In astrobiology and planetary astrophysics, the galactic habitable zone is the region of a galaxy in which life is most likely to develop. The concept of a galactic habitable zone analyzes various factors, such as metallicity (the presence of elements heavier than hydrogen and helium) and the rate and density of major catastrophes such as supernovae, and uses these to calculate which regions of a galaxy are more likely to form terrestrial planets, initially develop simple life, and provide a suitable environment for this life to evolve and advance.
According to research published in August 2015, very large galaxies may
favor the birth and development of habitable planets more than smaller
galaxies such as the Milky Way. In the case of the Milky Way, its galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs (33,000 ly) and an inner radius close to the Galactic Center (with both radii lacking hard boundaries).
Galactic habitable-zone theory has been criticized due to an
inability to accurately quantify the factors making a region of a galaxy
favorable for the emergence of life.
In addition, computer simulations suggest that stars may change their
orbits around the galactic center significantly, therefore challenging
at least part of the view that some galactic areas are necessarily more
life-supporting than others.
History
Background
The idea of the circumstellar habitable zone was introduced in 1953 by Hubertus Strughold and Harlow Shapley and in 1959 by Su-Shu Huang
as the region around a star in which an orbiting planet could retain
water at its surface. From the 1970s, planetary scientists and
astrobiologists began to consider various other factors required for the
creation and sustenance of life, including the impact that a nearby supernova may have on the development of life. In 1981, computer scientist Jim Clarke proposed that the apparent lack of extraterrestrial civilizations in the Milky Way could be explained by Seyfert-type outbursts from an active galactic nucleus, with Earth alone being spared from this radiation by virtue of its location in the galaxy. In the same year, Wallace Hampton Tucker analyzed galactic habitability in a more general context, but later work superseded his proposals.
Modern galactic habitable-zone theory was introduced in 1986 by L.S. Marochnik and L.M. Mukhin of the Russian Space Research Institute, who defined the zone as the region in which intelligent life could flourish. Donald Brownlee and palaeontologist Peter Ward expanded upon the concept of a galactic habitable zone, as well as the other factors required for the emergence of complex life, in their 2000 book Rare Earth: Why Complex Life is Uncommon in the Universe.
In that book, the authors used the galactic habitable zone, among other
factors, to argue that intelligent life is not a common occurrence in
the Universe.
The idea of a galactic habitable zone was further developed in 2001 in a paper by Ward and Brownlee, in collaboration with Guillermo Gonzalez of the University of Washington. In that paper, Gonzalez, Brownlee, and Ward stated that regions near the galactic halo would lack the heavier elements required to produce habitableterrestrial planets, thus creating an outward limit to the size of the galactic habitable zone.
Being too close to the galactic center, however, would expose an
otherwise habitable planet to numerous supernovae and other energetic
cosmic events, as well as excessive cometary impacts caused by perturbations of the host star's Oort cloud. Therefore, the authors established an inner boundary for the galactic habitable zone, located just outside the galactic bulge.
Considerations
In
order to identify a location in the galaxy as being a part of the
galactic habitable zone, a variety of factors must be accounted for.
These include the distribution of stars and spiral arms, the presence or
absence of an active galactic nucleus, the frequency of nearby supernovae that can threaten the existence of life, the metallicity of that location, and other factors. Without fulfilling these factors, a region of the galaxy cannot create or sustain life with efficiency.
Chemical evolution
The metallicity of the thin galactic disk is far greater than that of the outlying galactic halo.
One of the most basic requirements for the existence of life around a star is the ability of that star to produce a terrestrial planet of sufficient mass to sustain it. Various elements, such as iron, magnesium, titanium, carbon, oxygen, silicon, and others, are required to produce habitable planets, and the concentration and ratios of these vary throughout the galaxy.
The most common benchmark elemental ratio is that of Fe/H, one of the factors determining the propensity of a region of the galaxy to produce terrestrial planets. The galactic bulge, the region of the galaxy closest to the Galactic Center, has an [Fe/H] distribution peaking at −0.2 decimal exponent units (dex) relative to the Sun's ratio (where −1 would be 1⁄10 such metallicity); the thin disk, in which local sectors of the local Arm
are, has an average metallicity of −0.02 dex at the orbital distance of
the Sun around the galactic center, reducing by 0.07 dex for every
additional kiloparsec of orbital distance. The extended thick disk has an average [Fe/H] of −0.6 dex, while the halo, the region farthest from the galactic center, has the lowest [Fe/H] distribution peak, at around −1.5 dex.
