The brute force of “Why is there something rather than nothing?”
Question 6C: Structured to support our current
biosphere (planetary level)
“Why -- rather than nothing–
-- is there something that looks
structured to allow ‘life’ as we know it?
-- and which contains basic elements which
can combine into compounds which form the basis for ALL lifeforms in our past
and present.
-- and which contains at least one
location in the universe that is CURRENTLY positively FAVORABLE to life as we
know it—and perhaps UNIQUELY SO? (In spite of it being anti-life for most of
its history?!)
This is the question of ‘how or why’ the configuration/location
of OUR PLANET SPECIFICALLY –and the ecological processes in/around it--is in
such an almost impossible synchronization as to positively FAVOR
the existence (once it ‘appeared’) and then to ‘finish building out’ the ecosystem
to support the variety, complexity, and interdependence of life forms currently
living on the earth.
This is called by some the ‘privileged
planet’ scenario.
Here we
have to examine some of the oddities of our planet Earth – in its current state
and location – that provide a stable base for the existence and development of
life forms.
This
would include planetary features, our neighbors in the solar system,
and our location in the galaxy and universe.
I will
cite from the same major works above:
·
(cited
in/from Miracle*) –
Denton,
Michael. The Miracle of the Cell (Privileged Species Series).
·
(cited
in/from Privileged*) –
Gonzalez,
Guillermo; Richards, Jay W.. The Privileged Planet: How Our Place in the
Cosmos Is Designed for Discovery
·
(cited
in/from Fit*) –
Rana, Fazale. Fit for a Purpose: Does the Anthropic Principle
Include Biochemistry?
·
(cited
in/from Improbable*) –
Ross,
Hugh. Improbable Planet: How Earth Became Humanity's Home
………………………………………………….
Planetary
Factors
One:
Our ‘cautious’ orbit and tilt – impact on temperature and habitability
(cited
in/from Privileged*)
“Finally,
there is Earth’s hospitable orbit and tilt. (See Plate 11.) As we
described in Chapter Two, even slight changes in these two variables affect a
planet’s climate. If either of these were only slightly greater, every part
of Earth’s surface would vary more in temperature, and would very probably be
less habitable. In fact, Earth would be likely to have a qualitatively
different biosphere, since the biodiversity of a region depends on the
productivity of its biological base. With a region experiencing larger
swings in temperature over the course of a year, fewer species would survive.
And the weaker the biosphere, the smaller its role in stabilizing the climate.
All told, Earth might support only an anemic microbial community, and nothing
more.24 This isn’t merely academic. Since Earth’s axial tilt and the
eccentricity of its orbit vary nearly as little as they can, both factors are
probably larger on most other terrestrial planets.
Two:
Our Magnetic Field
(cited
in/from Privileged*)
“A
terrestrial planet with plate tectonics is also more likely to have a strong
magnetic field, since both depend on convective overturning of its interior.
And a strong magnetic field contributes mightily to a planet’s habitability
by creating a cavity called the magnetosphere, which shields a planet’s
atmosphere from direct interaction with the solar wind. If solar wind
particles—consisting of protons and electrons—were to interact more directly
with Earth’s upper atmosphere, they would be much more effective at
“sputtering” or stripping it away (especially the atoms of hydrogen and
oxygen from water). For life, that would be bad news, since the water would be
lost more quickly to space.
“Just
as Star Trek’s Enterprise uses a force field to protect it from incoming photon
torpedoes, Earth’s magnetic field serves as the next line of defense against
galactic cosmic ray particles, after the Sun’s magnetic field and solar wind [the
Astrosphere] deflect the lower-energy cosmic rays. These cosmic
ray particles consist of high-energy protons and other nuclei, which, together
with highly interacting subatomic particles called mesons, interact with nuclei
in our atmosphere. These secondary particles can pass through our bodies,
causing radiation damage and breaking up nuclei in our cells.
Three:
The Atmosphere
(cited
in/from Privileged*)
“The
combined circumstance that we live on Earth and are able to see stars—that the
conditions necessary for life do not exclude those necessary for vision, and
vice versa—is a remarkably improbable one. This is because the medium in
which we live is, on the one hand, just thick enough to enable us to
breathe and to prevent us from being burned up by cosmic rays, while, on the
other hand, it is not so opaque as to absorb entirely the light of the stars
and block any view of the universe. What a fragile balance between the
indispensable and the sublime.” —Hans Blumenberg [1]
[1] Hans Blumenberg, The
Genesis of the Copernican Revolution, translated by Robert M. Wallace
(Cambridge: MIT Press, 1987), 3, originally published as Die Genesis der kopernikanischen Welt in 1975.
“Consider
the atmosphere’s transparency, which is actually just part of the story.
Our atmosphere participates in one of the most extraordinary coincidences
known to science: an eerie harmony among the
range of wavelengths of light emitted by the Sun, transmitted by Earth’s
atmosphere, converted by plants into chemical energy, and detected by the human
eye. The human eye perceives light of different wavelengths as
different colors, ranging from violet blue (the shortest wavelengths in the
visible spectrum) to red (the longest). Looking at a diagram of the
electromagnetic spectrum, we see the visible light emerging gracefully from the
ultraviolet, differentiating into the familiar colors of the rainbow, and
disappearing seamlessly into the warm, invisible infrared.
