the scientists.^[5][6] The CHZ is also of particular interest to the emerging field of habitability of natural satellites, because planetary-mass moons in the CHZ might outnumber planets.^[8]

In subsequent decades, the CHZ concept began to be challenged as a primary criterion for life. Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now believed to occur outside the circumstellar habitable zone. Sustained by other energy sources, such as tidal heating^[9][10] or radioactive decay^[11] or pressurized by other non-atmospheric means, the basic conditions for water-dependent life may be found even in interstellar space, on rogue planets, or their moons.^[12] Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on Equatorial Mars,^[13] or ammoniates,^[14] due to its different colligative properties. In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.^[15]

History

The concept of a Circumstellar Habitable Zone was first introduced in 1953 by Hubertus Strughold, who in his treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars coined the term "ecosphere" and referred to various "zones" in which life could emerge.^[3][16] In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same theory in further scientific detail. Both works stressed the importance of liquid water to life.^[17] Su-Shu Huang, an American astrophysicist, first introduced the term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life.^[18][19] A major early contributor to habitable zone theory, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems.^[20]

The theory of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man, in which he covered the circumstellar habitable zone itself as well as various other determinants of planetary habitability, eventually estimating the number of habitable planets in the Milky Way to be about 600 million.^[21] At the same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization.^[22] The term "Goldilocks zone" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase.^[23] In 1993, astronomer James Kasting introduced the term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone.^[18]

An update to habitable-zone theory came in 2000, when astronomers Peter Ward and Donald Brownlee introduced the idea of the "galactic habitable zone", which they later developed with Guillermo Gonzalez.^[24][25] The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements, but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers.^[24]

Subsequently, several planetary scientists have criticized the circumstellar habitable zone theory for its "carbon chauvinism", proposing that the concept be extended to other solvents, such as ammonia or methane, which could be the basis of life based on an alternative biochemistry.^[15] In 2013, further developments in habitable zone theory were made with the proposal of a circumplanetary habitable zone, also known as the "habitable edge", to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away.^[26]

Determination of the circumstellar habitable zone

The range of published estimates for the extent of the Sun's CHZ. The conservative CHZ^[21] is indicated by a dark-green band crossing the inner edge of the aphelion of Venus, whereas an extended CHZ,^[27] extending to the orbit of the dwarf planet Ceres, is indicated by a light-green band.

Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth or Venus can remain habitable, complemented by a larger "extended habitable zone" in which super-Earth planets, with stronger greenhouse effects, can have the right temperature for liquid water to exist at the surface.^[28]

Solar System estimates

Estimates for the habitable zone within the Solar System range from 0.5 to 3.0 astronomical units,^[29] though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches the inner edge of the zone and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to 462 °C (864 °F) at which water can only exist as vapour.^[30] The entire orbits of the Moon,^[31] Mars,^[32] and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods.^[33] At Hellas Basin, for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero (around the triple point for water) for 70 days in the Martian year.^[33] Despite indirect evidence in the form of seasonal flows on warm Martian slopes,^{[34][35][36][37]} no confirmation has been made of the presence of liquid water there. While other objects orbit partly within this zone, including comets, Ceres^[38] is the only one of planetary mass. A combination of low mass and an inability to mitigate evaporation and atmosphere loss against the solar wind make it impossible for these bodies to sustain liquid water on their surface. Most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus.

According to extended habitable zone theory, planetary mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth (Super-Earth class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study and not enough is known about the nature of atmospheres of these kinds of extrasolar objects and the net temperature effect of such atmospheres including induced albedo, anti-greenhouse or other possible heat sources cannot be determined by their position in the habitable zone.