In addition, ratios such as [C/O], [Mg/Fe], [Si/Fe], and [S/Fe] may be
relevant to the ability of a region of a galaxy to form habitable
terrestrial planets, and of these [Mg/Fe] and [Si/Fe] are slowly
reducing over time, meaning that future terrestrial planets are more
likely to possess larger iron cores.
In addition to specific amounts of the various stable elements that comprise a terrestrial planet's mass, an abundance of radionuclides such as 40K, 235U, 238U, and 232Th is required in order to heat the planet's interior and power life-sustaining processes such as plate tectonics, volcanism, and a geomagnetic dynamo. The [U/H] and [Th/H] ratios are dependent on the [Fe/H] ratio; however, a general function for the abundance of 40K cannot be created with existing data.
Even on a habitable planet with enough radioisotopes to heat its
interior, various prebiotic molecules are required in order to produce
life; therefore, the distribution of these molecules in the galaxy is
important in determining the galactic habitable zone. A 2008 study by Samantha Blair and colleagues attempted to determine the outer edge of the galactic habitable zone by means of analyzing formaldehyde and carbon monoxide emissions from various giant molecular clouds scattered throughout the Milky Way; however, the data is neither conclusive nor complete.
While high metallicity is beneficial for the creation of terrestrial extrasolar planets, an excess amount can be harmful for life. Excess metallicity may lead to the formation of a large number of gas giants in a given system, which may subsequently migrate from beyond the system's frost line and become hot Jupiters, disturbing planets that would otherwise have been located in the system's circumstellar habitable zone. Thus, it was found that the Goldilocks principle
applies to metallicity as well; low-metallicity systems have low
probabilities of forming terrestrial-mass planets at all, while
excessive metallicities cause a large number of gas giants to develop,
disrupting the orbital dynamics of the system and altering the
habitability of terrestrial planets in the system.
Catastrophic events
The impact of supernovae on the extent of the galactic habitable zone has been extensively studied.
As well as being in a region of the galaxy that is chemically
advantageous for the development of life, a star must also avoid an
excessive number of catastrophic cosmic events with the potential to
damage life on its otherwise habitable planets.
Nearby supernovae, for example, have the potential to severely harm
life on a planet; with excessive frequency, such catastrophic outbursts
have the potential to sterilize an entire region of a galaxy for
billions of years. The galactic bulge, for example, experienced an
initial wave of extremely rapid star formation, triggering a cascade of supernovae that for five billion years left that area almost completely unable to develop life.
In addition to supernovae, gamma-ray bursts, excessive amounts of radiation, gravitational perturbations
and various other events have been proposed to affect the distribution
of life within the galaxy. These include, controversially, such
proposals as "galactic tides" with the potential to induce cometary
impacts or even cold bodies of dark matter that pass through organisms and induce genetic mutations. However, the impact of many of these events may be difficult to quantify.
Galactic morphology
Various
morphological features of galaxies can affect their potential for
habitability. Spiral arms, for example, are the location of star
formation, but they contain numerous giant molecular clouds and a high
density of stars that can perturb a star's Oort cloud, sending avalanches of comets and asteroids toward any planets further in.
In addition, the high density of stars and rate of massive star
formation can expose any stars orbiting within the spiral arms for too
long to supernova explosions, reducing their prospects for the survival
and development of life. Considering these factors, the Sun is advantageously placed within the galaxy because, in addition to being outside a spiral arm, it orbits near the corotation circle, maximizing the interval between spiral-arm crossings.
Spiral arms also have the ability to cause climatic changes on a
planet. Passing through the dense molecular clouds of galactic spiral
arms, stellar winds
may be pushed back to the point that a reflective hydrogen layer
accumulates in an orbiting planet's atmosphere, perhaps leading to a snowball Earth scenario.
A galactic bar
also has the potential to affect the size of the galactic habitable
zone. Galactic bars are thought to grow over time, eventually reaching
the corotation radius of the galaxy and perturbing the orbits of the
stars already there.
