Gerald Joyce suggests a different perspective, which analyzes the information content of a system. In that sense, biological systems have a kind of molecular memory, their genotype, that is maintained by self-reproduction and is affected by experience and environment. The information content of this memory can be calculated from the number of possible combinations, minus the number of actually realized combinations, or ‘phenotypes.’ A fictive molecule that self-replicates and also uses substances in its environment to create copies of itself would have information content of 0 according to this calculation. Every possible combination—one—has also been realized. Conclusion: Not life. According to this view, life could develop in different ways: directly from chemistry, for instance, after a sufficient molecular complexity was reached to form an information memory. Life could also develop from earlier forms of life, without taking information from them. That is probably what happened on Earth: The earlier RNA-based forms cleared the path for DNA-based life.
In his paper, Joyce develops a complex method to determine the information content of potential life forms. At the same time, the method also provides clues where it might be worth searching for life. In order to develop a permanent memory, certain prerequisites concerning the potential information density must be fulfilled. Joyce calculates, for example, that in a small pond, even under ideal conditions, 27 kilograms of RNA would have to aggregate chemically in order that there is a high probability of life developing. The first real alternative to life as we know it, Joyce thinks, might not therefore, be found on Mars or on Saturn’s moon Titan, but rather in labs on Earth.
Finding Aliens faster
If ET doesn’t come from Mars, but from a faraway solar system, he might not even notice Earth is an inhabited planet. From several thousand light-years away our home planet can’t be seen. This rock is so small compared to Jupiter or Saturn that it does not visibly affect the movement of the sun. The radio waves our civilization sends into space are useless as a cosmic beacon, as we only started building the first radios a few generations ago. And mankind has not yet succeeded in destroying our whole planet in an explosion.
Of course these restrictions also work the other way, when we ourselves start searching for alien life forms. How should we recognize them from a great distance? There is one substance that astronomers consider a typical, though not conclusive, sign of life—and we have been voluntarily releasing more of it into the atmosphere for several centuries: Methane, which under Earth-like conditions is a gas, can have a biological origin.
In order to find out which substances are in a planet’s atmosphere, we need a spectrum of the light that is emitted or reflected by it. Back in school we learned how spectra are created: an excited electron moves to a lower energy level and emits a photon. Its color—or its wavelength and energy—depends on the energy difference of the two levels the electron is switching between.
Reality is basically like this lesson from school, but a bit more complicated. While methane only consists of one carbon and four hydrogen atoms, and is highly symmetrical, even this modest multi-body system causes considerable difficulties for scientists. So even at room temperature the spectrum of methane has not yet been completely characterized. However, if temperatures rise, for example to the level on a brown dwarf, millions of different transitions become possible. The current knowledge about the methane spectrum in various sectors is fragmentary, partially calculated from the structure of the molecules, partially determined semi-empirically, and partially measured in experiments.
In a paper in Proceedings of the National Academy of Sciences, a British team presents new spectral lines for methane with much higher resolution. Their experiments were conducted in a temperature range up to 1500 degrees Kelvin—with 9.8 billion transitions. For the same range, previous experiments had suggested only 340,000 transitions. For this project the researchers simulated the methane molecule in the COSMOS supercomputer. They also prove that the newly-calculated spectra correspond to reality. This shows that through our lack of knowledge the methane content of celestial bodies seems to have been systematically underestimated. This is not only relevant for searching out possible aliens, but also for our understanding of the structure and evolution of these bodies.
How do We Distinguish Inhabited Planets from Uninhabited Ones?
It was not long ago that the idea of identifying planets orbiting other suns was considered pure science fiction. But now, astronomers find hundreds of new planets every year. At the same time, they collect incredibly meticulous information about these alien worlds, analyze their surface temperatures, and try to determine the consistency of the surfaces. Just 50 years ago we knew less about Saturn’s moon Titan than we know today about some exoplanets.
Of course scientists are most interested in whether a world is habitable. They don’t shy away from a purely-human perspective, as we don’t have another one, and they search for life as we know it. At least this has the advantage that the necessary conditions are well-researched.
Which of the exoplanets fulfill these conditions? This is determined by a kind of exclusion system. Based on the fact that water should exist in liquid form, the heat emitted by the mother star determines a maximum and a minimum distance for the planetary orbit. Yet that is not enough. Some stellar types, for example, emit above-average amounts of dangerous radiation, so that a planet in the habitable zone might become uninhabitable.
Previously, the fact that water vapor existed in the upper air layers—the stratosphere—was seen as a sure sign that the planet was no longer habitable. If a planet and its sun were in the right position, astronomers on Earth could receive the light of the star after it had passed through the planetary atmosphere. When the spectrum contained indications of water vapor, it was assumed that the ground was hotter than 66 degrees, and that the planet was an uninhabitable ‘moist greenhouse.’