Estimates of the circumstellar-habitable-zone boundaries of the Solar System

Inner edge (AU)	Outer edge (AU)	Year	Notes
0.725	1.24	Dole 1964[21]	Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone.
	1.385–1.398	Budyko 1969[39]	Based on studies of ice albedo feedback models to determine the point at which Earth would experience global glaciation. This estimate was supported in studies by Sellers 1969[40] and North 1975.[41]
0.88–0.912		Rasool and De Bergh 1970[42]	Based on studies of Venus's atmosphere, Rasool and De Bergh concluded that this is the minimum distance at which Earth would have formed stable oceans.
0.95	1.01	Hart et al. 1979[43]	Based on computer modelling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications.
	3.0	Fogg 1992[27]	Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone.
	1.37	Kasting et al. 1993[18]	Noted the cooling effect of cloud albedo.
	2.0	Spiegel et al. 2010[44]	Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity.
0.75		Abe et al. 2011[45]	Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth.
0.77—0.87	1.02—1.18	Vladilo et al. 2013[46]	Inner edge of circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 millibar.
0.99	1.688	Kopparapu et al. 2013[1]	Revised estimates using updated runaway greenhouse and water loss algorithms. According to this measure Earth is at the inner edge of the HZ and close to, but just outside, the runaway greenhouse limit. This applies to a planet with Earth-like atmospheric composition and pressure.
0.5		Zsom et al. 2013 [47]	Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere.

Extrasolar extrapolation

Astronomers use stellar flux and the inverse-square law to extrapolate cirumstellar-habitable-zone models created for the Solar System to other stars. For example, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun,^[1] a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at , or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of the CHZ concept is more complex.

Spectral types and star-system characteristics

Play media

A video explaining the significance of the 2011 discovery of a planet in the circumbinary habitable zone of Kepler-47.

Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain spectral types. Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital-stability concerns inherent with a three-body configuration.^[48] If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU.^[49][50]

With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions.^[51] Studying ultraviolet emissions, Andrea Buccino found that only 40 percent of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones.^[52] Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. Michael Hart, for example, proposed that only main-sequence stars of spectral class K0 or brighter could possess habitable zones, an idea which has evolved in modern times into the concept of a tidal locking radius for red dwarfs. Within this radius, which is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no ability to support life.^[53]

Others maintain that circumstellar habitable zones are more common and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modelling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking.^[54] Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration.^[55] At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs.^[53]

Stellar evolution

Natural defenses against space weather, such as the magnetosphere depicted in this artistic rendition, may be required for planets to sustain surface water for prolonged periods.

Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the main sequence for fewer than 10 million years,^[56] would have rapidly changing habitable zones not conducive to the development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve.^[57][58] Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther and farther out; our Sun, for example, was only 75 percent as bright in the Archaean as it is now,^[59] and in the future continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the red giant phase.^[60] In order to deal with this increase in luminosity, the concept of a continuously habitable zone has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for a given period of time. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region.^[60]

In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes^[61] and huge starspots which can cover 20 percent of the star's surface area,^[62] have the potential to strip an otherwise habitable planet of its atmosphere and water.^[63] As with more massive stars, though, stellar evolution changes their nature,^[64] so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life.^[63][65]

Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size. For example, the Sun is expected to engulf the previously-habitable Earth as a red giant.^[66] However, once a red giant star reaches the horizontal branch, it achieves a new equilibrium and can sustain a circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU.^[67] At such stage, Saturn's moon Titan would likely be habitable in Earth's sense.^[68] Given that this new equilibrium lasts for about 1 Gyr, and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants.^[67] However, around such a helium-burning star, important life processes like photosynthesis could only happen around planets where the atmosphere has been artificially seeded with carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide.^[69]

Desert planets

A planet's atmospheric conditions influence its ability to retain heat, so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.^[70][71]

Other considerations

Earth's hydrosphere. Water covers 71% of Earth's surface, with the global ocean accounting for 97.3% of the water distribution on Earth.