High-metallicity stars like the Sun, for example, at an intermediate
location between the low-metallicity galactic halo and the
high-radiation galactic center, may be scattered throughout the galaxy,
affecting the definition of the galactic habitable zone. It has been
suggested that for this reason, it may be impossible to properly define a
galactic habitable zone.
Boundaries
The
galactic habitable zone is often viewed as an annulus 7-9 kpc from the
galactic center, shown in green here, though recent research has called
this into question.
Early research on the galactic habitable zone, including the 2001
paper by Gonzalez, Brownlee, and Ward, did not demarcate any specific
boundaries, merely stating that the zone was an annulus encompassing a
region of the galaxy that was both enriched with metals and spared from
excessive radiation, and that habitability would be more likely in the
galaxy's thin disk.
However, later research conducted in 2004 by Lineweaver and colleagues
did create boundaries for this annulus, in the case of the Milky Way
ranging from 7 kpc to 9 kpc from the galactic center.
The Lineweaver team also analyzed the evolution of the galactic
habitable zone with respect to time, finding, for example, that stars
close to the galactic bulge had to form within a time window of about
two billion years in order to have habitable planets.
Before that window, galactic-bulge stars would be prevented from having
life-sustaining planets from frequent supernova events. After the
supernova threat had subsided, though, the increasing metallicity of the
galactic core would eventually mean that stars there would have a high
number of giant planets, with the potential to destabilize star systems
and radically alter the orbit of any planet located in a star's
circumstellar habitable zone. Simulations conducted in 2005 at the University of Washington, however, show that even in the presence of hot Jupiters, terrestrial planets may remain stable over long timescales.
A 2006 study by Milan Ćirković
and colleagues extended the notion of a time-dependent galactic
habitable zone, analyzing various catastrophic events as well as the
underlying secular evolution of galactic dynamics.
The paper considers that the number of habitable planets may fluctuate
wildly with time due to the unpredictable timing of catastrophic events,
thereby creating a punctuated equilibrium in which habitable planets are more likely at some times than at others. Based on the results of Monte Carlo simulations on a toy model
of the Milky Way, the team found that the number of habitable planets
is likely to increase with time, though not in a perfectly linear
pattern.
Subsequent studies saw more fundamental revision of the old
concept of the galactic habitable zone as an annulus. In 2008, a study
by Nikos Prantzos
revealed that, while the probability of a planet escaping sterilization
by supernova was highest at a distance of about 10 kpc from the
galactic center, the sheer density of stars in the inner galaxy meant
that the highest number of habitable planets could be found there.
The research was corroborated in a 2011 paper by Michael Gowanlock, who
calculated the frequency of supernova-surviving planets as a function
of their distance from the galactic center, their height above the galactic plane,
and their age, ultimately discovering that about 0.3% of stars in the
galaxy could today support complex life, or 1.2% if one does not
consider the tidal locking of red dwarf planets as precluding the development of complex life.
Criticism
The
idea of the galactic habitable zone has been criticized by Nikos
Prantzos, on the grounds that the parameters to create it are impossible
to define even approximately, and that thus the galactic habitable zone
may merely be a useful conceptual tool to enable a better understanding
of the distribution of life, rather than an end to itself.
For these reasons, Prantzos has suggested that the entire galaxy may be
habitable, rather than habitability being restricted to a specific
region in space and time.
In addition, stars "riding" the galaxy's spiral arms may move tens of
thousands of light years from their original orbits, thus supporting the
notion that there may not be one specific galactic habitable zone. A Monte Carlo simulation, improving on the mechanisms used by Ćirković in 2006, was conducted in 2010 by Duncan Forgan of Royal Observatory Edinburgh.
The data collected from the experiments support Prantzos's notion that
there is no solidly defined galactic habitable zone, indicating the
possibility of hundreds of extraterrestrial civilizations in the Milky Way, though further data will be required in order for a definitive determination to be made.
The habitable zone is also called the Goldilocks zone, a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears",
in which a little girl chooses from sets of three items, rejecting 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, many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets. Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sizedplanets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way. About 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The HZ is also of particular interest to the emerging field of habitability of natural satellites, because planetary-mass moons in the HZ might outnumber planets.
In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving. Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres,
like Earth's, that exist independently of stellar energy, are now
generally accepted in astrobiology given the large amount of liquid
water known to exist in lithospheres and asthenospheres of the Solar System. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons. 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, or ammoniates, 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.
