Little children the world over dream of flying in space ships and meeting aliens from distant galaxies. Indeed many adults share the same fantasies! We all have our idea of what aliens might look like, mostly influenced by media and movies. We might imagine little green men with giant skulls, or perhaps we think of ultra-thin gray beings with bulbous, black, cat-shaped eyes. But if extraterrestrial life does actually exist, and the odds that it does are quite good, would it look like we imagine? Would it resemble humans or other terrestrial creatures, living or dead, from Earth’s history? Would alien life even be carbon-based, like life on Earth? If so, would we be able to cohabitate with aliens on their planet or our own? Does this have implications for the future of human life? Determining the answers to these questions, and more, is the responsibility of a little known scientific specialty called Astrobiology.
According to the National Aeronautics and Space Administration (NASA), Astrobiology is “the study of the origin, evolution, distribution, and future of life in the universe.” Such a broad description means that Astrobiologists have an unbelievable amount of data to study. Their first source of data is, of course, the only planet that scientists have truly intimate knowledge of, Earth. Starting here, scientists collect and examine data and compare what is discovered to what we know of other solar bodies.
Astrobiology grew out of a slightly older science called exobiology, which focuses on discovering the presence of biosignatures, or unambiguous signs of life, on other planets and planetoids in our solar system and beyond. Starting in 1996, NASA founded the Astrobiology Program and in 1998 established the Astrobiology Institute. Research in exobiology is now a major component of the NASA Astrobiology Program. This program’s overarching goals are to determine the origin of life on Earth and in the Universe, the habitability of bodies within our solar system and beyond, and what the future might hold for Earthlings. This last concern is especially important to many, as the looming fear that Earth will not remain habitable forever – or even for much longer – hangs overhead like a dark cloud.
To address the above-mentioned concerns, Astrobiologists must ask and seek the answers to three fundamental questions. The first question concerns the origin of life in the universe: “How did, and does, life originate and evolve?” The second question concerns the habitability of other extraterrestrial environments and whether or not we could even find them: “Does life exist beyond Earth and, if so, what are the ways that we might be able to detect it?” Finally, the third question is the one that is too often handled by the science-fiction community: “What does the future hold for life beyond planet Earth and other planets in the universe?” Each of these questions must be dealt with carefully, incorporating known data sets, integrating new data collection techniques, reexamining old questions and answers, and relying on the ingenuity and creativity of a multitude of scientists, including astronomers, biologists, anthropologists, and geologists. The above-mentioned NASA Astrobiology Institute, or NAI, supports the use of “focus groups,” made up of teams of scientists all focusing their attention in one area to gather evidence for and answer a specific question or set of questions regarding the origin of life in the universe.
In order to answer the question of how life began and how it evolves into new and dynamic life forms, including intelligent life forms, Astrobiologists use information from their home planet. Due to advances in modern genetics, we know that the biological set of instructions for all life on Earth, or DNA, is comprised of four basic amino acids: adenine, cytosine, guanine, and thymine. The term nucleic acid is used as a general term for these four building blocks of DNA and, along with some other amino acids that serve special purposes, such as uracil, RNA. All living beings on Earth are carbon-based organic beings – that is to say, they are made up of complex molecules based on carbon which are significantly more intricate than carbon monoxide (CO) and the very common carbon dioxide (CO2). In fact, looking at all life on the planet, despite the differences in appearance, the underlying structures between all creatures are actually very similar. As famed scientist and educator Carl Sagan put it, “There are not many different kinds [of life]; there’s only one kind."
Modern scientists have a good grasp on what makes up life but there are differing theories on how it exactly started. Through research on comets and other celestial bodies and through missions to other planets, we know that organic material is in no short supply in the universe. As far back as the late 19th century, Sir William Huggins discovered C2 was in abundance around a comet using a spectroscope. The C2 discovered by Huggins was leftover after the comet had reacted with solar wind and energy and the parent molecules were dispersed.
Wherever the comet came from was carbon-rich. Observances of stars and planets in more recent times tells us that not only is carbon abundant throughout the universe, but so is nitrogen, hydrogen, methane, and even water vapor. In this sense, the universe is crawling with organic building blocks. What’s more, carbon-based nucleic acids have been shown to replicate themselves in the laboratory!
Through geological studies of rock formations, and paleontological and biological studies of fossils, scientists have a determined the approximate age of the Earth and of the first complex life forms. The Earth is approximately 4.5 billion years old, while the oldest complex fossils discovered, stromatolites, are approximately 3.5 billion years old. Stromatolites are not quite as complex as many of the life forms found on Earth today, but are not simple single-celled organisms either. Since the Earth has a rather tumultuous beginning and wasn’t habitable until around 4 billion years ago, it only took around 500 million years for complex carbon-based life to form. Famed geneticist and evolutionary biologist J.B.S. Haldane suggests that this rapid origin of life comes from the process of fermentation, or life without oxygen. He surmises that Earth’s early atmosphere had high amounts of carbon dioxide, supplying the carbon needed for life, and low amounts of oxygen, which allowed ultra-violet, or UV, rays from the Sun to reach the oceans. The Sun’s energy synthesized organic material when UV rays penetrated the oceans, a combination of carbon dioxide, ammonia, and water, and began to produce a food-filled environment Haldane calls a hot dilute soup. This soup produced so much organic material to feed the process of fermentation it would have been difficult for life to not persist. In fact, it must have synthesized numerous times within the hot soup, using the power Sun’s energy, and any process that happens numerous times, especially in a relatively short amount of time, must be a somewhat simple process (see Occam's Razor).
