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2016
Narrowing the Search
for Life
THE WHAT AND WHERE OF HABITABILITY
TODD IODICE
Are we alone? This great philosophical question has fascinated humans for centuries
and has yet to be answered. As mankind expands its knowledge of the universe, the search for
life beyond our own planet remains a constant issue. Of course, with billions of planets in the
Milky Way alone, the answer to that question may seem impossible to answer. How can we
possibly examine every planet in the hopes of discovering extraterrestrial life? Nevertheless,
there are a number of criteria we can use to narrow the search.
The Basics
In the search for life, scientists almost have no other choice but to use the Earth as a
guideline for determining the conditions necessary to support life. It is, after all, the only
example we have, but more importantly, “comparative planetology in the Solar System
indicates that the Earth’s habitability may be near optimal (especially for complex life)”
(Gonzalez 186). Therefore, according to NASA, “for a planet to be habitable, it must have water,
carbon sources, energy sources, and a dynamic geologic past” (Gibson 1). The first three seem
self-evident. After all, every organism known to man is composed of carbon-based organic
molecules and requires liquid water during at least part of their life cycle and some form of
energy with which it can operate (Kasting 1). The role of a ‘dynamic geologic past,’ usually
traced to plate tectonics, is less evident but no less critical.
Plate tectonics are largely responsible in maintaining long-term climate stability, for
they drive the CO2-rock cycle (see image below). Carbon in the air accumulates as sediments on
the ocean floor via acid rain and is subsequently sub-ducted by the movement of tectonic
plates. From there, it is released back into the atmosphere by volcanoes in the form of CO 2 (Des
Marais 304). The process of removing CO2 from the atmosphere by chemical weathering is very
temperature-dependent, so hotter temperatures remove carbon dioxide from the air. With
lower levels of carbon in the air, the greenhouse effect decreases, causing temperatures to
lower. If temperatures are low instead, less carbon dioxide is removed from the atmosphere,
keeping the greenhouse effect intact. Thus, this process provides negative feedback that
opposes major global temperature variations (Gonzalez 188). Long-term climate stability allows
for the development of life without extreme temperature fluctuations that could threaten
unadjusted organisms.
Planetary Mass
Only terrestrial planets are capable of holding the liquid water necessary for life on or
near their surfaces. By definition, a terrestrial planet is composed of a large rocky core and a
comparatively negligible atmosphere, resulting in either a liquid or solid surface (Traub 6). The
development of such conditions relies almost entirely on the planet’s mass, which, according to
some experts, may be the “single most important factor in building a habitable terrestrial
planet” (Gonzalez 187). Planetary formation models have shown that a range of .1 to 10 Earth
masses holds the correct amount of gravity to form such a planet (Traub 6). A planet more than
ten times more massive than the Earth will accumulate a very large atmosphere very quickly
during its formation, developing into a gas giant similar to Jupiter or Uranus. On the other hand,
Mars, at about a tenth the mass of Earth, can barely hold onto any atmosphere at all.
However, the range of planetary masses suitable for life may be far narrower than this.
Terrestrial planets less massive than Earth have a larger surface area to mass ratio, meaning
their interior cools at a faster rate. Earth’s hot interior fuels its geological activity and tectonic
plate movement, which, as discussed before, is vital for maintaining stable climates and
recycling carbonates. Thus, smaller terrestrial planets remain geologically active for shorter
periods of time (Lissauer 6). Moreover, a smaller planet would have a much lower surface
pressure than Earth due to its lower gravity and higher surface area to mass ratio. In
combination with this low pressure, the reduced levels of greenhouse gases in the area due to
the lack of a CO2-rock cycle would result in lower surface temperatures than an Earth-sized
planet, perhaps freezing any liquid water on the planet’s surface (11).
On the other end of the spectrum, larger planets have far greater impact probabilities
(the chances of being struck by asteroids, comets, etc.). Impact probability is proportional to
the square of a planet’s radius, which increases with mass, so planets more massive than Earth
are far more susceptible to cataclysmic, possibly life-eradicating events (Gonzalez 187-188).
However, higher surface temperatures (for the opposite reasons above) would not necessarily
result in the evaporation of all surface water due to the increased pressure because of the
carbon cycle, unless the planet’s greenhouse was strong enough to boil the water away
(Lissauer 11).
