Habitable Zones: A New Look at “Are We Alone?”
Since the dawn of man, humans looked to the heavens and queried, “Are we
alone?” These queries have continued through the advent of modern technology and the
arrival of the modern scientific age. In 1600, Bruni’s early version of the Copernican
Mediocrity Principle, the theory that many Suns and Earths exist and thus extraterrestrial
life likely does as well, led to his execution for heresy.
In 1960, the Search for
Extraterrestrial Intelligence, (SETI), an organization dedicated to the search for
extraterrestrial life (Whitmire & Mattese, 2009) was founded.
In opposition to the
Copernican Mediocrity Principle, geologist Peter Ward and astrobiologist Donald
Brownlee proposed the Rare Earth Hypothesis in 1999, stating that simple life probably
exists elsewhere in the universe because of the resilience displayed by primitive life,
while it is highly unlikely that complex life resembling that found on Earth exists due to
how unique the environment on earth is (Bounama, et al., 2007). A major step toward a
quantifiable effort to find life came in F.D. Drake’s creation of the fabled Drake equation
and his initiation of Project Ozma, which some consider SETI’s first effort. This
involved probing the night sky for hypothetical messages from alien civilizations with
radio receivers (Drake, 1961). Modern science has progressed far beyond this point, so
that it is possible to quantify the area around a specific star in which extraterrestrial life
might be present and thus estimate the habitability or likelihood of life existing in that
star system (Shock & Holland, 2007; Jones, et. al, 2006). It is important to define HZs in
order to analyze the quality of certain targets for exploration.
An HZ, or Goldilocks zone, is defined as the area between the inner and outer
limits of habitability. These boundaries are defined by a variety of factors such as the
existence of water as described by Raymond et al. (2007) but are all influenced by
temperature, which is determined by the luminosity (energy output) of the central star,
making luminosity the most important factor and “last word” in terms of stellar factors.
The inner limit of the HZ is the point where temperature is at its maximum. This high
temperature affects the presence of water—if temperature is too high, water is broken
down by light into hydrogen and oxygen (photolysis), and hydrogen escapes because of
weak attraction to the planet and low density. Thus, high temperature eliminates water in
this area. The outer limit, on the other hand, is the area with minimum temperature; CO2
solidifies as a result of low temperature, scattering light and further decreasing the
temperature of the planet (von Bloh, et al., 2007; Lammer, 2007). Thus, while there are
many case-reliant factors such as local geology, luminosity plays a large part in defining
the HZ.
Another, narrower analysis of habitability is the pHZ (photosynthesis-sustaining
HZ), the area where photosynthetic life is possible and thus where complex life is likely
to exist. The pHZ would thus typically fall within HZs —the pHZ applies to a specific
form of life, while the HZ applies to all life, so prokaryotic life could easily exist outside
the pHZ. The variables defining the pHZ are CO2 and H2O levels, as these are the
reactants of photosynthesis. Therefore, pHZs usually envelop planets containing large
amounts of water (von Bloh, et al., 2009). One main factor controlling CO2 content is the
rate of silicate rock weathering—weathering consumes CO2. For example, on UMa47,
photosynthetic life is impossible after an estimated 5970-7820 million years (von Bloh, et
al., 2007). Thus, a relationship between natural conditions pertaining to habitability of a
planet, the time necessary for evolution, and the duration of life creates a temporal as
well as a spatial window for the existence of the pHZ.
To understand the definition of the HZ, individual cases must be analyzed. An
interesting scenario exists in the star system Gliese 581, which contains one gaseous
planet and two Jupiter-sized, Earth-like–(terrestrial etc.) planets, dubbed Gliese 581b,
581c and 581d respectively. 581c lies very close to the star such that it is uninhabitable.
However, 581d is within the HZ, though it departs the HZ at certain key points in its
orbit, but it has been suggested that microbial life can exist on planets in the HZ for the
majority of their orbits. Thus, following with the Rare Earth Hypothesis, it is possible
that Gliese 581d is a host for microbial life (von Bloh, et al., 2007). Another candidate
for habitability is UMa 47, a solar system in the constellation Sirius. It is considered
similar to our solar system since it has two gas giants analogous to our outer gas giants of
Jupiter, Uranus, and Neptune and possibly terrestrial planets closer to the star. This
illustrates the complexity of estimating habitability—the probability of an Earthlike
planet must first be determined, and then within this condition other factors, including
conditions of the star, as well as the possible geology of the planet are analyzed. For
example, the two Jupiter-sized planets of UMa47 could easily cause gravitational
disruption, pushing the inner planet(s) out of the HZ. However, given current
information, UMa47 is one of the most promising possible hosts of life (Franck, et al.,
2003).
The sheer amount of conjecture involved in calculating HZs begs the question of
the significance of these studies. Simply put, it is all that is known; were conditions on
extrasolar planets to be unprecedented and unknown, the HZ would be inapplicable.
Until other forms of life such as theorized silicon-based organisms are discovered, the HZ
and pHZ standards can be used to prioritize targets for future space missions, though the
HZ, even in its complexity, could be oversimplification, as it does not take into account
factors such as the moon’s influence on the Earth’s climate, without which humans would
not exist.
Thus, while there is more to life than the HZ, perhaps to the extent that
habitability can never be accurately quantified, it is the closest today’s technology can
come to definitively narrowing the search for life.
WORD COUNT: 996
N.B.:
Earthlike=terrestrial…http://www.sciencedaily.com/releases/2010/01/100106093642.ht
m
.
References
Bounama, C., von Bloh, W., & Franck, S. (2007). How Rare is Complex Life in
the Milky Way?. Astrobiology 7(5), 745-755.
Drake, F. D. (1961). "Project Ozma," Physics Today, 14, 40-46.
Franck, S., Cuntz, M., von Bloh, W., & Bounama, C. (2003). The habitable zone
of Earth-mass planets around 47 UMa: results for land and water worlds.
International Journal of Astrobiology, 2(1), 35-39.
Jones, B.W., Sleep, N.P., & Underwood, D.R. (2006). Habitability of Known
Exoplanetary Systems Based on Measured Stellar Properties. The Astrophysics
Journal, 649, 1010-1019.
Lammer, Helmut. (2007). M Star Planet Habitability. Astrobiology, 7(1), 27-29.
Raymond, S.N., Scalo, J., & Meadows, V.S. (2007). A Decreased Probability of
Habitable Planet Formation Around Low-Mass Stars. The Astrophysics Journal,
669, 606-614.
Shock, E.L. & Holland, M.E. (2007). Quantitative Habitability. Astrobiology,
7(6), 839-851.
Von Bloh, W., Bounama, C., Cuntz M., & Franck, S. (2007). The habitability of
super-Earths in Gliese 581. Astronomy and Astrophysics, 476, 1365-1371.
Von Bloh, W., Cuntz, M., K.-P. Schröder, Bounama, C., & Franck, S. (2009).
Habitability of Super-Earth Planets Around Other Suns: Models Including Red
Giant Branch Evolution. Astrobiology, 9(6), 593-602.
Whitmire, D.P., & Matese, J.J. (2009). The Distribution of Stars Most Likely to
Harbor Intelligent Life. Astrobiology, 9(7), 617-621.