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Waltham, Dartnell: Meeting report
Is the Earth special?
Meeting report Earth is the only inhabited planet we know of – so
far – but is that the only distinguishing feature of our planet? Dave
Waltham and Lewis Dartnell report from an RAS meeting that
considered how and why our home planet is unusual.
I
f very special preconditions are necessary
for the eventual evolution of observers on a
planet, then the Earth must be an oddball.
If true, this “anthropic selection” concept is as
important for a clear understanding of our planet’s properties as the concept of plate tectonics!
But is it true? On 9 December 2011 more than
A&G • August 2012 • Vol. 53 90 geologists, biologists and astronomers gathered at an RAS Specialist Discussion Meeting on
“Is the Earth special?” to debate this issue at a
meeting organized by the Astrobiology Society
of Britain (an affiliate organization of the RAS).
The meeting was divided into three sessions
(on astrophysics, planetary science and biol-
1: Earth as it would appear from 35 000 km,
using data from MODIS on NASA’s Terra
satellite for land surface data and NOAA’s
GOES for the clouds. Life is an unusual
and significant element of our planetary
environment, but oceans, plate tectonics,
magnetic field and even our large Moon may
also play a part in producing and maintaining
our habitable planet. (R Stöckli, N El Saleous
and M Jentoft-Nilsen, NASA GSFC)
ogy) but began with a keynote lecture from Jim
Kasting (Pennsylvania State University) explaining why he believes the “Rare Earth” hypothesis to be too pessimistic. In this hypothesis,
Peter Ward and Donald Brownlee suggested
that the circumstances that have produced
complex life on Earth are each unlikely, and
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Waltham, Dartnell: Meeting report
Life in the universe: extrasolar planets
2: Artist’s impression of Kepler-22b,
a relatively small planet discovered
by NASA’s Kepler mission that has
been confirmed orbiting in its star’s
habitable zone. The planet is 2.4
times the size of Earth, and orbits a
star like our Sun. It is not yet known
if the planet is predominantly
rocky, gaseous or liquid. It could be
like Earth, or it could be an ocean
world, possibly with clouds in its
atmosphere, as depicted here.
(NASA/Ames/JPL-Caltech)
their combination extremely unlikely, making
habitable planets like Earth very rare. Kasting
disagrees. The key parameter, in his view, is the
fraction of stars with planets in the habitable
zone (HZ) and, because the climate stabilizing
effect of silicate weathering produces a relatively
wide HZ, up to a third of all stars are likely to
have rocky planets at the appropriate distance
(figure 2). This parameter is called hEarth and
we’ll have a more reliable value for it in two
to three years when the Kepler dataset is more
complete. However, hEarth is not the only factor;
even if it turns out to be high there are other
important constraints such as planet mass, stellar mass and the question of whether planetary
formation mechanisms typically deliver the
necessary proportion of volatile compounds
(especially CO2 and H 2O) to planets in the HZ.
So, many different factors go into determining
whether a planet may be habitable.
Andrew Liddle (University of Sussex) then
outlined the cosmological context of the discussion. Anthropic selection arguments are more
fully developed in cosmology than they are at
the planetary level and have recently been given
increased credibility by the fact that multiple
universes are a natural consequence of inflationary models for the early universe. Inflationary
models, in turn, are spectacularly effective at
predicting the power spectrum of the cosmic
microwave background and are therefore widely
accepted by cosmologists. Crucially, inflation
predicts that our universe – with fundamental
constants suitable for life as we know it – is
much larger than the small part we can see and
thus, even if Earth-like planets are vanishingly
rare, they are all but inevitable somewhere in
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the cosmos.
The meeting then moved on to planetary formation theory. Richard Nelson (Queen Mary
University of London) provided an up-to-date
survey of N-body simulations of planet formation and, in particular, how migration of both
low- and high-mass planets affects the formation of terrestrial worlds. Migration of giant
planets from beyond the snow-line through the
terrestrial zone was shown to endow terrestrial
planets with abundant water and other volatiles, such that “ocean worlds” would be most
likely to form. To date, however, no models exist
that self-consistently treat the formation and
migration of terrestrial and giant planets, so
theoretical explanations about the formation
of the solar system and predictions about the
nature of alien worlds remain rather tentative.
