But, how much water? Researchers have now calculated this. According to their finding, about 15 percent of the total mass of inner planets b and c is water—for comparison, on Earth it is only 0.02 percent. So, 15 percent is a lot—750 times greater than on Earth. Half— meaning about 50 percent—of outer planets f and g must be water, though, an equivalent amount to hundreds of Earth oceans.
A large part of the water is most likely ice rather than occurring in liquid form. Considering the high pressures in the interior of a planet, that is not unusual. What is amazing is that these planets are located inside the ice line—aka snow line or frost line—of their system. Inside the ice line of a protostar, water is gaseous, and outside of the line it is solid. Ice worlds like Neptune and Uranus can thus only have formed outside of the ice line. This also applies to the planets of Trappist-1. During their history, they must have slipped closer to their star—by at least half of their initial distance.
What does this mean for the development of life? The planets of Trappist-1 might simply have too much water. On Earth, life developed in the boundary regions between rock and water. Only there were all the necessary preconditions met. A purely water world is therefore hardly suitable for developing Earth-like life. This might be bad news for all red dwarfs of the M class, the most common type of star in the Milky Way. The planets circling them might, in general, suffer from an oversupply of water.
Energy!
Life is an incredible phenomenon, from a physical perspective, as it at least temporarily reverses the most important trend during the development of the universe: It creates order from disorder. In fact, the cosmos has been moving the other way ever since the Big Bang. Entropy, the measure of disorder, is constantly increasing. With life, though, the degree of order is increased through evolution. This can’t happen without a very important ingredient: energy! It must be available energy, which exists wherever there is an imbalance.
That is, for instance, the case near a star. On the one side we have the star, which produces energy, and on the other its planets, which receive this energy in the form of light. Life must find a way to use and store the energy of light. Plants do this through photosynthesis, but other methods are imaginable. Animals can then use the energy stored in plants for their metabolism.
The energy received from the star also has another advantageous side effect: It allows the planet to heat its atmosphere so much that the solvent on its surface—mostly water—occurs in liquid form. The definition of a habitable zone around a star applies to this very fact.
This does not mean that a planet outside the habitable zone must by definition be uninhabitable. The worst factor would be too much energy. If it gets too hot, because the planet is too close to its star, it will sooner or later lose its atmosphere. Perhaps silicon-based life might stand a chance at high temperatures, but that is mere speculation. If it is too cold, other energy sources might play a role. Life does not necessarily have to be located on the surface. Conditions might be more favorable in the mantle or core of a planet. Biologists are surprised at the depths on Earth where we can discover traces of biological life. This might also apply to Mars, on whose surface conditions have been inconducive for life for three billion years.
Origin and Birth
Since its birth 13.8 billion years ago, the universe had to change significantly to become today’s haven for life. Rocky planets like Earth could not have formed around the first stars, because the heavy elements of which they consist did not yet exist. The first star generations had to produce these elements in their interiors and then fling them into space when they died in supernova explosions, which also simultaneously created elements heavier than iron. All of us consist of atoms created during the deaths of these stars.
Scientists are often amazed at how all of this worked. If only a single physical parameter had been different, we would not exist today. The simplest explanation for this is: We could not describe a universe in which the parameters are not suitable for the development of life, because then we would not exist. This is called the ‘anthropic principle.’ There is another formulation for this, which is much stronger: It demands that the development of life must be possible in all universes. One argument for this comes from quantum physics, stating that reality only comes into being through an observer.
Yet it is basically pointless to think about these issues. The only verifiable—and verified—fact is that in our universe the probability for the development of life must be greater than zero.
How high is this probability, though? Many people have wondered about this. There is the famous Drake equation, which calculates the number of galactic civilizations willing to communicate. However, it contains so many unknown factors that the result varies between one and four million, based on the specific interpretation. This tells us little about our chance of finding extraterrestrial life, because the Drake equation focuses on civilizations, on intelligent life. On Earth alone there are about 50 billion species, and only one of them developed higher intelligence.
Which stars should you concentrate on while looking for life? Over the last few years, many firm beliefs dissolved into nothing while discussing this. Currently we assume that on average there is one planet per star. Quite a few of these planets have the correct size and are at a suitable distance in relation to their star. In the Milky Way, which has up to 300 billion stars, our current estimate assumes about 40 billion rocky planets of Earth size in the habitable zone of their respective stars. Of these, 11 billion orbit sun-like stars. And the Milky Way is only one of the billions of galaxies in the universe.
Some stars have characteristics that make them appear less suitable at first sight. For red dwarfs—like Proxima Centauri in our story—the habitable zone is so close to the star that the planet is tidally locked, always pointing the same side toward the star. Yet life might have developed in the transition zones. These zones would also be shielded by the planet against frequent flares of the red dwarf, or a well-dimensioned magnetic field might protect against them.