A planet cannot have a hydrosphere—a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The origin of water on Earth is still not completely understood; possible sources include the result of impacts with icy bodies, outgassing, mineralization, leakage from hydrous minerals from the lithosphere, and photolysis.^[72][73] For an extrasolar system, an icy body from beyond the frost line could migrate into the habitable zone of its star, creating an ocean planet with seas hundreds of kilometers deep^[74] such as GJ 1214 b^[75][76] or Kepler-22b may be.^[77]

Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorised to outgassing, impact degassing and ingassing.^[78] Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape.^[79] In a 2013 study led by Italian astronomer Giovanni Vladilo, it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure.^[46] Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained^[46] because even a small shift in pressure or temperature could render water unable to form a liquid.^[80]

In the case of planets orbiting in the CHZs of red dwarf stars, the extremely close distances to the stars cause tidal locking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbital period, causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was believed to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, a 2013 paper written by geophysicist Jun Yang of the University of Chicago and collaborators, using three-dimensional climate models, showed that the side of a red dwarf planet facing the host star would have extensive cloud cover, increasing its Bond albedo and reducing significantly temperature differences between the two sides.^[54]

Planetary-mass natural satellites have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets.^[26] More specifically, planets need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like Io,^[26] but must still remain within the Hill radius of the planet so that they are not pulled out of orbit of their host planet.^[81] Red dwarfs that have masses less than 20 percent of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to a star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability.^[26]

Artists concept of a planet on an eccentric orbit that passes through the CHZ for only part of its year

A planetary object that orbits a star with high orbital eccentricity may spend only some of its year in the CHZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation (cryptobiosis) and/or hyperthermostability. Tardigrades, for example, can survive in a dehydrated state temperatures between 0.150 K (−273 °C)^[82] and 424 K (151 °C).^[83] Life on a planetary object orbiting outside CHZ might hibernate on the cold side as the planet approaches the apastron where the planet is coolest and become active on approach to the periastron when the planet is sufficiently warm.^[84]

Extrasolar discoveries

Among exoplanets, a review in 2015 came to the conclusion that Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable.^[85] These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is in similar size to Earth with its 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf sun. Among nearest terrestrial exoplanet candidates, Tau Ceti e is merely 11.9 light-years away. It's in the inner edge of its solar system's habitable zone, giving it an estimated average surface temperature of 68 °C (154 °F).^[86]
Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put η_e, the fraction of stars with planets in the CHZ, at 0.48,^[1] meaning that there may be roughly 95–180 billion habitable planets in the Milky Way.^[87] However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.^[88]

Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way.^[89] NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, concluding that about "1.4 to 2.7 percent" of all stars of spectral class F, G, and K are expected to have planets in their CHZs.^[90][91]

Early findings

The first discoveries of extrasolar planets in the CHZ occurred just a few years after the first extrasolar planets were discovered. One of the first discoveries was 70 Virginis b, a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold." Later study revealed temperatures analogous to Venus ruling out any potential for liquid water.^[92] 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that causes extreme seasonal effects on the planet's surface. In spite of this, simulations have suggested that it is possible for a terrestrial natural satellite to support water at its surface year-round.^[93]
Gliese 876 b, discovered in 1998, and Gliese 876 c, discovered in 2001, are both gas giants discovered in the habitable zone around Gliese 876. Although they are not thought to themselves possess significant water at their surfaces, both may have habitable moons.^[94] Upsilon Andromedae d, discovered in 1999, is a gas giant in its star's circumstellar habitable zone considered to be large enough to favor the formation of large, Earth-like moons.^[95]

Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone^[96] and has a low orbital eccentricity, comparable to that of Mars in the Solar System.^[97] Tidal interactions suggest that HD 28185 b could harbor habitable Earth-mass satellites in orbit around it for many billions of years,^[98] though it is unclear whether such satellites could form in the first place.^[99]

HD 69830 d, a gas giant with 17 times the mass of Earth, was in 2006 found orbiting within the circumstellar habitable zone of HD 69830, 41 light years away from Earth.^[100] The following year, 55 Cancri f was discovered within the CHZ of its host star 55 Cancri A.^[101][102] Although conditions on this massive and dense planet are not conducive to the formation of water or life as we know it, a hypothetical moon of this planet with the proper mass and composition could be able to support liquid water at its surface.^[103]

Habitable super-Earths

The habitable zone of Gliese 581 compared with our Solar System's habitable zone.

