One of the principal gaps in our knowledge of whether to
expect life on exoplanets is often phrased as the probability that life can
spontaneously arise on a planet with certain parameters, such as temperature
and size. This phrasing is terribly
inappropriate. We would better ask what
are the conditions under which self-replicating organisms can spontaneously form. There are some good theories that generally
indicate a location and a pathway by which self-replicating chemoautotrophs (organisms
which use chemical energy present in the environment plus carbon dioxide and
water to replicate) can form, but so far no proof and little evidence confirms
the details.
There are three sources of knowledge which might be tapped in
the future to fill the gap in our knowledge of the preconditions for life to
form. One is the exploration of other
planets and satellites in our own solar system.
Europa is touted as a good place to start for life exploration, as it
has an ocean of water under its icy surface, and the ocean may have an
interesting chemical composition.
Checking other planets may test the assumption that liquid water is
needed. Finding life in a gas giant’s atmosphere or a methane sea would instantly
throw off all the previous calculations and perhaps cause the astronomy
community to revise the definition of ‘habitable’.
Another source of knowledge about the probability of life
emerging spontaneously from a chemical solution is laboratory experiments. There is an odd contrast between the large
sums spent on looking for habitable planets and the amount spent on trying to
create the chemical building blocks of life in laboratories. If we really want to figure out how much life
is likely to be present in our galaxy, we should be sponsoring these studies with
a program of similar size. Some
preliminary work has been done, but the field is wide open at this point. So far, some basic organic chemicals are
known to form under the right conditions, but no one has developed the
necessary insights sufficiently to calculate what is needed, how long it might
take, and what stages are followed. Detailed
theories about the chemical pathways that might be involved have been developed
by Günter
Wächtershäuser, who discovered a stream of chemical reactions that utilize
an iron sulfide surface in a saturated CO2 water stream to produce
free energy, which then might lead to organic compounds that form a layer. The layer may lead to a primitive cell wall
which is the source of energy for the other reactions needed to reproduce and
evolve. Reproduction would be by growth
and division, and evolution by the usual process of mutation and selection, but
the step from a chemoautotroph to RNA remains to be speculated on.
The third source of knowledge about this probability is the
emergence of life here on this planet.
If life has emerged here many, many times, in a discoverable way, we
could begin to better assess where it might emerge in other chemical soups on
exoplanets and in our own solar system. We
could also assess more precisely what are the local conditions under which it
could form. The conditions that dominate
the differentiation between exoplanets with life and those without it might be
completely separate from just temperature and size, or even the presence of bodies
of water. Factors like crustal thickness
or core composition could be the make-or-break ones. These yet-to-be-discovered exoplanet conditions
might be observable, or might be unobservable indirectly, but could be
estimated from other observable conditions, if we only knew what they
were.
Wächtershäuser ‘s theory of the origin of life has it
emerging in hydrothermal
vents. A hydrothermal vent is a
location where hot crustal material beneath the surface takes seawater circulating
through porous areas and heats it – then it moves upward through cracks in the
crust to make a hot water spigot on the deep seafloor. These often have chemoautotrophic bacteria
present, which are the lowest level on the food chain. Many other types of organisms feed on them or
feed on organisms which feed on them.
Obviously, the chemoautotrophic bacteria have to be present before the
higher levels can be. These vents may
have been much more prevalent in different geologic eras, but there are many of
them known now, and more are being discovered each decade. The first was found in 1977, and they appear
in all of the oceans. They are located where
there is a circulation of sea water into a sufficiently hot area of the
crust. They can appear as a result of tectonic
activity.
A key question needs to be asked: “Are the chemoautotrophic bacteria evolving directly
from only chemical precursors on new vents, reverse evolving from other
bacteria, or are they migrating up to thousands of kilometers from vent to vent
and settling there?” We know that the
first fossils, which are layered biofilms of microorganisms such as cyanobacteria, appear at
least 3.5 billion years ago, only one billion years since the condensation of
the Earth from the dust disk. The
cyanobacteria were responsible for turning the atmosphere’s CO2 to O2. They are likely not the first bacteria of
life, as photosynthesis probably evolved after cells were already evolved. Earlier forms have not yet been discovered.