“As
it happens, our atmosphere strikes a nearly
perfect balance, transmitting most of the radiation that is useful for life
while blocking most of the lethal energy. Water vapor in the
atmosphere is likewise accommodating,3 a fact that even the fifteenth edition
of the staid Encyclopaedia Britannica picks up on: “Considering the
importance of visible sunlight for all aspects of terrestrial life, one cannot
help being awed by the dramatically narrow window in the atmospheric absorption
. . . and in the absorption spectrum of water.”4 The oceans transmit an
even narrower window of the spectrum, mainly the blues and greens, while
halting the other wavelengths near the surface, where they nourish the marine
life that figures prominently in Earth’s biosphere (we’ll return to this point
in Chapter Seven).
Four:
Our planetary SIZE and MASS
(cited
in/from Privileged*)
“Also
vitally important is a planet’s mass.23
A planet’s habitability depends on its mass in many ways; terrestrial planets
significantly smaller or larger than Earth are probably less habitable. Because
its surface gravity is weaker, a less massive Earth twin would lose its
atmosphere more quickly, and because of its larger surface-area-to-volume
ratio, its interior might cool too much to generate a strong magnetic field.
And as we will show in Chapter Five, smaller planets also tend to have more
dangerously erratic orbits.
In
contrast, without getting more habitable, a more massive Earth-twin would have
a larger initial inventory of water and other volatiles, such as methane and
carbon dioxide, and would lose less of them over time. Such a planet might
resemble the gas giant Jupiter rather than our terrestrial Earth. In fact, Earth
may be almost as big as a terrestrial planet can get. While life needs
an atmosphere, too much atmosphere can be bad. For example, high surface
pressure would slow the evaporation of water and dry the interiors of
continents. It would also increase the viscosity of the air at the surface,
making it more difficult for big-brained, mobile creatures like us to breathe.
23
M. H. Hart, “The Effect of a Planet’s Size on the Evolution of Its Atmosphere,”
Southwest Regional Conference on Astronomy & Astrophysics 7 (1982):
111–126; G. Gonzalez, D. Brownlee, and P. D. Ward, “The Galactic Habitable
Zone: Galactic Chemical Evolution,” Icarus 152 (2001): 185–200.
Five:
Sunlight energy fitness to chemical processes
(cited
in/from Privileged*)
“In
other words, because of the basic properties of matter, the typical energy
involved in chemical reactions corresponds to the typical energy of optical
light photons. Otherwise, photosynthetic life wouldn’t be possible.
Photons with too much energy would tear molecules apart, while those with too
little energy could not trigger chemical reactions.”
Six:
Protection from too much sunlight – the albedo
(cited
in/from Privileged*)
“Earth’s
water results
in skies that, on average, are about 68 percent cloudy.14 Clouds help
balance the global energy by contributing to the global albedo, that fraction
of sunlight reflected back into space. Earth currently
reflects about 30 percent of the sunlight that strikes it. Four major types
of reflecting surfaces contribute to the global albedo: land, ice, oceans, and
clouds, each with its own particular reflective properties.
“Earth’s
four albedo types operate on different timescales. Clouds are the
rapid responders. Trees and plants react more slowly, changing as
quickly as a single season or as slowly as several decades. Changes in sea
level may take hundreds or even thousands of years. And geological
changes may take a million years or more. With such variety available, the
climate can respond to changes lasting anywhere from hours to billions of
years. This is vital, because the Sun does change its energy output over
those timescales. A large sunspot group produces short-term changes against
a backdrop of slower variations produced over the prominent eleven-year sunspot
cycle and other, subtler cycles that last hundreds of years. Over billions of
years the Sun has slowly brightened as its core has heated up.
Seven:
Plate tectonics and the carbon cycle
(cited
in/from Privileged*)
“Plate tectonics plays
at least three crucial roles in maintaining animal life: It promotes biological
productivity; it promotes diversity (the hedge against mass extinction); and it
helps maintain equable temperatures, a necessary requirement for animal life.
It may be that plate tectonics is the central requirement for life on a planet
and that it is necessary for keeping a world supplied with water.” —Peter D. Ward and Donald
Brownlee 1
1
Brownlee and Ward, Rare Earth (New York: Copernicus, 2000), 220.
“THE
CARBON CYCLE
(cited in/from Privileged*)
“As
already suggested, this [plate tectonics] means more than just
headaches and high insurance fees for people living close to major fault lines.
Plate tectonics makes possible the carbon cycle,
which is essential to our planet’s habitability. This cycle is actually
composed of a number of organic and inorganic subcycles,
all occurring on different timescales. These cycles regulate the exchange of
carbon-containing molecules among the atmosphere, ocean, and land.
Photosynthesis, both by land plants and by phytoplankton near the ocean
surface, is especially important, since its net effects are to draw carbon
dioxide from the atmosphere and make organic matter. Zooplankton, such as the forams mentioned in Chapter Two, consume much of the
organic matter produced in the sunlight-rich surface. The carbonate and
silicate skeletons of the marine organisms settle obligingly on the ocean
floor, to be eventually squirreled away beneath the continents.