History
An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton'sPrincipia (Book III, Section 1, corol. 4). The philosopher Louis Claude de Saint-Martin speculated in his 1802 work Man: His True Nature and Ministry,
"... we may presume, that, being susceptible of vegetation, it [the
Earth] has been placed, in the series of planets, in the rank which was
necessary, and at exactly the right distance from the sun, to accomplish
its secondary object of vegetation; and from this we might infer that
the other planets are either too near or too remote from the sun, to
vegetate."
The concept of a circumstellar habitable zone was first introduced
in 1913, by Edward Maunder in his book "Are The Planets Inhabited?". The concept was later discussed 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. In the same year, Harlow Shapley
wrote "Liquid Water Belt", which described the same concept in further
scientific detail. Both works stressed the importance of liquid water to
life. 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.
A major early contributor to the habitable zone concept, 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.
The concept of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man,
in which he discussed the concept of the circumstellar habitable zone
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. 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. 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. 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. Kasting was the first to present a detailed model for the habitable zone for exoplanets.
An update to habitable zone concept 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.
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.
Subsequently, some astrobiologists propose that the concept be
extended to other solvents, including dihydrogen, sulfuric acid,
dinitrogen, formamide, and methane, among others, which would support
hypothetical life forms that use an alternative biochemistry. In 2013, further developments in habitable zone concepts were made with the proposal of a circum planetary
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.
It has been noted that the current term of 'circumstellar
habitable zone' poses confusion as the name suggests that planets within
this region will possess a habitable environment. However, surface conditions are dependent on a host of different individual properties of that planet. This misunderstanding is reflected in excited reports of 'habitable planets'. Since it is completely unknown whether conditions on these distant HZ worlds could host life, different terminology is needed.
Determination
Thermodynamic
properties of water depicting the conditions at the surface of the
terrestrial planets: Mars is near the triple point, Earth in the liquid;
and Venus near the critical point.The range of published estimates for the extent of the Sun's HZ. The conservative HZ is indicated by a dark-green band crossing the inner edge of the aphelion of Venus, whereas an extended HZ, extending to the orbit of the dwarf planetCeres, 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 can remain habitable, complemented by a larger
"extended habitable zone" in which a planet like Venus, with stronger greenhouse effects, can have the right temperature for liquid water to exist at the surface.
Solar System estimates
Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units,
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 in most estimates 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 vapor. The entire orbits of the Moon, Mars,
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. At Hellas Basin,
for example, atmospheric pressures can reach 1,115 Pa and temperatures
above zero Celsius (about the triple point for water) for 70 days in the
Martian year. Despite indirect evidence in the form of seasonal flows on warm Martian slopes,
no confirmation has been made of the presence of liquid water there.
While other objects orbit partly within this zone, including comets, Ceres
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.
Despite this, studies are strongly suggestive of past liquid water on the surface of Venus, Mars, Vesta and Ceres,
suggesting a more common phenomenon than previously thought. Since
sustainable liquid water is thought to be essential to support complex
life, most estimates, therefore, are inferred from the effect that a
repositioned orbit would have on the habitability of Earth or Venus as
their surface gravity allows sufficient atmosphere to be retained for
several billion years.
According to the extended habitable zone concept, 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; not enough is known about the nature of atmospheres of
these kinds of extrasolar objects, and their position in the habitable
zone cannot determine the net temperature effect of such atmospheres
including induced albedo, anti-greenhouse or other possible heat sources.
For reference, the average distance from the Sun of some major
bodies within the various estimates of the habitable zone is: Mercury,
0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU;
Ceres and Pallas, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU. In the
most conservative estimates, only Earth lies within the zone; in the
most permissive estimates, even Saturn at perihelion, or Mercury at
aphelion, might be included.
Estimates of the circumstellar habitable zone boundaries of the Solar System
Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone.
1.005–1.008
1969, Budyko
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 and North 1975.
0.92–0.96
1970, Rasool and De Bergh
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.
0.958
1.004
1979, Hart
Based on computer modeling and simulations of the evolution of
Earth's atmospheric composition and surface temperature. This estimate
has often been cited by subsequent publications.
3.0
1992, Fogg
Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone.
0.95
1.37
1993, Kasting et al.