What about the neighbours?
The second session of the morning began with
back-to-back talks on “Where did it all go
wrong for Mars?” by Monica Grady (Open
University) and “Venus, the Earth that never
was” from Richard Ghail (Imperial College
London), which contrasted Earth’s evolution
with those of our nearest neighbours. Both talks
emphasized that Venus, Earth and Mars were
made from similar starting materials by similar
processes, but that both our neighbours became
permanently unsuitable for complex life early in
their history. In the case of Mars, the key factor
seems to be its relatively small size and hence
rapid cooling which resulted in the loss of any
form of plate tectonics by 3.5 billion years ago.
Venus, on the other hand, although much
closer in size to Earth, is significantly less dense,
perhaps indicating a smaller core that is entirely
liquid. Earth’s magnetic field is generated by
thermal and chemical convection primarily
driven by the formation of its inner core. Without an inner core, Venus lacks the driving force
for a magnetic field. The influence of a magnetic
field on atmospheric evolution is uncertain, but
may have significantly reduced atmospheric
erosion by coronal mass ejections, particularly
during the early part of the solar system. The
lack of a magnetic field at Venus may have significantly accelerated the rate of water loss from
its atmosphere and an ocean of water could have
been lost within a billion years. Alternatively,
because it is closer to the Sun, Venus may have
accreted significantly less water in the first
place. Without oceans and their role in the precipitation of CO2 as limestone, the present-day
extreme greenhouse effect and hostile environment for life on Venus became inevitable.
The final three talks of the morning looked
at what little data we have and what we can
infer from it. Andrew Watson (University of
East Anglia) began with the observation that
complex life evolved late on Earth. Large, multi­
cellular life began to emerge about 1 billion
years ago, and intelligence only very recently,
about 0.001 billion years ago. This is 80% of the
time available between the origin of life (around
4 billion years ago) and a time (roughly 1 billion
years in the future) when the slowly evolving
Sun will become too warm for life on Earth.
The timescale for complex life to emerge contrasts with the rapid establishment of bacterial
life, which appears to have thrived only a few
hundred million years after Earth formed. This
suggests that, while prokaryote styles of life
A&G • August 2012 • Vol. 53
Waltham, Dartnell: Meeting report
might evolve relatively easily, getting to complex life is hard. We can be more quantitative; the timescales can be explained
by assuming that several extremely
unlikely but critical steps were
required for the evolution of
intelligence. The best estimate is that four steps
would be needed. Each
of the steps has very
low probability of
occurrence within
the habitable lifetime of a planet and
the implication is
that intelligent life
(which requires all
four steps) is vanishingly rare. Habitable planets might be
quite common, but if
they are inhabited at
all it will be by bacteria,
not by animal- or plantlike life.
Where are they?
Ian Crawford’s (Birkbeck) talk
could well be described as based upon
the absence of data! Fermi’s Paradox – the
question that, if there are other forms of intelligent life, then where are they? – stems from
the observation that there is no evidence of any
outside interference in the solar system at any
point in its 4.5 billion year history. This is significant because it is easy to show that a sufficiently
advanced technological civilization would be
able to colonize every suitable planet in our galaxy in a very much shorter time than the galaxy’s age. The fact that our planet has not been
interfered with, despite having being wide open
to such interference for the past 4.5 billion years
(during which time it has circumnavigated the
galaxy approximately 20 times) places severe
constraints on the prevalence of technological
civilizations. Although there are many conceivable explanations for why other civilizations may
not have interfered with us, almost all require
implausible assumptions about the consistency
of alien psychology (e.g. that advanced civilizations always wipe themselves out, or that they
all subscribe to the same set of ethical codes
regarding non-interference with more primitive life forms). The most plausible explanation
that doesn’t suffer from these difficulties is that
advanced civilizations are rare.
However, Crawford argued that this is
unlikely to be due to an absence of suitable
planets. Warm, wet, rocky planets similar
to the Earth as it was 4 billion years ago are
probably very common in the galaxy, as recent
results from the Kepler mission seem to confirm.