Star Metallicity
The metallicity of a star is simply the fraction of that star composed of elements heavier
than hydrogen and helium. These heavier elements compose the terrestrial planets like Earth
that have the potential to host life. According to Charles Lineweaver, an astrophysicist at the
University of New South Wales in Australia, there is a sort of “Goldilocks zone of metallicity” for
the formation of planets similar to Earth (Lineweaver 60). On the one hand, too little metallicity
results in Earth-mass planets being unable to form, since a minimum abundance of heavy
elements are necessary. According to Gonzalez, a metallicity at least half that of the Sun is
necessary for a habitable terrestrial planet to form (Gonzalez 185). On the other hand,
Lineweaver notes a “strong correlation between the presence of large close-orbiting massive
planets and high metallicity of the host star” (Lineweaver 59). Such massive planets may
destroy Earth-mass planets with their gravity. In fact, any planet within a multiple-planet
system can suffer from gravitational perturbations that can result in collisions and ejections
(Lissauer 3). Only, single planet systems are indefinitely stable, but the risk of perturbations in
multiple-planet systems is reduced with larger spacing between the planets and smaller
eccentricities in their orbits. For this reason, Lissauer declares that “large numbers of habitable
planets per system are unlikely,” and one could expect to find one or two at most in any given
system, if any at all (3).
These are some of the vital criteria used in determining the habitability of celestial
bodies. Now the question arises: where can we find these habitable planets?
Circumstellar Habitable Zone
Habitable planets cannot exist anywhere. Indeed, they are limited to specific regions of
space that allow for the requisite conditions. These regions are called ‘habitable zones’, and the
most well-known is the Circumstellar Habitable Zone (CHZ). The main criterion for this region,
representing an annulus around a star, is the presence of liquid water. A team at Penn State led
by physicist James Kasting conservatively estimated the habitable zone of the Sun for an Earthlike planet to be between .95 and 1.37 Astronomical Units, with 1 AU representing the mean
distance of the Earth from the Sun (Kasting 108). In this model, the inner boundary is
determined by the loss of water due to photodissociation, the breaking down of a chemical
compound by photons, and hydrogen escape, in which molecules of hydrogen accumulate
enough energy to reach escape velocity. Put in simple terms, if a planet is too hot, it can’t hold
life because water will disassociate, allowing the free hydrogen molecules to escape into space.
In contrast, the formation of carbon dioxide clouds sets the outer limit of this model. Such
accumulations of CO2 in the atmosphere occur because not enough energy is present to run the
CO2-rock cycle (see above). These clouds prevent sunlight from reaching the planet’s surface,
thereby lowering surface temperatures unsuitable for life (108). Within the range calculated by
Kasting and his colleagues, the carbon cycle maintains a climate stable enough to support liquid
water and, therefore, life.
What about other stars? Stars of different sizes, emitting different levels of energy,
naturally would have different CHZ’s than the Sun, with those of larger stars occurring farther
away than that of the Sun and those of smaller stars occurring closer. It is important to note
that habitable zones are affected by solar evolution. All stars increase in luminosity during their
lifetime on the main sequence (the main part of their lifespan), meaning their HZ’s are
constantly changing (Kasting 109). For instance, Kasting estimates that the Sun was 30%
dimmer at the onset of the Solar System, and it will triple in luminosity by the time it exits the
main sequence. Therefore, scientists must account for this movement of the HZ in order to
determine whether planets have resided there long enough to develop life. For this reason,
stars more massive than the Sun, which run through the main sequence faster than small stars,
are less likely to be orbited by planets containing life as their habitable zones shift very quickly,
leaving relatively little time for life to develop (Lissauer 3). On the other hand, smaller stars can
live for trillions of years, so their habitable zones are rather unchanging. However, their low
luminosity can have huge variations due to flares and large starspots, which is far from ideal for
maintaining a stable environment. Furthermore, planets in the habitable zone of small stars are
so close that their rotations can become tidally-synched (like the Moon is with the Earth). This
eliminates the day-night cycle and leaves an entire half of the planet dark and cold while the
other is constantly exposed to solar emissions, making it that much harder for life evolve (3).
Thus, very massive and very small stars are less likely to host a truly habitable planet than a star
similar to the Sun.