Founded the most common working definition of the habitable zone used today. Assumes that CO2 and H2O are the key greenhouse gases as they are for the Earth. Argued that the habitable zone is wide because of the carbonate–silicate cycle. Noted the cooling effect of cloud albedo. Table shows conservative limits. Optimistic limits were 0.84–1.67 AU.
2.0
2010, Spiegel et al.
Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity.
0.75
2011, Abe et al.
Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth.
10
2011, Pierrehumbert and Gaidos
Terrestrial planets that accrete tens-to-thousands of bars of
primordial hydrogen from the protoplanetary disc may be habitable at
distances that extend as far out as 10 AU in the Solar System.
0.77–0.87
1.02–1.18
2013, Vladilo et al.
Inner edge of the circumstellar habitable zone is closer and outer
edge is farther for higher atmospheric pressures; determined minimum
atmospheric pressure required to be 15 mbar.
0.99
1.67
2013, Kopparapu et al.
Revised estimates of the Kasting et al. (1993) formulation using
updated moist 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 moist greenhouse limit. As with Kasting et al. (1993), this
applies to an Earth-like planet where the "water loss" (moist
greenhouse) limit, at the inner edge of the habitable zone, is where the
temperature has reached around 60 Celsius and is high enough, right up
into the troposphere, that the atmosphere has become fully saturated
with water vapor. Once the stratosphere becomes wet, water vapor
photolysis releases hydrogen into space. At this point cloud feedback
cooling does not increase significantly with further warming. The
"maximum greenhouse" limit, at the outer edge, is where a CO2 dominated atmosphere, of around 8 bars, has produced the maximum amount of greenhouse warming, and further increases in CO2 will not create enough warming to prevent CO2
catastrophically freezing out of the atmosphere. Optimistic limits were
0.97–1.67 AU. This definition does not take into account possible
radiative warming by CO2 clouds.
0.38
2013, Zsom et al.
Estimate based on various possible combinations of atmospheric
composition, pressure and relative humidity of the planet's atmosphere.
0.95
2013, Leconte et al.
Using 3-D models, these authors computed an inner edge of 0.95 AU for the Solar System.
0.95
2.4
2017, Ramirez and Kaltenegger
An expansion of the classical carbon dioxide-water vapor habitable zone assuming a volcanic hydrogen atmospheric concentration of 50%.
0.93–0.91
2019, Gomez-Leal et al.
Estimation of the moist greenhouse threshold by measuring the water
mixing ratio in the lower stratosphere, the surface temperature, and the
climate sensitivity on an Earth analog with and without ozone, using a
global climate model (GCM). It shows the correlation of a water mixing
ratio value of 7 g/kg, a surface temperature of about 320 K, and a peak
of the climate sensitivity in both cases.
Astronomers use stellar flux and the inverse-square law
to extrapolate circumstellar habitable zone models created for the
Solar System to other stars. For example, according to Kopparapu's
habitable zone estimate, although the Solar System has a circumstellar
habitable zone centered at 1.34 AU from the Sun, 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 HZ concept is more complex.
Spectral types and star-system characteristics
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.
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.
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.
Studying ultraviolet emissions, Andrea Buccino found that only 40% of
stars studied (including the Sun) had overlapping liquid water and
ultraviolet habitable zones.
Stars smaller than the Sun, on the other hand, have distinct
impediments to habitability. For example, Michael Hart proposed that
only main-sequence stars of spectral classK0 or brighter could offer 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
hospitable environment for life.
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 modeling from 2013 supports the idea that
red dwarf stars can support planets with relatively constant
temperatures over their surfaces in spite of tidal locking. Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration. At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs.
Also, a habitable zone in the outer parts of stellar systems may exist
during the pre-main-sequence phase of stellar evolution, especially
around M-dwarfs, potentially lasting for billion-year timescales.
Stellar evolution
Natural shielding 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,
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.
Even while stars are on the main sequence, though, their energy output
steadily increases, pushing their habitable zones farther out; our Sun,
for example, was 75% as bright in the Archaean as it is now,
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. 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. Like the general circumstellar
habitable zone, the continuously habitable zone of a star is divided
into a conservative and extended region.
In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes and huge starspots which can cover 20% of the star's surface area, have the potential to strip an otherwise habitable planet of its atmosphere and water. As with more massive stars, though, stellar evolution changes their nature and energy flux, so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life.