“Also
central to the carbon cycle is the chemical weathering of silicate rocks on
the continents.21 This occurs
when rainwater, made acidic with dissolved carbon dioxide from the atmosphere,
dissolves minerals in exposed rock. The rivers eventually carry dissolved
silica (SiO2), calcium, and bicarbonate ions (derived from the carbon dioxide)
to the oceans. Phytoplankton and zooplankton—and to a lesser extent, corals and
shellfish—then remove these dissolved chemicals from the ocean to build their
silicate and calcium carbonate skeletons. The carbon cycle is completed when
the subducted carbonates are pressure-cooked deep in the crust, releasing
carbon dioxide that eventually finds its way to the surface through volcanoes
and springs.
21
J. C. G. Walker, P. Hays, and J. F. Kasting, “A Negative Feedback Mechanism for
the Long-Term Stabilization of Earth’s Surface Temperature,” Journal of
Geophysical Research 86 (1981): 9776–9782.
Eight:
The amazing balancing act – and how simple life contributes
(cited
in/from Privileged*)
“Plate
tectonics and the carbon and water cycles work together to produce a planet
that is hospitable to life. Heat from Earth’s interior makes its way to the
surface, setting the mantle in motion. The dynamic interior builds mountains
and releases carbon dioxide into the atmosphere through volcanoes. Energy from
the Sun evaporates water into clouds, which precipitates, returning water and
CO2 to plants, the soil, and the oceans. Transformed into other chemical forms,
the carbon originally in the atmosphere makes its way to the seafloor. The
carbon-enriched oceanic crust is then subducted into the mantle, and eventually
reheated and returned to the surface as CO2. The entire process keeps
nutrients, water, and land available for life, and regulates the global
temperature on timescales of millions of years.
“Negative
feedback loops maintain the whole cycle in balance. Perhaps the most important
stabilizing feedback is the dependence of the rate of chemical weathering on
temperature. Here’s how it works: Suppose a prolonged period of large
volcanic eruptions rapidly increased the amount of carbon dioxide in the
atmosphere. Through the greenhouse effect, the upsurge in carbon dioxide would
raise the global temperature. The higher temperature and carbon dioxide level,
in turn, would speed up chemical weathering, and thus the removal of carbon
dioxide from the atmosphere. Eventually, the carbon dioxide and temperature
would return to their pre-eruption levels. Conversely, a drop in carbon
dioxide would slow chemical weathering, allowing carbon dioxide to build
up in the atmosphere. In either case, the loop comes full circle.
“As
we’ll discuss in Chapter Four, a rise in atmospheric carbon dioxide
increases plant growth. Rocks fuzzy with plants weather chemically about
five times faster than bare rocks, allowing Earth to tuck carbon away under its
surface all the more quickly. In other words, this accelerated weathering
reinforces this stabilizing feedback loop, allowing Earth’s climate system to
respond much more effectively to perturbations than could a lifeless world.
Thus, plate tectonics, together with plant life, makes a planet much more
nurturing for all life.
“Recall
that a key part of the carbon cycle is the rate of chemical weathering of
surface rocks by carbon dioxide dissolved in rainwater, which forms a weak acid.
This reaction can occur without life, but plants, trees, and tiny marine life
greatly accelerate it.19 Since both
rising temperature and the concentration of carbon dioxide increase chemical
weathering, somewhat paradoxically, they speed up the removal of carbon dioxide
from the atmosphere, which reduces greenhouse warming. And like Daisyworld, Earth’s trees and plants alter the local albedo
with their dark foliage and by evaporating more water, which cools Earth’s
surface by drawing the curtains—that is, by increasing cloud cover. Scientists
have only recently recognized other important links between life and the
global climate. One such process involves the formation of cloud
condensation nuclei (CCN), small particles in the atmosphere around which water
can condense to form cloud droplets. … Phytoplankton, such as Emiliana huxleyi, produce dimethyl sulfide, the first step in a
chemical chain to build the CCN. Phytoplankton
respond to a warming ocean surface by producing more dimethyl sulfide, which
concentrates more CCN and enhances the albedo of marine stratus clouds.
Reflecting more light back into space cools the ocean below. Higher
carbon dioxide levels also stimulate production of dimethyl sulfide and CCN.21
19
R. A. Berner,
“Paleozoic Atmospheric CO2: Importance of Solar Radiation and Plant Evolution,”
Science 26 (1993): 68–70.
21
Recent studies
of dimethyl sulfide (DMS) production by phytoplankton lend strong empirical
support to Lovelock’s theory. See G. P. Ayers and R. W. Gillett, “DMS and Its
Oxidation Products in the Remote Marine Atmosphere: Implications for Climate
and Atmospheric Chemistry,” Journal of Sea Research 43 (2000): 275–286.
(cited
in/from Privileged*)
“In
taking basic mineral elements and energy sources to produce organic compounds, autotrophs
[organisms that make their own food] make their environment more
habitable for all life. For example, marine organisms deposit carbonates—an
important part of the carbon cycle—on the ocean floor. In addition, marine
phytoplankton produce most of the oxygen in the atmosphere. We and
our animal cohabitants also depend on simple life directly: as food sources,
digestive aids, and decomposers.”