Tidally Heated Habitable Zone
While the Circumstellar Habitable Zone of a star remains the likeliest place to find life in
a given solar system, it has been postulated that there may exist so-called ‘tidally heated
habitable zones’ around very large planets. According to researchers at NASA, “tidal dissipation
in the satellite of a giant planet may provide sufficient heating to maintain an environment
favorable to life” (Reynolds 125). While on Earth sunlight is the only source of heating for the
planetary surface, tidal friction over the surface of a natural satellite may dissipate enough of
that satellite’s orbital and rotational energy in the form of heat to allow life to form. Such
natural satellites can not only be the size of a planet but can also contain compounds necessary
for life. For instance, Saturn’s moon Titan is larger than Mercury, and over nine organic
molecules have been detected in its atmosphere (125). In fact, Jupiter’s moon Europa may have
the greatest chance of hosting life out of any celestial body in the Solar System other than the
Earth. Those same NASA researchers have hypothesized that the moon could have a liquid
ocean that, though covered in ice, could receive enough sunlight through cracks in its shell to
support organisms similar to those found under the Antarctic ice shelf (125). After all, Europa is
about 6% water. Nevertheless, this theory remains unlikely, so tidally heated habitable zones
are probably more applicable to other star systems.
Some problems do arise in this model however. These natural satellites orbiting massive
planets have far greater impact rates than regular planets due to the strong gravity of their gas
giant hosts (Gonzalez 187). Moreover, if the satellite has a far weaker gravitational field than its
host planet, it will suffer from increased levels of particle radiation that, over time, could lead
to the destruction of its atmosphere (187). Still, these difficulties do not rule out the possibility
that life may arise in a star system beyond the Circumstellar Habitable Zone.
Galactic Habitable Zone
Many scientists have also proposed the idea of a ‘Galactic Habitable Zone’ (GHZ), the
region of the Milky Way where an Earth-like planet can form and provide a long-term habitat
for the development of complex life. According to Lineweaver, “of the factors that determine
the location of the GHZ, the abundance of elements heavier than hydrogen and helium
(metallicity) is particularly crucial because these elements are what terrestrial planets are
composed of” (Lineweaver 60). Indeed, there exists a radial Galactic metallicity gradient,
meaning that heavier elements, and especially iron, are more abundant the closer to the center
of the galaxy you go. Thus, the outer limit of the GHZ is primarily determined by the minimum
metallicity required to form large terrestrial planets (Gonzalez 185). On the other hand, as
mentioned above, higher metallicity results in the orbital decay of planets and increased impact
rates, affecting the inner limit of the GHZ. However, another important factor for this inner
boundary is the probability of high energy events like supernovae and gamma ray bursts (186).
These events increase greatly in frequency toward the center of the galaxy. It is important to
note that all this means the GHZ is probabilistically defined. It does not represent set
boundaries outside of which habitable planets cannot form. Rather it is a probability gradient
that Lineweaver has calculated to be centered at about 8 kiloparsecs (26,096 light years) from
the center of the Milky Way (Lineweaver 62).
Conclusion
Humanity must keep these factors in mind as we explore the cosmos and search for life
on other worlds. We can certainly narrow the search with these parameters. Of course, perhaps
our view of life is far too narrow. Perhaps we will someday encounter life that does not require
water, or an inhabited planet outside a star’s habitable zone. Nevertheless, this is all
speculation, and it seems best to continue to go by what we know about life on Earth.
Bibliography
Des Marais, David. “Tectonic control of the crustal organic carbon reservoir during the
Precambrian.” Chemical Geology 1994: 303-314. Print.
Gibson, Everett et al. “Conditions on Early Mars Might Have Fostered Rapid and Early
Development of Life.” NASA Johnson Space Center.
Gonzalez, Guillermo, Donald Brownlee, and Peter Ward. “The Galactic Habitable Zone: Galactic
Chemical Evolution.” Icarus 22 January 2001: 185-200. Print.
Kasting, James, Daniel Whitmore, and Ray Reynolds. “Habitable Zones around Main Sequence
Stars.” Icarus 2 October 1992: 108-128. Print.
Lineweaver, Charles, Yeshe Fenner, and Brad Gibson. “The Galactic Habitable Zone and the Age
Distribution of Complex Life in the Milky Way.” Science 2 January 2004: 59-62. Print.
Lissauer, Jack. “How Common Are Habitable Planets?” NASA Ames Research Center.
Reynolds, Ray, Christopher McKay, and James Kasting. “Europe, Tidally Heated Oceans, and
Habitable Zones around Giant Planets.” Adv. Space Res. 1987: 125-132. Print.
Traub, Wesley. “Terrestrial, Habitable‐Zone Exoplanet Frequency from Kepler.” Boston
University. 13 October 2011. Lecture.
Images:
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