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. However, once a red giant star reaches the horizontal branch,
it achieves a new equilibrium and can sustain a new circumstellar
habitable zone, which in the case of the Sun would range from 7 to 22
AU. At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense. 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. However, around such a helium-burning star, important life processes like photosynthesis
could only happen around planets where the atmosphere has 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. Moreover, as Ramirez and Kaltenegger (2016)
showed, intense stellar winds would completely remove the atmospheres
of such smaller planetary bodies, rendering them uninhabitable anyway.
Thus, Titan would not be habitable even after the Sun becomes a red
giant.
Nevertheless, life need not originate during this stage of stellar
evolution for it to be detected. Once the star becomes a red giant, and
the habitable zone extends outward, the icy surface would melt, forming
a temporary atmosphere that can be searched for signs of life that may
have been thriving before the start of the red giant stage.
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.
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. 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 such as GJ 1214 b or Kepler-22b may be.
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.Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape. 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. Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained because even a small shift in pressure or temperature could render water unable to form as a liquid.
Although traditional definitions of the habitable zone assume
that carbon dioxide and water vapor are the most important greenhouse
gases (as they are on the Earth), a study led by Ramses Ramirez and co-author Lisa Kaltenegger
has shown that the size of the habitable zone is greatly increased if
prodigious volcanic outgassing of hydrogen is also included along with
the carbon dioxide and water vapor. The outer edge in the Solar System
would extend out as far as 2.4 AU in that case. Similar increases in the
size of the habitable zone were computed for other stellar systems. An
earlier study by Ray Pierrehumbert and Eric Gaidos had eliminated the CO2-H2O
concept entirely, arguing that young planets could accrete many tens to
hundreds of bars of hydrogen from the protoplanetary disc, providing
enough of a greenhouse effect to extend the solar system outer edge to
10 AU. In this case, though, the hydrogen is not continuously
replenished by volcanism and is lost within millions to tens of millions
of years.
In the case of planets orbiting in the HZs 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 thought to cause
extreme heat on the star-facing side and bitter cold on the opposite
side, making many red dwarf planets uninhabitable; however,
three-dimensional climate models in 2013 showed that the side of a red
dwarf planet facing the host star could have extensive cloud cover,
increasing its bond albedo and reducing significantly temperature differences between the two sides.
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.
More specifically, moons need to be far enough from their host giant
planets that they are not transformed by tidal heating into volcanic
worlds like Io, but must remain within the Hill radius of the planet so that they are not pulled out of the orbit of their host planet.
Red dwarfs that have masses less than 20% 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 the
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.
Artist's concept of a planet on an eccentric orbit that passes through the HZ for only part of its orbit
A planetary object that orbits a star with high orbital eccentricity
may spend only some of its year in the HZ 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 temperature between 0.150 K (−273 °C) and 424 K (151 °C). Life on a planetary object orbiting outside HZ 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.
A 2015 review concluded that the exoplanetsKepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable. These are at a distance of 990, 490 and 1,120 light-years
away, respectively. Of these, Kepler-186f is closest in size to Earth
with 1.2 times Earth's radius, and it is located towards the outer edge
of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates, Tau Ceti e
is 11.9 light-years away. It is in the inner edge of its planetary
system's habitable zone, giving it an estimated average surface
temperature of 68 °C (154 °F).
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 ηe, the fraction of stars with planets in the HZ, at 0.48, meaning that there may be roughly 95–180 billion habitable planets in the Milky Way. However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.
Previous studies have been more conservative. In 2011, Seth
Borenstein concluded that there are roughly 500 million habitable
planets in the Milky Way. NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, estimating 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 HZs.
The first discoveries of extrasolar planets in the HZ occurred just a
few years after the first extrasolar planets were discovered. However,
these early detections were all gas giant-sized, and many were in
eccentric orbits. Despite this, studies indicate the possibility of
large, Earth-like moons around these planets supporting liquid water.
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. 16 Cygni Bb,
also discovered in 1996, has an extremely eccentric orbit that spends
only part of its time in the HZ, such an orbit would causes extreme seasonal
effects. In spite of this, simulations have suggested that a
sufficiently large companion could support surface water year-round.