…………………….
Our neighbors in
the solar system – helpers!
One:
Our Moon
(cited
in/from Privileged*)
“As
we noted in Chapter One, the Moon also makes Earth friendlier to life.
Studies of the Solar System have demonstrated that the Moon stabilizes
Earth’s axial tilt. It now varies by a mere 2.5 degrees. Such a small
variation produces, over thousands of years, the mild seasonal temperature
changes we now enjoy. Temperature swings would be far greater without the
Moon.
Two:
Protection from catastrophic impacts – The reality of these
(cited
in/from Privileged*)
“The
same processes that form a planetary system also leave surplus asteroids and
comets. A terrestrial planet with protective planetary neighbors
is preferable to one in isolation around its host star. Too many planets,
however, will make a system less stable. The most habitable and measurable
system will be one with the most planets allowed by stability constraints. It
appears that Earth belongs to such a system.
Three:
Protection from catastrophic impacts –the Danger level
(cited
in/from Privileged*)
“For
example, the transparency of the atmosphere, especially with respect to
high-precision astrometry (measuring position on the sky) of faint objects, allows
astronomers to catalog the population of near earth objects, or NEOs, which
include both near Earth asteroids and comets. NEOs have Earth-crossing orbits, which
means they can hit us. Currently, about 1,250 NEOs are known, most
discovered in the past few years.25 The extent of damage caused by a NEO impact
with Earth depends on its size. A NEO two
kilometers in diameter is considered a “civilization ender,” while one ten to
fifteen kilometers in diameter is a “K/T event,” like the one that probably
killed off the dinosaurs sixty-five million years ago.26 K/T events are
believed to occur once every fifty to one hundred million years. There is about
a 1 percent chance of a two-kilometer NEO impacting in a 10,000-year time
period, and a 50 percent chance for a two-hundred-meter impact over the same
time period. In the latter case, enormous tsunamis could cause severe damage by
flooding coastal areas.
“Comets seem to impact Earth
less frequently than asteroids, but the typical comet impact may be
more devastating because of comets’ greater velocity. Hale-Bopp, the
intrinsically brightest visible comet since the sixteenth century, inspired
people worldwide in 1997. (See Plate 9.) It’s understandable why such a sight
both inspired and frightened ancient peoples. Today, we fear comets for a
different reason. If Hale-Bopp had hit Earth, with an energy perhaps one
hundred times greater than the K/T extinction event, it would have wiped out everything but the hardiest
microbes.
Four: Protection from
catastrophic impacts –Planets as Protective Shields
(cited
in/from Privileged*)
“Jupiter
and Saturn are probably the most significant planetary protectors, since they
shield the inner Solar System from excessive comet bombardment. And
although this should be the subject of intense future research, we suspect the
other terrestrial planets have played a role in maintaining Earth’s
habitability as well. First, the mere presence of other planets in the inner
Solar System reduces the number of asteroids and comets hitting Earth, for
the simple reason that an object that hits one of these other planets is no
longer around to slam Earth. How much protection the other planets add depends
on their combined surface areas and their proximity to Earth. Thus, Venus, the
closest planet, and nearly the same size as Earth, offers the greatest protection
in the inner Solar System. Mars, though a little farther than Venus, is closer
to the main asteroid belt; it has almost certainly taken a few asteroids and
comet hits on our behalf. The Moon, another Solar System vacuum cleaner, has
only about 7 percent of Earth’s surface area, so the protection it offers has
been small but not insignificant. To get a sense of the impacts that Earth
might have endured, just look at the Moon’s scarred face through a small
telescope.Yikes!
……………………..
Looking outward
– how odd is this in our galaxy and others?
Astronomers and
astrobiologists work with the concepts of ‘habitable zones’ in their estimation
and search for extra-terrestrial life.
There are four
relevant terms/concepts here, in our attempt to ascertain OUR uniqueness:
One: The Circumstellar
Habitable Zone (in our solar system)
(cited
in/from Privileged*)
“In
the late 1950s, astronomers introduced the concept of the Circumstellar Habitable Zone (CHZ).16 While its definition has varied somewhat
since then, they’ve generally defined it as that region around a star where
liquid water can exist continually on the surface of a terrestrial planet for
at least a few billion years. This definition is based on the assumption
that life will flourish if this minimum requirement is met. Modelers have
tended to focus quite narrowly, considering only a planet’s distance from its
host star, the composition of its atmosphere, and how these relate to the
heating of its surface. They usually mark the inner boundary of the Circumstellar Habitable Zone as the point
where a planet loses its oceans to space through a runaway greenhouse effect,
and define its outer boundary as the point where oceans freeze or
carbon dioxide clouds form, both of which increase a planet’s albedo and
trigger a vicious cycle of increasing coldness until the oceans freeze over
completely. (The location of the outer boundary is more difficult to
determine, since it’s hard to model the effects of carbon dioxide clouds.)
16.