Thus, in this sense, the Earth as a planet is probA&G • August 2012 • Vol. 53 Life in the universe: solar system
3: The icy moons of the outer planents,
especially Jupiter, may have environments
suitable for life, because tidal heating could
lead to subsurface water. Here Ganymede
displays varied terrains, including fractured
regions, flows and a bright ray system
probably caused by a relatively recent
impact. In May 2012 the European Space
Agency announced plans for the Jupiter Icy
Moon Explorer (JUICE) spacecraft, which
will study Ganymede, Callisto and Europa.
(NASA/JPL/Ted Stryk)
ably not special. Rather, the apparent rarity of
technological civilizations is more likely to be a
consequence of the complexities of evolutionary
biology. The evolution of intelligent creatures
capable of developing advanced technology is
contingent on a number of intermediate steps.
Examples include the origin of life in the first
place, the evolution of complex (eukaryotic)
cells, the origin of multicellularity, and the evolution of intelligence itself. Each individual step
may be unlikely to occur, making the end result
– technological civilizations – very rare even if
broadly Earth-like planets are not.
Dave Waltham (Royal Holloway University of
London) then discussed the limited astronomical data currently available for assessing how
unusual our planet is compared to the general
population of exoplanets. In particular, he
pointed out that 95% of all stars in our
neighbourhood are smaller than the
Sun and 95% of our galaxy’s stars
are closer to the centre of the
galaxy than we are. The unusually large size of our star
is particularly surprising
since, if the evolution of
intelligent life requires
a long period of habitability as suggested
by Andrew Watson
and Ian Crawford,
we should expect
intelligent organisms to arise more
often around stars
with long mainsequence lifetimes.
This may imply that
planets orbiting small
stars are less habitable
for some other reason (e.g.
tidal-locking or large and
frequent flares).
However, the key point of
Waltham’s presentation was that
probability distribution functions of
exoplanet properties, together with Bayes’
theorem and the “principle of mediocrity” (i.e.
that the Earth should be a typical inhabited
planet), allow the habitable range for properties to be estimated. Using the stellar mass and
galactic location data discussed above, the best
estimates are that 10% of stars have masses in
the habitable range and 11% of stars are within
the galactic habitable zone. Unfortunately, current data are very sparse and biased and the formal uncertainties in the results are very large.
Hence, at present, it is hard to draw more than
tentative conclusions, but Bayesian analysis
should come into its own as better data become
available. This should allow the investigation
of properties such as planetary mass, orbital
eccentricity and atmospheric composition.
Alternatives to Earth
Most of the morning’s talks produced a rather
gloomy assessment of the prevalence of habitable worlds, but our afternoon keynote lecture
from Helmut Lammer (Austrian Academy of
Sciences) gave a more upbeat approach by considering alternative habitats such as icy moons
and ice giants that have migrated into the habitable zone. Depending on the formation age of
the planet, its mass and size, as well as the lifetime of the extreme ultraviolet radiation early
phase of its host star, many terrestrial planets
may not get rid of their proto-atmospheres and
could end as water worlds with CO2 atmo­
spheres and hydrogen- or oxygen-rich upper
atmospheres. If an atmosphere of a terrestrial
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Waltham, Dartnell: Meeting report
planet evolves to an N2-rich atmosphere too
early in its lifetime, the atmosphere may be lost
by various escape processes. This talk demonstrated that the route to Earth-like habitability
is easily disrupted by early stellar activity, differences in volatile content and by the role of
impacts. As a result, habitats unlike Earth may
be more common than Earth-like ones!
The next talk continued the theme of alternatives to Earth by considering the question of
whether Earth-like biospheres, characterized
by complex organisms living upon the surface
of a planet, are the exception rather than rule
(Sean McMahon, University of Aberdeen). The
deep biosphere is a relatively recent discovery
on Earth and includes diverse microbes reliant
on geochemical energy sources and nutrients,
living in complete isolation from sunlight and
photosynthetic organic matter.