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 that may also have large moons. Another gas giant, Upsilon Andromedae d was discovered in 1999 orbiting Upsilon Andromidae's habitable zone.
Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone and has a low orbital eccentricity, comparable to that of Mars in the Solar System. Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years, though it is unclear whether such satellites could form in the first place.
HD 69830 d, a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of HD 69830, 41 light years away from Earth. The following year, 55 Cancri f was discovered within the HZ of its host star 55 Cancri A. Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.
Though, in theory, such giant planets could possess moons, the
technology did not exist to detect moons around them, and no extrasolar
moons had been discovered. Planets within the zone with the potential
for solid surfaces were therefore of much higher interest.
The habitable zone of Gliese 581 compared with the Solar System's habitable zone.
The 2007 discovery of Gliese 581c, 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 extreme surface conditions that may resemble Venus.
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, but only for a short time. As
of 2015, the planet has no newer disconfirmations. 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, and astronomers are divided about its existence.
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 speculated to be habitable,
but the new circumstellar habitable zone criteria devised by Kopparapu
et al. in 2013 place the planet outside the circumstellar habitable
zone.
Kepler-22 b, discovered in December 2011 by the Kepler space probe, is the first transiting exoplanet discovered around a Sun-like star. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet. Gliese 667 Cc, discovered in 2011 but announced in 2012, is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C. It is one of the most Earth-like planets known.
Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163 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. HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307. In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a Sun-like star 12 light years away. Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone;
however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar
habitable zone criteria established by the 2013 Kopparapu study. It is now considered as uninhabitable.
Near Earth-sized planets and Solar analogs
Comparison of the HZ 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 thought to be
similar in size or mass to Earth. "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 these planets have a significant fraction of volatiles by
volume overlying a rocky core. A genuinely Earth-like planet – an Earth analog
or "Earth twin" – would need to meet many conditions beyond size and
mass; such properties are not observable using current technology.
A solar analog
(or "solar twin") is a star that resembles the Sun. No solar twin with
an exact match as that of the Sun has been found. However, some stars
are nearly identical to the Sun and are considered solar twins. An exact
solar twin would be a G2V star with a 5,778 K temperature, be
4.6 billion years old, with the correct metallicity and a 0.1% solar luminosity variation.
Stars with an age of 4.6 billion years are at the most stable state.
Proper metallicity and size are also critical to low luminosity
variation.
Using data collected by NASA's Kepler space telescope and the W. M. Keck Observatory,
scientists have estimated that 22% of solar-type stars in the Milky Way
galaxy have Earth-sized planets in their habitable zone.
On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-size exoplanet candidate (1.7 times the radius of Earth) orbiting Kepler-69, a star similar to the Sun, in the HZ and expected to offer habitable conditions. 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.
With a radius estimated 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 though its mass remains unknown and its parent star is not a Solar analog.
Kapteyn b,
discovered in June 2014 is a possible rocky world of about 4.8 Earth
masses and about 1.5 Earth radii were found orbiting the habitable zone
of the red subdwarf Kapteyn's Star, 12.8 light-years away.
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.[166] However, Kepler-438b is found to be a subject of powerful flares, so it is now considered uninhabitable. 16 January, K2-3d a planet of 1.5 Earth radii was found orbiting within the habitable zone of K2-3, receiving 1.4 times the intensity of visible light as Earth.
Kepler-452b,
announced on 23 July 2015 is 50% bigger than Earth, likely rocky and
takes approximately 385 Earth days to orbit the habitable zone of its G-class (solar analog) star Kepler-452.
The discovery of a system of three tidally-locked planets orbiting the habitable zone of an ultracool dwarf star, TRAPPIST-1, was announced in May 2016.
The discovery is considered significant because it dramatically
increases the possibility of smaller, cooler, more numerous and closer
stars possessing habitable planets.
Two potentially habitable planets, discovered by the K2 mission in July 2016 orbiting around the M dwarf K2-72 around 227 light years from the Sun: K2-72c and K2-72e are both of similar size to Earth and receive similar amounts of stellar radiation.
Announced on the 20 April 2017, LHS 1140b is a super-dense super-Earth
39 light years away, 6.6 times Earth's mass and 1.4 times radius, its
star 15% the mass of the Sun but with much less observable stellar flare
activity than most M dwarfs.
The planet is one of few observable by both transit and radial velocity
that's mass is confirmed with an atmosphere may be studied.