The 2007 discovery of Gliese 581 c, the first super-Earth in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have surface conditions that likely resemble Venus more than Earth.^[104] Gliese 581 d, another planet in the same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014. Gliese 581 g, yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence was also disconfirmed in 2014.^[105]

A diagram comparing size (artist's impression) and orbital position of planet Kepler-22b within Sun-like star Kepler 22's habitable zone and that of Earth in the Solar System

Discovered in August 2011, HD 85512 b was initially believed to be habitable,^[106] but the new circumstellar-habitable-zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.^[88] With an increase in the intensity of exoplanet discovery, the Earth Similarity Index was devised in October 2011 as a way of comparing planetary properties, such as surface temperature and density, to those of Earth in order to better gauge the habitability of extrasolar bodies.^[107]

Kepler-22 b, discovered in December 2011 by the Kepler space probe,^[108] is the first transiting exoplanet discovered around a sunlike star. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet.^[109] Gliese 667 Cc, discovered in 2011 but announced in 2012,^[110] is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C. Subsequently in June 2013, two other habitable super-Earths orbiting the same star, Gliese 667 Cf and Gliese 667 Ce, were discovered in the CHZ.^[111]

Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163^[112] is located 49 light years from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about 60° C.^{[113][114][115]} HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307.^[116] In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a sunlike star just 12 light years away.^[117] Although more massive than Earth, they are among the least massive planets found to date orbiting in the zone;^[118] however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar-habitable-zone criteria established by the 2013 Kopparapu study.^[119]

Earth-sized planets and Solar analogs

Comparison of the CHZ position of Earth-radius planet Kepler-186f and the Solar System (17 April 2014)

While larger than Kepler 186f, Kepler-452b's orbit and star are more similar to Earth's.

Recent discoveries have uncovered planets that are believed to be similar in many ways to the Earth (that is Earth analogs, or terrestrial planets relatively high Earth Similarity Indexes). While there is no universal definition of "Earth-sized", ranges are typically defined by mass. The lower range used in many definitions of the Super-Earth class is 1.9 Earth masses, likewise, Sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above 1.5 R_⊕ the average planet density rapidly decreases with increasing radius, indicating that these planets have a large fraction of volatiles by volume overlying a rocky core.^[120]

On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-like exoplanet candidate (1.7 times the radius of Earth) orbiting Kepler-69, a star similar to our Sun, in the CHZ and a "prime candidate to host alien life".^{[121][122][123][124]} The discovery of two planets orbiting in the habitable zone of Kepler-62, by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f, are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively.^{[123][125][126]}

With a radius measured at 1.1 Earth, Kepler-186f, discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method^{[127][128][129]} though its mass remains unknown and its parent star is not a Solar analog.

On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth.^[130]{ Announced 16 January, EPIC 201367065 d is a planet of 1.5 Earth radii found to orbit within a habitable zone (as calculated by Selsis, Kasting et al.) of EPIC 201367065, receiving 1.4 times the intensity of visible light as Earth.^[131]

Kepler-452b, publicly announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days orbit in the habitable zone of its G-class (solar analog) star Kepler-452.^[132][133]

Notable exoplanets – Kepler Space Telescope
Confirmed small exoplanets in habitable zones. (Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b) (Kepler Space Telescope; January 6, 2015).[130]

Habitability outside the CHZ

The discovery of hydrocarbon lakes on Saturn's moon Titan has begun to call into question the carbon chauvinism that underpins CHZ theory.

