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.