Discovered by radial velocity in June 2017, with approximately three times the mass of Earth, Luyten b orbits within the habitable zone of Luyten's Star just 12.2 light-years away.
At 11 light-years away, the second closest planet, Ross 128 b,
was announced in November 2017 following a decade's radial velocity
study of relatively "quiet" red dwarf star Ross 128. At 1.35 times
Earth's mass, is it roughly Earth-sized and likely rocky in composition.
Discovered in March 2018, K2-155d is about 1.64 times the radius of Earth, is likely rocky and orbits in the habitable zone of its red dwarf star 203 light years away.
One of the earliest discoveries by the Transiting Exoplanet Survey Satellite (TESS) announced on July 31, 2019, is a Super-Earth planet GJ 357 d orbiting the outer edge of a red dwarf 31 light years away.
K2-18b is an exoplanet 124 light-years away, orbiting in the habitable zone of the K2-18, a red dwarf. This planet is significant for water vapor found in its atmosphere; this was announced on September 17, 2019.
The discovery of hydrocarbon lakes on Saturn's moon Titan has begun to call into question the carbon chauvinism that underpins HZ concept.
Liquid-water environments have been found to exist in the absence of
atmospheric pressure and at temperatures outside the HZ temperature
range. For example, Saturn's moons Titan and Enceladus and Jupiter's moons Europa and Ganymede, all of which are outside the habitable zone, may hold large volumes of liquid water in subsurface oceans.
Outside the HZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water.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.
With some theorising that life on Earth may have actually originated in stable, subsurface habitats, it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'. On Earth itself, living organisms may be found more than 6 km (3.7 mi) below the surface.
Another possibility is that outside the HZ organisms may use alternative biochemistries that do not require water at all. Astrobiologist Christopher McKay, has suggested that methane (CH 4)
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.
This distance is coincident with the location of Titan, whose lakes and
rain of methane make it an ideal location to find McKay's proposed
cryolife. In addition, testing of a number of organisms has found some are capable of surviving in extra-HZ conditions.
Significance for complex and intelligent life
The Rare Earth hypothesis
argues that complex and intelligent life is uncommon and that the HZ is
one of many critical factors. According to Ward & Brownlee (2004)
and others, not only is a HZ 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) and biochemical (the role of radiant energy in supporting photosynthesis for necessary atmospheric oxygenation). 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 HZ. Intelligent life outside the HZ may have evolved in subsurface environments, from alternative biochemistries or even from nuclear reactions.
On Earth, several complex multicellular life forms (or eukaryotes)
have been identified with the potential to survive conditions that
might exist outside the conservative habitable zone. Geothermal energy
sustains ancient circumvent ecosystems, supporting large complex life
forms such as Riftia pachyptila.
Similar environments may be found in oceans pressurised beneath solid
crusts, such as those of Europa and Enceladus, outside of the habitable
zone. Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the Milnesium tardigradum, which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space. In addition, the lichens Rhizocarpon geographicum and Xanthoria elegans
have 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. The fungi Cryomyces antarcticus and Cryomyces minteri are also able to survive and reproduce in Mars-like conditions.
Species, including humans, known to possess animal cognition require large amounts of energy,
and have adapted to specific conditions, including an abundance of
atmospheric oxygen and the availability of large quantities of chemical
energy synthesized from radiant energy. If humans are to colonize other
planets, true Earth analogs
in the HZ are most likely to provide the closest natural habitat; 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 or space habitat analogs on the surface may be eliminated, and complex Earth life can thrive.
Planets in the HZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe. The Drake equation, sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter ne,
which is the average number of planetary-mass objects orbiting within
the HZ of each star. 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 Copernicanmediocrity principle, the view that habitability—and therefore life—is common throughout the Universe. A 1971 NASA report by Drake and Bernard Oliver proposed the "water hole", based on the spectral absorption lines of the hydrogen and hydroxyl components of water, as a good, obvious band for communication with extraterrestrial intelligence that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, HZ candidates are the priority targets to narrow waterhole searches and the Allen Telescope Array now extends Project Phoenix to such candidates.
Because the HZ 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 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 HZ. The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its HZ. A Message from Earth in 2008, and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three planets in the HZ—Gliese 581 c, d, and the unconfirmed g.