During the first portion of those first billion years,
condensation was finishing up, and chemical separation and tectonic plate
formation, asteroid bombardment, massive volcanism had started. Some fraction of this period was finally
quiescent enough for something alive to evolve, possibly these chemoautotrophic
bacteria. How much time did they need? If
they only evolved at hydrothermal vents, and if the vents didn’t last long in
that environment, the initial origination of self-reproducing entities had to
be short. Was it ten million years or a hundred
years? There is no information or theory
available to advise on this. There is no
reason to think that, just because it takes tens of thousands of years for new
species to emerge presently, that the initial origination of life needed
anything like that. It could have been
more or less. If it was less, we may be
able to make some very important discoveries that will go a long way toward
answering questions about the prevalence of life on exoplanets.
A team of oceanographers and microbiologists needs to answer
the following question: “Is the DNA of
the chemoautotrophic bacteria in hydrothermal vents of the different oceans the
same, or are there some, perhaps hard-to-detect, unique codings there that
would indicate the chemoautotrophic bacteria evolved independently from
chemicals present at the vent site?” If
the evolution of the precursor chemicals and then the metabolic layers and then
the detached protocell and then RNA and more takes hundreds of millions of
years, we would find only migratory or de-evolved cells at newer vents. But if these estimates are gross exaggerations,
and it takes much, much less, there might be unique organisms on sites that
were far from each other. Some estimate
of the time of migration might be useful here or a detection of the presence of
living chemoautotrophic bacteria in some sort of suspended state everywhere,
but the finding of unique signatures in the DNA of the chemoautotrophic
bacteria would be a startling clue that life is very easy to make, despite our
anticipation that it might be very hard. It will also be necessary to check that
reverse evolution of other organisms back to chemoautotrophic cells can not be
the source of these bacteria. Unique DNA
codings would help eliminate this.
Reverse evolution would seem to be a difficult route to follow.
A result showing re-evolution of life would clarify our exploration plans for our own solar system, and perhaps could lead to a better understanding of what to look for in our hunt for exoplanets. The implications are startling. If we definitively find newly evolved life on some hydrothermal vents, we can further investigate to determine what conditions are necessary for it to happen and how long it takes. For example, suppose hypothetically that some of Wächtershäuser ‘s theory is true, and vents with iron sulfide are necessary, and a minimum temperature of 300°C is needed, and it happens in a hundred years at an average vent.
Theoretical planetologists would need to determine what
chemical constituents of a dust disk would be necessary to produce a planet
with oceans, crust and atmosphere that match these requirements. Maybe red
dwarfs rarely have disks meeting the conditions for this, but O-type stars
do. We could change our emphasis on the
class of stars to be monitoring for planet discovery and investigation. At this time, we expect that red dwarfs, the
most prevalent class of star, will have planets about as regularly as larger
stars. But if we want to find planets
with life, we might have to ignore the most prevalent class and concentrate on
the ones which usually have the right composition of rocky planets.
Our Europa subsurface ocean probes might be focused on
finding and exploring vents. Most
certainly, looking for these would be on the agenda of any probe capable of reaching
the seafloor there. But if our
Earth-based studies indicate life almost pops into existence right after a
particular type of vent forms, the whole program might be aimed at finding if such
vents exist and taking and analyzing samples there.
Sometimes in science, we get lucky. Maybe there is a chance to draw a royal flush
in the life formation game here, and to do it soon without much new science
needed, just some experiments of the kinds already done. We know how to go down to hydrothermal vents
and take samples there, we know how to decode DNA, and we certainly can do
detailed comparisons of DNA from chemoautotrophs from different vent areas once
decoded. Not much more is needed. The payoff would be large compared with the
effort required. Knowing life can be
expected on certain classes of exoplanets would change our perspective on what
missions we want to undertake, both short-term, like observations, and
long-term, like mankind’s plans for its interaction with the rest of the
galaxy.
If we do find that life starts up easily, and should be on a substantial fraction of the exoplanets we find to be ‘habitable’ (liquid water temperature), do we still want to go? Or should we simply leave them all alone. If we make this choice, where is left for us to go? If there is no need to populate the galaxy because it is full of various kinds of populations already, what becomes of the role for us that some propose?
Similarly, Fermi’s paradox takes a different slant. Many potential answers could go away. Others poke their heads up a little
higher. Perhaps everybody just lets
everybody else stay alone and do whatever they choose to with their own home
planet. Or perhaps there is a huge
barrier between life and intelligent life, and most Earth-like planets are full
of their own ecologies, lacking the capstone of intelligence. At any rate, if the results of the proposed hydrothermal
vent DNA research are positive, we are in for a lot of new thinking and
planning.