However, a new atmospheric model by NASA researchers shows this might not always be the case. Under certain conditions, such as in the case of a tidally locked planet, which always faces the same side toward its sun, clouds can form and function as an umbrella. What happens depends on the energy emission of the star in the near-infrared, and thus the type of star. Red dwarfs in particular emit a lot of their energy in near-infrared. Water vapor absorbs this infrared radiation very well. Researchers’ calculations show that the planet should have a climate somewhat warmer than the tropics on Earth. Because most stars around which we have found planets are red dwarfs, NASA emphasizes that this significantly increases our chances during the search for life.
With Darwin into Space: How Alien Life Develops
The Milky Way contains about 100 billion planets. Between 20 and 40 percent of these orbit in the habitable zone of their stars. If only one-thousandth of one percent of these celestial bodies develops life, which is a very conservative estimate, there would still be 200,000 inhabited planets, or one planet in each cubic region of space with an edge length of 230 light-years.
What might this alien life look like? Researchers and science fiction authors love to speculate. Here’s the problem: We only have one example, life on Earth, so we tend to use it as a measuring stick. Even the concept of a habitable zone is based on it. But who could say whether silicon-based life forms might not find a very different environment optimal? On Earth, an eye-like visual organ has developed independently more than 40 times—therefore astrobiologists assume that aliens also would have eyes.
A research team who published an article in the International Journal of Astrobiology considers this a mechanistic perspective. It is not necessarily wrong to make such assumptions, but this approach is severely limited by the fact that, so far, we only know one form of life. Therefore the authors suggest to also use theory-based predictions. There is one theory that has worked very well in explaining the wide variety of life on Earth—the theory of natural selection.
This is a fundamental element of Darwin’s theory of evolution. As this is a theory verifie
d multiple times, it should also apply on other planets, just as quantum physics is universally valid. So if we find life somewhere in space, it will owe its shape to natural selection, unless it is so primitive that it creates an exact copy during each reproduction.
But in all other cases the theory of evolution will apply. Over time it causes a rise in the complexity of life. This is particularly true when there was a major transition. A complex organism can only develop from individual, independent cells, if these cells exist without conflict, but dependent on each other, without being able to reproduce individually. The cells in your hand, for example, are dependent on the cells in your foot, and they are not in conflict. Quite the opposite, as they can only reproduce and pass on their genes communally. Nature has to offer certain conditions before such a state can be reached. This is also something that would apply to life in space. Major transitions, for instance, are provoked when certain resources, such as food and land, become scarce.
What do researches deduce from this as regards alien life forms? Three points:
•They will be individuals consisting of smaller units. There might still be variants of the sub-units living individually in the wild.
•There must be something to balance the interests of these sub-units so that they work for the common good.
•Some process must limit the growth of the population so that different interests can adapt to each other.
Good Living Conditions on Enceladus, the Moon of Saturn
Carboxydothermus hydrogenoformans is a bacterium, or more precisely the endospore of a bacterium that helps it survive bad times. For carboxydothermus hydrogenoformans, ‘bad times’ mean disgusting fresh air that contains not a bit of carbon monoxide, which is poisonous for humans, as well as what it considers icy temperatures, anything below 100 degrees Celsius. This bacterium grows and flourishes in hot springs on the Russian volcanic island Kunashir, where it was discovered in 1991. Carboxydothermus hydrogenoformans was never bothered by the fact that the conditions there seemed, at first sight, to be hostile to life—and it is a very fast-growing species of bacteria.
It secures its survival—which it seems to have been doing for millions, if not billions of years—with the aid of a process which is now very rarely used by lifeforms on Earth: Carboxydothermus hydrogenoformans gains energy from carbon monoxide (CO), which during the same process turns carbon monoxide into carbon dioxide (CO2), as the organism breathes water (H2O) and molecular hydrogen (H2) is created. On Earth that is a narrow niche, as there is plenty of water but little carbon monoxide. This was not always the case, though. In the early period of our planet, life had to make do without oxygen. Back then there were more ways to secure one’s daily bread, whether with the help of ammonia (bacterium Planctomycetes), or of sulfur (Thiobacillus denitrificans) or even iron (like Acidithiobacillus ferrooxidans). All these microorganisms still live on Earth, even though they probably feel discriminated against by us oxygen breathers.
On other worlds, we might be the aliens, because we don’t yet know any other planet with an oxygen atmosphere. But what about the living conditions of these species leading a niche existence on Earth? Carboxydothermus hydrogenoformans might do quite well on the Saturn moon Enceladus, as scientists proved in a recent paper published in Science. The decisive clues come—once again—from the Cassini probe, which crashed into Saturn on September 15, 2017.
Cassini recorded the data a while ago, during its last close fly-by in 2015. At that time the probe flew as low as 49 kilometers over the so-called Tiger Stripes, through which gas from the ocean below the ice crust vents into space. The INMS, or Ion and Neutral Mass Spectrometer, measured the composition of the gas clouds. This is the exciting part: It recorded up to 1.4 percent molecular hydrogen H2, which is exhaled by Carboxydothermus hydrogenoformans. Incidentally, there was also 0.8 percent CO2, which is not quite as telling.