S.-S. Huang, “Occurrence of Life in the Universe,” American Scientist 47
(1959): 397–402; J. S. Shklovsky and C. Sagan, Intelligent Life in the Universe
(Holden-Day: San Francisco, 1966); M. H. Hart, “Habitable Zones About Main
Sequence Stars,” Icarus 37 (1979): 351–357; J. Kasting, D. P. Whitmire,
and R. T. Reynolds, “Habitable Zones Around Main Sequence Stars,” Icarus
101 (1993): 108–128; S. Franck et al., “Habitable Zone for Earth-like Planets
in the Solar System,” Planetary and
Space Science 48 (2000): 1099–1105.
“Habitability
varies dramatically,
depending on the sizes of a planet and its host star and their separation.
There are good reasons to believe that a star similar to the Sun is necessary
for complex life.13 A more massive star has a shorter lifetime and
brightens more rapidly. A less massive star radiates less energy, so a
planet must orbit closer in to keep liquid water on its surface. (The band
around a star wherein a terrestrial planet must orbit to maintain liquid
water on its surface is called the Circumstellar
Habitable Zone.) Orbiting too close to the host star,
however, leads to rapid tidal locking, or “rotational synchronization,” in
which one side of the planet perpetually faces its host star. (The Moon,
incidentally, is so synchronized in its orbit around Earth.) This leads to
brutal temperature differences between the day and night sides of a planet.
Even if the thin boundary between day and night, called the terminator, were
habitable, a host of other problems attend life around a less massive star
(more on this in Chapter Seven).
Two:
The Circumstellar
Continuously Habitable Zone (CCHZ)
But
the CHZ moves – it moves outward as changes to the sun/star happen.
(cited
in/from Privileged*)
“For
example, by the 1970s astronomers recognized that the Sun’s gradually
increasing luminosity would cause the zone to move outward. And starting
in the late 1990s modelers began treating plate tectonics as a changing rather
than a steady process in equilibrium. 19
19 S. A. Franck et al.,
“Reduction of Biosphere Life Span as a Consequence of Geodynamics,” Tellus 52B
(2000): 94–107.
“The
Circumstellar Habitable Zone (CHZ) is that temperate region around a star
wherein liquid water can exist on the surface of a terrestrial planet for
extended periods. Since the luminosity of even a stable star like the Sun
changes over billions of years, however, a star’s CHZ will move outward over
time. The Circumstellar Continuously Habitable Zone (CCHZ) is the overlapping
region of various instantaneous CHZs.
“Earth’s
geological record indicates that liquid water
has been present somewhere on its surface continuously over most of its
history. That means Earth must have stayed within the habitable zone
even as the Sun brightened and the zone moved outward. The region over
which all the instantaneous habitable zones overlap for some extended period of
time is called the Circumstellar Continuously Habitable
Zone (CCHZ). The CCHZ is narrower than the CHZ, especially if the time
interval under consideration is a substantial fraction of the main sequence
lifetime of the host star. Of course, the position of this zone varies from
star to star. Low-mass main-sequence stars, being less luminous than the Sun,
have small, close-in CCHZs; the opposite is true for more massive stars.
Three: The Galactic
Habitable Zone
(cited
in/from Privileged*)
“Like
the Circumstellar Habitable Zone in
our Solar System, there is also a Galactic Habitable Zone (GHZ). And its first requirement is to
maintain liquid water on the surface of an Earth-like planet. But it’s also
about forming Earth-like planets and the long-term survival of
animal-like aerobic life.18 The boundaries of the Galactic
Habitable Zone are set by the needed planetary building blocks
and threats to complex life in the galactic setting.”
18.
The material in
this section is based on the following paper: G. Gonzalez, D. Brownlee, and P.
D. Ward, “The Galactic Habitable Zone: Galactic Chemical Evolution,” Icarus
152 (2001): 185–200; a less technical summary is given in G. Gonzalez, D.
Brownlee, and P. D. Ward, “The Galactic Habitable Zone,” Scientific American
(October 2001): 60–67.
For
A Galactic Habitable Zone, beyond the water requirement is the metallicity factor.
(cited
in/from Privileged*)
“As
we move from the center to the edge of the galaxy, the gas density drops, and
with it, the rate of star formation. Since stars are the source of most heavy
elements in the galaxy, this means that metallicity drops as we move from
the center to the edge.
“Thus,
a minimum amount of metals is required so that the rocky core forms
quickly before most of the gas is lost from the system (on a timescale near ten
million years).
“Conversely,
too much metal in the mixture can pose a different set of problems. The
higher the initial allotment of metals in its birth cloud, the more
planetesimals—early asteroid-sized planetary building blocks—and planets will
form in a system, greatly complicating the orbits. In a planetary version of
pinball, the giant planets might migrate inward by scattering the many tiny
bodies. And if being metal-rich increases the chances that more giant planets
will form, then they will also be more likely to perturb each other and destabilize
the system, even dumping terrestrial planets into the host star or flinging
them out of the system entirely.25 Even if the giant planets don’t migrate
much, frequent bombardment from the many remaining asteroids and comets—which
will be more abundant in a metal-rich system—will threaten planetary life.
“Indeed,
in the extreme case of a (star-system birth) cloud without metals, where
not even fist-sized rocks will form, making the right size terrestrial
planets becomes impossible. On the other hand, with a high initial
inventory of metals you’re likely to end up with a busy, chaotic system
with a bevy of large planets constantly irritating one another into a very
inhospitable state.