The Birth of Life
Let’s assume we found one of the 40 billion rocky planets in the Milky Way. Let’s further assume there is water on the surface that was deposited there by comet impacts. The planet has a proto-atmosphere. What next?
Today, scientists don’t know for sure. The most important relevant experiment was performed in 1952 by Stanley Miller and Harold Urey at the University of Chicago. They mixed simple chemical substances in a hypothetical early Earth atmosphere—water, methane, ammonia, hydrogen, and carbon monoxide, but no oxygen—and exposed this mixture to electrical discharges modeling the energy supplied by lightning strikes. The result: After a while, organic molecules formed in the experiment.
Back then, the Miller-Urey experiment caused quite a stir, because it showed that the building blocks necessary for life can spontaneously develop under the right conditions. However, it did not solve the question about the development of life. Geologists later showed that early Earth exhibited slightly different conditions than the experiment presupposed. In addition, the experiment resulted in amino acids, but not in carbohydrates, another important building block of life. Those must have formed in a second phase. The same holds true for lipids, i.e. fatty acids.
By now, chemists have found indirect ways in which these components might have formed from the original chemicals. What is missing is the scenario that unites them all. Very often the conditions under which one biological building block is supposed to have developed are different from those for others. For practical purposes we must assume that all of this happened simultaneously four billion years ago at a specific location on Earth. Perhaps it did not occur in the atmosphere, but in a volcanic vent in the ocean, or deep inside the Earth’s crust.
How the actual spark of life came into the world is still a hotly-debated research subject. There are basically two opposing theories. The first assumes it all started with the metabolism, the energy cycle. This might have occurred in
volcanic areas of the ocean, either based on iron and sulfur, or on zinc. At first there would have only been a primitive metabolism. Hereditary transmission via RNA molecules—a primitive precursor of DNA—would have been added later.
The second theory proposes that life started when RNA molecules came into being by accident within encapsulated proto-cells. We know that some variants of RNA both encode information and serve as a catalyst for certain reactions. Just the sudden appearance of the RNA molecule could have boosted the metabolism of the proto-cell, giving it an advantage over other cells.
There are also discussions on whether RNA might have been preceded by even more primitive code molecules such as PNA, TNA, or GNA. Back then, the conditions for the development of life might have been better on Mars than on Earth—meteorites could have afterward transported those early forms of life to Earth.
If you check the Wikipedia entry for ‘Abiogenesis’ you will find more than a dozen more theories about how life might have come into existence. For biologists, only one of them can be correct. Which one it is will be decided based on the exact conditions on Earth four billion years ago, a subject which is still being researched.
From the perspective of astrobiology, all of these theories might be correct—if the conditions on any of the exoplanets fit any specific theory. Life does not have to develop the same way everywhere. Earth is one example of it, nothing more and nothing less.
On Earth the results at some point in time became LUCA, the Last Universal Cellular Ancestor. This is proven by the DNA analysis of current life. LUCA, the archetype of all current species, from bacteria to humans, lived about 3.5 billion years ago. At that time it probably was not the only form of life—but the descendants of all others living back then have become extinct. LUCA was already a rather biologically complex single-cell organism. It certainly was already the product of evolution. The organism stored genetic information in RNA and used ribosomes as protein factories. LUCA must have existed before Earth’s atmosphere was enriched with oxygen. This is an interesting piece of information for astrobiology—even a planet without oxygen in its atmosphere can harbor life.
The Classification of Life
Life on Earth seems to have divided early on into three sections—called domains—namely bacteria, archaea, and eukaryota. You know bacteria as pathogens. You yourself are a eukaryote—like any other multi-cellular being. Recent research even suggests there are only two domains. Bacteria would then have branched off from archaea.
Archaea are very fascinating from an astrobiological viewpoint, because among the still-living species there are many extremophiles, which thrive under particularly difficult conditions which might mimic those on alien worlds. There are organisms that grow at temperatures higher than 80° C, others living in highly concentrated saline solutions, under high pressure, or in very acidic or alkaline surroundings. Even methane-generating archaea are extreme in a certain way: They only grow without oxygen and often need molecular hydrogen for their metabolism.
Archaea species are relatively common and occur in fresh water, the ocean, and the soil, but also in the intestinal tracts of animals and humans. Archaea have even been found in the folds of the human belly button.
The greatest change of life on Earth occurred with photosynthesis. It probably developed first among bacteria and archaea. This is not always connected with the production of oxygen. Sulfur bacteria create the carbohydrates they need for their metabolism from carbon dioxide by creating elementary sulfur from sulfur compounds. Others use hydrogen or small organic molecules. We are lucky these species did not succeed worldwide, otherwise the atmosphere of today’s Earth would be unbreathable for us. Instead, organisms thrived that convert the oxygen atom in water into molecular oxygen, in order to produce carbohydrates. Plants later inherited this system from cyanobacteria.