In general, planetary and lunar surfaces
are subject to extremes of cold, desiccation,
radiation and other hazards, while subsurface
environments are more hospitable. Crucially,
almost all the liquid water belonging to planets
and moons is likely to occur well outside traditional circumstellar HZs, in geothermally or
tidally heated subsurface environments. Such
environments may commonly meet other criteria for habitability, including the provision of
long-term chemical disequilibrium. However,
it is not clear whether they can host independent origins of life. If they can, then subsurface
biospheres are probably far more common in
the universe than surface-dominated ones. If
not, there may nevertheless be many instances
in which transient surface biospheres inoculate
subsurface environments that remain inhabited
long after surfaces become sterile, a scenario
that is widely discussed in relation to Mars. It
may be impossible to detect deep biospheres on
exoplanets. In our own solar system, however,
their signatures might be found in rocks and
minerals formed at depth and later exposed
by impacts, tectonics or erosion; or preserved
in ices, rocks and minerals formed where
deeply sourced fluids reach the surface; or even
entrained in cryo­volcanic plumes amenable to
remote spectroscopic analysis.
Geochemical systems
In the penultimate talk of the day Nick Lane
(University College London) returned to the
earlier theme of whether there are intrinsically
unlikely steps in the evolution of complex organisms. Nick demonstrated that redox and proton
gradients across membranes are “as universal as
the genetic code”, and are central to all forms
of cell respiration and photosynthesis. Under
primordial Earth conditions, in the absence of
electron acceptors such as oxygen that derive
ultimately from photosynthesis, the most likely
electron donor for life is hydrogen, and the most
likely electron acceptor is carbon dioxide. This
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Life in the universe: extreme environments
4: This alkaline thermal vent emits fluids at 91 °C with
a pH of 11. It is part of the “Lost City” vent system off
the mid-Atlantic ridge. Alkaline vents are formed from
the reaction of water with olivine in the oceanic crust,
producing large quantities of hydrogen, dissolved in
alkaline hydrothermal fluids, as well as heat. These
vents offer simple geochemical equivalents to the
biological processes still operating in bacteria living
there today. For scale, the red laser spots are 10 cm
apart. (Deborah Kelley, Univ. Washington)
reaction is the basis of metabolism in phylogenetically ancient cells such as methanogens
and acetogens, which glean both the energy
and reduced carbon they need for growth from
H 2 and CO2 alone, but can only do so through
the use of proton gradients (chemiosmotic coupling). Chemiosmotic coupling appears to be
strictly necessary for growth on thermodynamic
grounds (because gradients permit substoichiometric conservation of energy).
The requirement for proton gradients points
to a very particular geochemical system as the
incubator of life: alkaline hydrothermal vents
(figure 4). These vent systems juxtapose reduced
alkaline fluids rich in H 2 with oxidized acidic
oceans rich in CO2 via catalytic microporous
mineral matrices, to produce steep redox and
proton gradients. Such systems are produced
by the serpentinization of ubiquitous ultramafic
minerals (notably olivine), and should have been
widespread on the early Earth or any other wet,
rocky planet. The remarkable congruence of
alkaline vents to living cells, and their likely
abundance on the Earth and throughout the universe, suggests that life should arise on a large
number of planets and moons, albeit within a
restricted set of geochemical conditions.
While natural proton gradients in alkaline
vents may have facilitated the origin of pro­
kary­otic cells, which respire across their plasma
membrane, they also engendered a major bio­
energetic obstacle to the evolution of complex
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Waltham, Dartnell: Meeting report
Life in the universe: extremophiles
5: Extremophiles are generally thought of as
microbial life forms, but larger creatures can
take advantage of unusual conditions. This is a
new kind of centipede-like worm discovered in
1997 living on and within mounds of methane ice
on the floor of the Gulf of Mexico, about 240 km
south of New Orleans. Methane ice, which is
a gas hydrate (clathrate), forms naturally at
the high pressure and low temperature of the
deep sea, but is usually buried deep in marine
sediment. The Gulf of Mexico is one of the few
places where hydrate is exposed, seeping
from the ocean floor to form mounds that can
be as much as 2 m across. These flat, pinkish
worms, between 2 and 5 cm long, were found
in colonies burrowing into methane mounds
seeping up from the sea floor. They were seen
to move across the clathrate using their many
appendages, as if rowing through water. They
may be grazing off chemosynthetic bacteria
that grow on the methane or may be living
symbiotically with them in some other way.