“In
summary, there weren’t enough basic elements to build Earth-mass planets
until the Milky Way was a few billion years old, and then there were enough
only in its inner regions. Today regions of its disk within the Sun’s orbit
should contain plenty of building blocks to make Earth-mass planets. The
formation of giant planets and comets, too, depends on the supply of ashes from
supernovae, and also affects the habitability of Earth-mass planets. The
relative abundance of magnesium, silicon, and iron also varies with place and
time, leading to differences in planetary geology. Additionally, the typical
Earth-mass planet forming in the future will probably have a smaller
concentration of geophysically important
radioisotopes and, as a result, weaker tectonic activity. By themselves,
these factors limit the time and place in our galaxy wherein Earth-like
planets can form.”
(cited
in/from Improbable*)
“… only 3 percent of all stars in our galaxy remain as
possible hosts for planets on which primitive life could briefly survive.”
Four: And beyond our
galaxy?
(cited
in/from Privileged*)
“As
if our galaxy’s habitable zone weren’t exclusive enough, the broader universe looks even less inviting.
About 98 percent of galaxies in the local universe are less luminous—and
thus, in general, more metal-poor—than the Milky Way.61 So entire galaxies could be devoid of
Earth-size terrestrial planets.62 In addition, stars in elliptical galaxies
have less-ordered orbits, like bees flying around a hive minus a bee’s capacity
to react to impending collisions. Therefore, they are more likely to visit
their galaxy’s dangerous central regions.63 They’re also more likely to pass
through interstellar clouds at disastrously high speeds (though such clouds are
less common in elliptical galaxies). In many
ways, ours is the optimal galaxy for life: a late-type, metal-rich, spiral
galaxy with orderly orbits, and comparatively little danger between spiral
arms.
61.
This is because the average metallicity of a galaxy correlates with its
luminosity. Since luminous galaxies contain more stars, galaxies at least as
luminous as the Milky Way galaxy contain about 23 percent of the stars. When we
compare our Galactic setting with other galaxies, it is not clear which is the
more appropriate statistic. If the total luminosity (or mass) of a galaxy is
also relevant (and not just the metallicity of a given star), then the smaller
statistic is more appropriate. For observational evidence of the correlation
between galaxy luminosity and metallicity, see D. R. Garnett, “The
Luminosity-Metallicity Relation, Effective Yields, and Metal Loss in Spiral and
Irregular Galaxies,” Astrophysical Journal 581 (2002), 1019–1031.
………………
Putting all this
together, the overlapping requirements for life (of any complexity) quickly
form such a constrained space, that the ‘oddness’ is readily apparent. And as
scientific research continues, this oddness seems to grow…
Although this is more
detailed than I would like to include (although very readable), Ross’ summary
of some of the various ‘zones’ required, is worth this extended quote (from Improbable*):
“Astronomers
and astrobiologists refer to that region about a star where some conceivable
sort of life could possibly exist as the circumstellar
habitable zone. Here a potential “carrier” planet or moon (see
appendix A for an explanation of why surface life is not possible on a moon)
might exist, and where life could reside for a reasonable length of time. The
extent of this zone depends entirely on assumptions about what characteristics
and resources life requires for its existence. Some scientists as well as
laypeople tend to see just one or two properties and supplies (such as a
certain radiation level and quantity of liquid water) as sufficient for
habitability. However, this picture comes into sharper focus as research
reveals how many more features and supplies—each representing a distinct
zone—life requires.
These
multiple zones, then, must converge to some extent and for some duration to
make life conceivable. Only in this much narrower region where all the
habitable zones overlap, do all life’s necessities come together. Of
course, the region of overlap for advanced life will be much smaller yet. The
following list describes what ongoing studies have revealed, to date, about the
kinds and sizes of eight distinct habitable zones.
1.
Liquid
Water Habitable Zone
The liquid water
habitable zone is that region about a star wherein liquid water can exist on a
planetary surface. For water to remain, of course, requires an appropriate
level of atmospheric pressure. (Where pressure is low, such as on Mars, a drop
of water would evaporate in a second.) The liquid water habitable zone may also
be called the temperature habitable zone. At least some part of a planetary
surface must range between 0–100°C (32–212°F)—assuming a surface air pressure
similar to Earth’s—to retain liquid water.
For
a long history of life, one that includes the possibility of advanced life, the
liquid water habitable zone by itself would be much narrower than the narrowest
limits described above. Advanced life requires more than merely a stable supply
of liquid water. As ongoing research tells us (see chs.
13–15), it requires a habitat in which frozen water, liquid water, and water
vapor exist simultaneously over long time periods. It also requires a habitat
in which water transitions efficiently from one of its states to the other two.
2.
Ultraviolet
Habitable Zone
The ultraviolet
habitable zone is that region about a star where incident UV radiation arriving
on a planet’s surface is neither too strong nor too weak to provide for life’s
needs. UV radiation is a double-edged sword. Without it several essential
biochemical reactions and the synthesis of many life-essential biochemical
compounds (such as DNA repair and vitamin D manufacturing) cannot occur. Too
much of it, however, will damage or destroy land-based life. Both the quantity
and the wavelength of incident UV radiation must fall within a certain range
for life to survive, and an even narrower range for life to flourish. The
acceptable range of UV radiation seems especially narrow for human beings.