Liquid-water environments have been found to exist in the absence of atmospheric pressure, and at temperatures outside the CHZ temperature range. For example, Saturn's moon Titan and Jupiter's Europa, both outside the habitable zone, may hold large volumes of liquid water in subsurface oceans.^[134]

Outside the CHZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water.^[9][10] Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on rogue planets as a result of radioactive decay-based heating and insulation by a thick surface layer of ice.^[12]

With some theorising that life on Earth may have actually originated in stable, subsurface habitats,^[135][136] it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'.^[137] Indeed, on Earth itself living organisms may be found more than 6 kilometres below the surface.^[138]

Another possibility is that outside the CHZ organisms may use alternative biochemistries that do not require water at all. Astrobiologists, including NASA's Christopher McKay, have suggested that methane may be a solvent conducive to the development of "cryolife", with the Sun's "methane habitable zone" being centered on 1,610,000,000 km (1.0×10⁹ mi; 11 AU) from the star.^[15] This distance is coincidental with the location of Titan, whose lakes and rain of methane make it an ideal location to find McKay's proposed cryolife.^[15] In addition, testing of a number of organisms has found some are capable of surviving in extra-CHZ conditions.^[139]

Significance for complex and intelligent life

The Rare Earth hypothesis argues that complex and intelligent life is uncommon and that the CHZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a CHZ orbit and surface water a primary requirement to sustain life but a requirement to support the secondary conditions required for multicellular life to emerge and evolve. The secondary habitability factors are both geological (the role of surface water in sustaining necessary plate tectonics)^[24] and biochemical (the role of radiant energy in support photosynthesis for necessary atmospheric oxygenation).^[140] But others, such as Ian Stewart and Jack Cohen in their 2002 book Evolving the Alien argue that complex intelligent life may arise outside the CHZ.^[141] Intelligent life outside the CHZ may have evolved in subsurface environments, from alternative biochemistries^[141] or even from nuclear reactions.^[142]

On Earth, complex multicellular life has been found with the potential to survive the conditions that might exist outside the CHZ. An animal example of such a life form is the tardigrade, which can withstand both temperatures well above the boiling point of water and the vacuum of outer space.^[143] In addition, the plant Rhizocarpon geographicum has been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize.^[144][145] If the human race, however, is to colonize other planets, true Earth analogs in the CHZ are most likely to provide the closest natural habitats for human beings; this concept was the basis of Stephen H. Dole's 1964 study. With suitable temperature, gravity, atmospheric pressure and the presence of water, the necessity of spacesuits may be eliminated and complex Earth-life can be allowed to flourish.^[21]

Planets in the CHZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe.^[146] The 1961 Drake equation, still used as means of calculating the number of intelligent civilizations in our galaxy, contains a parameter η_e, which is generally considered to imply the fraction of stars that have planetary mass objects orbiting within the CHZ. A low value lends support to the Rare Earth hypothesis, which posits that intelligent life is a rarity in the Universe, whereas a high value provides evidence for the Copernican mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe.^[24] A 1971 NASA report by Drake and Bernard Oliver proposed the "waterhole", based on the spectral absorption lines of the hydrogen and hydroxyl components of water, as a good, obvious band for communication with extraterrestrial intelligence^[147][148] that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, CHZ candidates are the priority targets to narrow waterhole searches^[149][150] and the Allen Telescope Array now extends Project Phoenix to such candidates.^[151]

Because the CHZ is considered the most likely habitat for intelligent life, METI efforts have also been focused on systems likely to have planets there. The 2001 Teen Age Message and the 2003 Cosmic Call 2, for example, were sent to the 47 Ursae Majoris system, known to contain three Jupiter-mass planets and possibly with a terrestrial planet in the CHZ.^{[152][153][154][155]} The Teen Age Message, and the later Wow! reply, were also directed to the 55 Cancri system, which has a gas giant in its CHZ.^[101] A Message to Earth in 2008, and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three planets in the CHZ—Gliese 581 c, d, and the unconfirmed g.^[156]