When our planet was not even two billion years old, its surface was like a paradise. A billion years after the development of Earth, temperatures finally dropped below 100 degrees. Due to this cooling, the water vapor in the atmosphere condensed to huge oceans, in whose pleasant warmth first organic molecules and then the first forms of life appeared. Approximately 3.2 billion years ago the oceans must have been filled with flourishing communities of primeval bacteria, a veritable paradise.
Then environmental villains showed up.
Blue-green algae—today they are correctly called cyanobacteria, because they lack the true nuclei which real algae possess—started to produce masses of a deadly poison: dioxygen. They used the energy of sunlight to turn carbon dioxide, which was plentiful, into this dangerous substance. Their competitors, all of them anaerobic bacteria, had not yet formed a defensive system that could have stopped this environmental destruction.
At first everything seemed to be fine. There were sufficient dissolved metal ions in the oceans to render the produced oxygen harmless, and active volcanoes constantly provided enough reaction mass. The ore formations that came into being then are now our most important sources of iron world-wide. Earth settled down, and the cyanobacteria developed further. The first multicellular organisms appeared—and that must have been the moment when the great oxygen disaster started. About 2.3 to 2.4 billion years ago the percentage of this gas in the atmosphere, which was lethal to most forms of life then, rose so much that most species were doomed. Inside these organisms, oxygen created very reactive peroxides, which damaged the cells.
At the same time the growing oxygen content caused a global climate catastrophe. The protective greenhouse gas methane reacted with O2 and became carbon dioxide—which has a much lower impact on the climate—and water. Earth, scientists believe, must have been completely covered with ice for 300 to 400 million years.
The reactive gas also accelerated the weathering of the surface. New compounds and minerals were created—and that is the fact used by an international team of researchers publishing in the scholarly journal Nature to date this catastrophe. The scientists examined deposits of chromium compounds in rocks in South Africa, which must have formed about 2.98 billion years ago.
These already turned out to be extremely weathered, meaning they were influenced by processes that could have only happened in the presence of significant percentages of oxygen. Researchers calculated that at that time the dioxygen percentage in Earth’s atmosphere was about three ten-thousandths of today’s value. Although that seems a very small amount, it is significantly more than had been assumed for this point in time. This fact indicates that blue-green algae must have been very common three billion years ago—and therefore photosynthesis as an efficient means of gaining energy.
We can still feel the effects all over the world today:
A large percent of anaerobic organisms populating Earth at that time was eliminated.
The oxidation of atmospheric methane to carbon dioxide caused an ice age covering the entire planet and lasting 300 million years.
The variety of minerals occurring on Earth nearly doubled.
Life received a new chance through the increased oxygen content. The energy cycle became easier, as with many substances the oxidation with oxygen releases much more energy than a metabolism without oxidation. This was ultimately a trigger that helped multicellular organisms succeed.
A different Life?
The search for extraterrestrial life does not just fascinate astronomers, but also philosophers, movie directors, authors—and most other people. In previous times, influenced by religion, the fear of losing our exclusive position in the universe influenced this search. Today, enlightened from a historical perspective, we seem to want a kind of big brother, a wise civilization like the Vulcans in the Star Trek universe, who would have solved our problems long ago by helping with their advanced technology.
An alternative is the overwhelming menace from space, which makes us forget our differences and work together, saving us at least politically and religiously this way. However, if we are realistic, unlike in Hollywood movies, mankind would have a
s little chance against attackers who are 200 years more advanced as Native Americans had against European settlers 200 years ago. And a technological difference of 200 years is probably not even enough to give a civilization interstellar travel.
So much for the dreams of Hollywood and science fiction. The reality in searching for extraterrestrial life looks quite different. We have not even solved the question of how we would recognize alien life. The molecular biologist Gerald Joyce has published an interesting paper about this, which can be found in the open-access journal PLOS Biology. The preconditions for the work of exobiologists seem to be very good: We already know of about 1,000 exoplanets. We know that a number of them are orbiting their stars in areas that we consider suitable for the development of life. Astronomers have calculated that within a radius of 1,000 light-years around Earth there should be tens of thousands of sister planets offering conditions similar to our world. We estimate that within ten years we should have atmospheric data about some of these planets.
Nevertheless, it is hard for us to identify alien life. One reason, of course, is our lack of experience—we only have one biological life form for comparison. Therefore we are inclined to assume that life is based on cells in which genetic information is translated into proteins. Accordingly, we define that life reproduces itself, passes information from generation to generation, and is subject to Darwinian evolution. Yet what would happen if a life form does not fulfill these prerequisites? What if, for instance, it was a complex chemical system that does not use heredity?
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