Of course this does not mean that there have to be life forms exhaling hydrogen in the Enceladus ocean. Perhaps the opposite is true: If there were bacteria like Cupriavidus metallidurans, which breathe hydrogen into water, then less hydrogen should escape. Researchers have wondered, though, where the H2 comes from—and they exclude geological deposits. The hydrogen could not have been stored in the ice, for example. It seems clear that it must be generated at the bottom of the Enceladus ocean, in processes that provide enough energy to enable the existence of life. Energy is a decisive prerequisite for the development of life—if it is available, life will somehow find a way, as the exploration of hostile environments on Earth proves again and again.
Alien Watch: Which Alien Civilizations can See Us?
To discover exoplanets, astronomers generally use one of two methods: the radial velocity method or the transit method. When considering the rotation of the Earth around the sun, one often imagines as if the sun were stationary, pulling the Earth around it on a string, so to speak. This image is incorrect. In reality, both Earth and Sun, planet and star, move around a common center of gravity. So the star in the telescope also turns in circles, though small ones, when it is influenced by its planet. We cannot see this circular motion from the Earth. But we can see that the star observed in the telescope moves back and forth, away from us and toward us.
The speed with which this happens is called the ‘radial velocity.’ Via the Doppler effect, this slightly shifts the star’s spectral lines. We can measure this shift with special instruments and then calculate how heavy the planet or planets pulling on this star must be—that is the so-called radial velocity method. If this technique alone is used, though, it yields a lower value for the planetary mass.
In order to calculate the exact mass, and thus the density, the planet would also have to be detected by the transit method. The transit method presupposes that the course of the planet moves directly across the axis between the Earth and the planet’s star. This reduces the brightness of the star at specific intervals, which can be measured by telescopes.
If an alien civilization wanted to discover Earth as a terrestrial planet, it would need both of these methods to gain all necessary information. This means that, as viewed from the alien home world, Earth and the sun would have to be on one plane, as otherwise the Earth would not move in front of the sun and obscure it.
In an exciting research project, astronomers from such institutions as the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, determined which alien worlds would fulfill this condition. Only a fraction of them do. In general, terrestrial worlds such as Mercury, Venus, Earth, and Mars are easier to discover than gas giants, in spite of their small size, since the closer a planet orbits to its star, the more frequently it obscures the sun. In addition, there is no place in space from which more than three planets of our solar system can be detected via the transit method. The average chance of finding at least one of the planets is 1 in 40, for 2 planets only 1 in 400 and for 3 planets just 1 in 4,000. Among the more than 3,600 known exoplanets there are 68 from which at least one of the planets in the solar system could be discovered. Nine of these planets have a direct view of Earth, but not one of the nine is in a habitable zone. Scientists estimate there might be ten as-yet-undiscovered worlds in space for which both are true: they could detect Earth, and they orbit in a habitable zone. That is not very many—and perhaps this is the reason we’ve never received a call from ET?
What is going to be the last life form on Earth?
People say cockroaches could survive any catastrophe. Yet, if a supernova in our vicinity bathed Earth with gamma rays, it would mean the end of virtually all lifeforms, including the local cockroach population. There would still be survivors, though, as astrobiologists described in a paper in Scientific Reports. They are less than one millimeter long and belong to the genus water bears, or tardigrades.
Tardigrades can live almost anywhere, both on land and in water. Biologists have known for a while that they can survive even in extreme situations. They make it through droughts, cold snaps
, strong fluctuations in salinity, and lack of oxygen using clever strategies. In 2007 the ESA put some of these animals aboard the satellite Foton-M3 where they were exposed to vacuum, cold, and cosmic radiation—and they still survived. Temperatures down to -273 degrees Celsius and dehydration cannot kill them.
These are good preconditions for becoming the ultimate survivors on Earth. Researchers showed that neither a supernova nor a giant asteroid strike would extinguish the tardigrades. Only a dying sun, which in its red giant stage would enshroud Earth and its atmosphere, would completely sterilize our planet.
This shows, as scientists say, that life might be hard to extinguish once it develops, both here and elsewhere. That is a good sign for our efforts at finding life on Mars or in the oceans of icy moons.
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Glossary of Acronyms
AI – Artificial Intelligence
COSMOS – Cluster Of Systems of Metadata for Official Statistics
DNA – DeoxyriboNucleic Acid
EEG – ElectroEncephaloGram
ESA – European Space Agency
ET – ExtraTerrestrial
GNA – Glycol Nucleic Acid
INMS – Ion and Neutral Mass Spectrometer
LUCA – Last Universal Cellular Ancestor
NASA – National Aeronautics and Space Administration
Proxima Dreaming Page 27