In this respect, it appears that the origin of life, under the right conditions, is a simple process. Once the originating forms begin popping into existence, competition for survival increases, and natural selection takes over. Some organisms may need to start synthesizing sugars and other organic material without the help of the Sun, while others may compete for particular food sources. As long as there are selective mechanisms (i.e., mutations, adaptations, or other variations between organisms in a particular environment) to work against, natural selection, and thus evolution, begins. In a nutshell, natural selection posits that those organisms with a physical advantage in a given environment will live longer and produce more. The result, of course, is a world rich in biodiversity.
Based upon what we know of the totality life on this planet, through living creatures and plants, and through the fossil record, complex carbon molecules can produce a seemingly unlimited number of possible life forms. However, we are only aware of one kind of life, and it is carbon-based, leaving scientists hard-pressed to even imagine other possible building blocks for life. With that in mind, scientists can search for environments with a carbon concentration, along with some other traits, in the hopes of possibly finding the early formation of life and to answer the second question of Astrobiology – that of habitability. A foremost indicator of potential habitability is the presence of water. Astrobiologists and Exobiologists focus a significant amount of time looking for water and organic material on planets and bodies within our solar system because they allow scientists to conduct in situ, or “in position,” observations.
The Viking 1 and Viking 2 missions to Mars in the 1970s are two famous examples of landing crafts with a biological mission component. Unfortunately (or perhaps fortunately, depending on your view), soil samples taken from two very different locations neglected to yield positive signs of life. In May 2008, the Mars Phoenix landed on the red planet bringing about new discoveries in the existence of water and ice including capturing images of Martian snowfall. More recent rover missions to Mars have yielded even more information on the planet’s current and past water activity and a number of geological processes that have occurred on the planet over time. In fact, the Spirit rover accidentally uncovered deposits of near pure silica, a sign that the planet likely had steam vents in its past, the presence of which is favorable to microbial life. Furthermore, large amounts of calcium carbonate were detected by the rover, another sign that wet, warm, and non-acidic conditions, again favorable for microorganisms, once existed on the now dry and rather frigid planet.
The existence of water is telling, but it doesn’t necessarily equal life. Other factors also contribute to the suitability of a planet to harbor life. Looking at Mars, scientists can say that life may have existed at some point, but the conditions of the planet, such as rapid climate changes, lack of a stable, oxygen-rich atmosphere, lack of a strong magnetic field similar to Earth’s, and distance from the Sun may not be conducive to any extended existence. Does this mean that Earth is unique in having all the right characteristics for life? Notorious “new atheist” and zoologist Richard Dawkins doesn’t think so. In his international bestseller, The God Delusion, Dawkins says:
It has been estimated that there are between 1 billion and 30 billion planets in our galaxy, and about 100 billion galaxies in the universe. Knocking a few noughts off for reasons of ordinary prudence, a billion billion is a conservative estimate of the number of available planets in the universe. Now, suppose the origin of life… really was a quite staggeringly improbable event… as to only occur on only one in a billion planets… even with such absurdly long odds, life will still have arisen on a billion planets – of which Earth, of course, is one.
He goes on to support his idea of life on a minimum billion planets by showing that the chemical recipe for life need only arise on one in a billion billion planets to be suitable for explaining life on Earth. The probability of the right conditions for habitability is almost certainly more probable than one in a billion. Especially considering that there is no reason to suspect that there is only one path to life. On Earth skeletal material can be comprised of calcium carbonate, like that of mollusks and other invertebrates, bone and cartilage, like mammals and other vertebrates, silica, like that of protozoa and slime molds, or chitin, such as is seen in lobsters, crabs, and other arthropods. From evidence gathered over years of biological research, it appears the evolution of the skeleton has occurred upwards of six times. The same should be expected of other organs that allow for species advancement. Take the human brain which, of course, allows our species to better understand the environment and anticipate upcoming events. We have no reason to believe that this is a particularly human characteristic or that it is a particularly improbable occurrence. Rather, we should assume the opposite, that it is just as likely to occur wherever life occurs, based on the theory of natural selection. However, even with the conservative estimate of a billion planets harboring life and no reason to believe that such life will not evolve into intelligent life, the universe being as vast as it is does not make the chances of finding such life especially favorable.
Finally, what many scientists may consider the hardest question to answer, by its very nature a prediction, the issue of the future of human life as resources become depleted on Earth hangs in the balance. NASA scientists are working on several projects to find potential future homes for Earthlings, including the SIM Lite Astrometric Observatory, formerly the Space Interferometry Mission, and the Terrestrial Planet Finder, or TPF. SIM Lite is planned to launch in 2015 with the goals of identifying terrestrial planets in habitable, or goldilocks, zones, determining the age of the Milky Way Galaxy, investigating energy streams and jets from celestial masses, and determining the size of the universe, among other things. The TPF mission, while still in the conceptual phase, has a specific focus to study every aspect of all known, and newly discovered, planets outside of our solar system. Scientists hope to determine the number of stars and planets, how they formed, and evidence of their habitability, to include potential for life in the future. Even if potentially Earth-like planets are found, the ability to reach them and to do so in a practical amount of time are still indeterminable factors at this point. This is not to mention whether the life that may live on the planet will welcome the introduction of a new intergalactic species as permanent guests or not.
It is easy to imagine little green men on a planet not too outrageously far away. Indeed, many people, particularly not-very-highly-educated Americans, who tend to live in the Midwestern states, believe they have seen unidentified flying objects or have been abducted by aliens (usually when no one else is around). The notion of a technologically advanced extraterrestrial species coming to Earth to probe colons is considerably far-fetched. However, the likelihood of life existing elsewhere in the Universe is not. Astrobiologists certainly have their work cut out for them in their search for such an existence. The future of human life could depend on it.