Shrimp and tubeworms that live at deep sea
vents also rely on chemosynthetic microbes that
can extract energy from the hot mineral-rich
water of hydrothermal plumes. (NOAA)
life. Surface-area-to-volume constraints, coupled with a requirement for genes to control
chemiosmotic coupling across bioenergetic
membranes, restricts cell volume and genome
size in bacteria. This explains why the majority
of Earth’s organisms remain microscopic with
genome sizes to match. The evolution of larger
cells and multicellular organisms turned on
a rare and stochastic endosymbiosis between
prokaryotes, which was most likely to have
been the only way of overcoming these bioenergetics constraints. Such an event gave rise to
eukaryotes, the group that includes all complex
life on Earth (animals, plants, fungi and algae),
on a single occasion in 4 billion years. The fact
that proton gradients were necessary for the
origin of life under Earth-like conditions, yet at
the same time prohibited the evolution of complex life except through a rare and stochastic
event, means that complex life is far from the
inevitable outcome of natural selection operating on enormous populations of bacteria over
billions of years.
Extremophiles
The discussion was drawn to a close by Lewis
Dartnell’s (UCL) presentation on extremophiles. These are organisms living in the most
hostile and seemingly inhospitable environments on Earth; they are “extreme-loving” life
forms. The survival of extremophiles therefore
define the outer boundaries of what are habitable conditions on Earth and, by extension,
what environments on other worlds could
support life. Some of the stranger organisms
discussed were psychrophilic, or cold-loving,
A&G • August 2012 • Vol. 53 grylloblatid insects that live on Arctic ice fields
at subzero temperatures (and would die even
with the warmth of your hand), bacteria which
metabolize uranium, and animals from 3 km
below the surface of the Mediterranean sea
that spend their entire lifecycle without using
oxygen.
The ranges of conditions tolerable by such
extremophiles define the boundaries of the
survival envelope of all life on Earth. A plot of
temperature, pH and salinity shows a “boot
shaped” survival envelope under these factors.
This region of habitability overlaps with environments believed to have existed on the surface
of early Mars, or perhaps deep subsurface martian aquifers today, in the cloud layers of Venus,
and in the ocean of liquid water lying beneath
the frozen surface of Europa.
Dartnell explained that extremophiles, and
the astounding range of conditions they can
tolerate, are often used within astrobiology to
argue that life may be commonplace beyond
Earth, thriving in a variety of extraterrestrial
environments. But he cautioned that they may
be a red herring in this context. Extremophiles
demonstrate the capacity for life to adapt to a
huge diversity of new environments once it has
arisen, but they don’t indicate how likely life is
to emerge in the first place. It can be expected
that the physicochemical conditions required
for the development of networks of prebiotic
chemical reactions and the origin of life are
much more limiting than those survivable by
cellular life once established. An encapsulating
membrane allows a cell to control its internal
environment and, indeed, many extremophiles
expend a lot of energy maintaining this against
the external conditions. The origin of the first
cells probably required conditions of mild
salinity, roughly neutral pH, and warm temperatures. From its origin in this small region
of the parameter space, cellular life has radiated
outwards to populate the survival envelope we
see today. Hence, while extremophiles inform
us on the habitability of various extraterrestrial
environments, the chemical processes leading to
the emergence of life are still poorly understood.
The general thrust of the talks at this discussion meeting was that, while simple life may
well be widespread, there are many possible barriers to the development of intelligent observers
in the universe. Suitable habitats may be rare
and, to make matters worse, there may be several critical and unlikely steps in the development of sentient life. However, given the current
paucity of hard data, we cannot know how rare
life is in general, and intelligent life is in particular, unless we keep looking. None of the factors
discussed at this meeting preclude the possibility
that simple life may well exist on planets around
nearby stars. Such planets could conceivably
be identified and characterized by space-based
telescopes such as NASA’s proposed Terrestrial
Planet Finder and ESA’s Darwin interferometer,
and all the meeting participants look forward to
seeing the results of such missions. ●
Dave Waltham is Head of the Department of
Earth Sciences at Royal Holloway University of
London; [email protected]. Lewis Dartnell
is a research associate in the Centre for Planetary
Sciences at University College London.
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