Although
the UV zone may prove relatively wide for the sake of more primitive
life-forms, it may not be wide enough even then to overlap with the liquid
water habitable zone.
The fact that the liquid water and UV habitable zones must
overlap for the sake of life eliminates most planetary systems as possible
candidates for hosting life.
This requirement effectively rules out all the M-dwarf and most of the
K-dwarf stars, as well as all the O-, B-, and A-type stars. All that remain
are F-type stars much younger than the Sun, G-type stars no older than the Sun,
and a small fraction of the K-type stars. As described in chapter 5, only stars
at a certain distance from the galactic core can be considered candidates for
life support. In the MWG, some 75 percent of all stars residing at this
appropriate-for-life distance are older than the Sun.[25] Once these and other noncandidate stars are ruled out,
only 3 percent of all stars in our galaxy remain
as possible hosts for planets on which primitive life could briefly survive.
[25].
Charles H. Lineweaver, Yeshe Fenner, and Brad K. Gibson, “The Galactic
Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” Science
303 (January 2004): 59–62.
[Tanknote:
these two factors alone make our little planet look VERY ODD INDEED…]
3. Photosynthetic
Habitable Zone
The photosynthetic
habitable zone refers to the range of distances from a host star within which a
planet could possibly possess the necessary conditions for photosynthesis to
occur. While some life-forms can exist in the absence of photosynthesis,
such life exhibits metabolic rates from hundreds to millions of times lower
than those of photosynthetic life. In other words, without photosynthesis,
large-bodied warm-blooded animals would not be possible.
Photosynthetic
life requires much more demanding constraints on the quantity, stability, and
spectral light range available on a planet’s surface. Limited photosynthetic
activity is possible for a planet where the UV and liquid water habitable zones
overlap. However, for the scope of photosynthetic activity advanced
life requires to endure and thrive, these seven factors must fall within
highly specific ranges:
1.
Light intensity
2.
Ambient temperature
3.
Carbon dioxide concentration
4.
Seasonal variation and stability
5.
Mineral availability
6.
Liquid water quantity
7.
Atmospheric humidity (for land-based life)
To
maintain these features within appropriate ranges presents an even greater
challenge due to ongoing changes in surface conditions. Over the past 3.9
billion years, Earth has undergone some dramatic variations in solar luminosity
and spectral response, and these variations impacted all seven conditions for
photosynthesis. Detailed models of the early history of the Sun and Earth (between
4.0 and 3.0 billion years ago) show that surface radiation levels were at
least thousands of times higher in the 2,000–3,000 angstrom (Ĺ) wavelength
range than current levels in this range. (One angstrom is equal to 0.1
nanometer.) This finding tells us that life-forms on Earth previous to 3
billion years ago had to endure far greater solar X-ray and UV radiation than
do life-forms today.
4. Ozone Habitable Zone
The
ozone habitable zone describes that range of distances from a star where an
ozone shield can potentially form. When stellar radiation impinges upon an
oxygen-rich atmosphere, it produces a quantity of ozone in that planet’s
atmospheric layers. This ozone, in turn, affects the amount of radiation
reaching the planetary surface.
Ozone,
a molecule composed of three oxygen atoms, forms in a planet’s stratosphere
as short wavelength UV radiation and, to a lesser degree, stellar X-ray
radiation react with dioxygen (O2). Meanwhile, its reaction with atomic
oxygen in the stratosphere destroys ozone (O3 + O →2O2). The quantity
of ozone in the stratosphere at any given time depends on the status of this
balancing act.
Currently,
ozone in Earth’s stratosphere absorbs 97–99
percent of the Sun’s short wavelength (2,000–3,150 Ĺ), life-damaging UV radiation while allowing much
of the longer wavelength (3,150+ Ĺ), beneficial
radiation to pass through to Earth’s surface. What makes this
life-favoring scenario possible is the combination of three main factors: (1)
the necessary quantity of oxygen in Earth’s atmosphere; (2) the just-right
intensity of UV radiation impinging on Earth’s stratosphere; and (3) the
relatively low variability of this UV radiation bath.
For
life protection purposes,
the ozone quantity in a planet’s troposphere (the atmospheric layer extending
from the surface up to a certain distance, in Earth’s case, from sea level to
six miles up) must amount to about 10 percent of that in the stratosphere. Too
much ozone in the troposphere would hinder respiration for large-bodied
animals while also reducing crop yields and wiping out many plant species. Insufficient
tropospheric ozone would lead to an ever-increasing buildup of biochemical
“smog” particles emitted by tree-like vegetation. These factors place
additional constraints on a host star’s UV radiation intensity and stability,
and on the host planet’s distance from the star, especially given that ozone
production in a planet’s troposphere receives a boost from lightning.
5. Rotation-Rate
Habitable Zone
A planet’s rotation
rate impacts the reflectivity of its clouds and, thus, how much of the host
star’s light penetrates to the planetary surface. Three-dimensional
atmospheric circulation models show that rapidly rotating planets compared to
slowly rotating planets would generate much narrower bands of clouds at low
latitudes (given the same atmospheric magnitude and composition). These
narrower tropical cloud belts would reflect much less of the host star’s light
and, consequently, allow the planet’s surface to reach much higher average
temperatures.
A
planet’s rotation rate actually affects the positions and sizes and potential
overlap of multiple habitable zones. The faster the rotation rate, the more
distant from the host star the water, UV, photosynthetic, and ozone habitable
zones would be. Rotation rate would also impact (in different ways) the
breadth of all these habitable zones.
6. Obliquity Habitable
Zone
The tilt of a
planet’s rotation axis relative to its orbital axis (i.e., its obliquity)
plays a significant part in determining the planet’s surface temperature.
Climate simulation studies demonstrate that the higher the obliquity, the
warmer a planet’s surface. Specifically, high obliquity warms the oceans and
cools the continents.
Just
as a planet’s rotation rate impacts the positions of several habitable zones,
so does the planet’s obliquity. Greater obliquity pushes certain habitable
zones—water, UV, photosynthetic, and ozone—outward from the host star. The
planet’s obliquity also affects the breadth of these habitable zones in
different ways.
7. Tidal Habitable Zone
The tidal habitable
zone refers to the distance range from a host star where the planet is near
enough for life-essential radiation but far enough to prevent tidal locking.
Due to gravity, a star exerts a stronger pull on the near side of its
surrounding planets than on the far side. Tidal force describes the difference
between the near-side tug and the far-side tug, a difference that carries great
significance. The tidal force a star exerts on a planet is inversely
proportional to the fourth power of the distance between them. Thus, shrinking
the distance by one half increases the tidal force by 16 times.
If
a planet orbits too close to its star, it becomes tidally locked (as the
Moon is tidally locked with Earth), which means one hemisphere faces
permanently toward its star. As a result of tidal locking, one face of
the planet would receive an unrelenting flow of stellar radiation while the
opposite side would receive none. On a tidally locked planet, then, the
only conceivable place where life could exist would be in the twilight
zone––that narrow region between permanent light and permanent darkness. If such
a planet happened to reside in the liquid water habitable zone and possess an
atmosphere, water would move via atmospheric transport from the day side to the
night side, where it would become permanently trapped as ice. So, no liquid
water would exist anywhere on its surface. For life to exist on a tidally
locked planet, it would have to be unicellular, exhibit extremely low metabolic
rates, and reside below the surface.
The
complex interaction of both solar and lunar tidal effects permits Earth to
sustain an enormous biomass and biodiversity at its seashores and on its
continental shelves. The tides on Earth are optimal for recycling
nutrients and wastes. They provide the potential for a rich and abundant
ecology.
8. Astrosphere Habitable
Zone
This is a recently
discovered astrophysical phenomenon that limits the region around a star where
a potential life-sustaining planet can reside. Researchers observe that a star’s
“wind” (release of radiation) pushes outward against the cosmic radiation all
around it, radiation emanating from its galaxy’s core and from nearby
supernova explosions. This wind creates a cocoon of charged particles
surrounding the star. This region, called the astrosphere, is where the
wind blows strongly enough to deflect cosmic rays. The astrosphere acts
as a buffer to screen the orbiting planet’s atmospheres and surfaces
from the high-energy cosmic radiation that would be deadly.
The
buffering, however, must be delicately balanced to yield a zone safe for
life. A powerful stellar wind will generate a large plasma cocoon,
but such a cocoon could blast nearby planets with so many stellar
particles as to either kill or seriously limit the prospects for life there. On
the other hand, a weak stellar wind produces a small plasma cocoon
inadequate for shielding its planets from blasts of deadly cosmic
radiation.
“To
allow for the existence of any life other than the most primitive unicellular
forms, all these habitable zones must overlap.
They can do so only in a relatively narrow band around a star with the same
mass as the Sun.”
……………………………………………………
It
is very obvious that our biosphere is odd in the extreme—when compared to the
‘rest of the SOMETHING’—and even more UNEXPECTED as coming ‘from nothing’.
And
if the numbers given above about the low probability of other planets theoretically
able to support advanced/complex lifeforms are even remotely close to reality,
my ‘alertness’ goes way up.
If
it were just clever patterns and designs of chemicals (which would be present
everywhere), the fact that they were in MY PLANET would not be that big of a
deal.
But
when science seems to indicate that they only DO THEIR WEIRDEST THINGS (e.g.
specific geometric 3D macromolecules, radiation filtering) in my backyard—due
to the way my planet and sun are configured and located in space—it is easy to
feel that ‘I am being set up’. It is almost a little like a version of the
Shakespeare quote: “Our SOMETHING is a stage, and we are (but) actors on it”.
And
although all this Goldilocks stuff could be interpreted ‘optimistically’
(without ANY evidential* warrant WHATSOVER, I hasten to point out), it would be
naďve in the extreme to presume that any post-mortem encounter with the
putative Outside-Other of such ability would invariably be a ‘pleasant
experience’ for a disembodied mind.
[And
we haven’t even gotten to the actually existence of life (in any form), nor the problem of ‘why consciousness’?!]
*There
are philosophical warrants that could be applied here, based on Ultimate
Reference Point concepts, but how much they could add to the ‘confidence level’
of somebody ‘risking this’ is very, very variable…