The Search for Life Beyond the Solar System
Arguably, the questions of whether extraterrestrial life, in general, and intelligent life, in particular, exist are two of the most intriguing questions in science today. The discovery of extraterrestrial complex life would undoubtedly usher in a revolution that will rival the Copernican and Darwinian revolutions combined. At the most basic level one could wonder what the probability is that the Earth is unique in harboring life. Recent observations with the Kepler spacecraft (Figure 1) and the Hubble and Spitzer Space Telescopes allow us to make at least a rough estimate of this probability. Astronomers using Kepler found that about one in five (22%) Sun-like stars in the Milky Way has an Earth-sized planet in the so-called Habitable Zone around that star.
The Habitable Zone (Figure 2) is that circular band around the star that is neither too hot nor too cold, so that liquid water can exist on the planet’s surface. This, of course, doesn’t mean that life would actually emerge on such a planet, but a planet in the habitable zone satisfies at least some of the necessary conditions for life. How many such planets exist in the Milky Way? There are about 100 billion Sun-like stars in our Galaxy, which puts the number of “habitable” planets around these stars at about 20 billion. Now, how many galaxies are there in the cosmos? Estimates from the Hubble Ultra Deep Field put that number at about 200 billion in the observable universe. Not all of these galaxies are as large as the Milky Way, but some are much larger. Assuming that on the average about one in ten galaxies is similar to the Milky Way in its contents, we obtain for the number of potentially habitable planets the staggering number of 4 × 1020—that is, four hundred million trillion. If we now use the law of large numbers, for there to be (on average) only one planet (Earth) with life on it, the probability for a planet to harbor life must be as small as one in four hundred million trillion, or 2.5 × 10-21. Furthermore, any deviation from this probability (say, by a factor of one thousand, which could easily be possible), would result in there either being no planets with life on them at all, or in there being one thousand such planets. This makes it highly unlikely that we are the only game in the universe.
The best news that emerges from the Kepler findings is that the abundance of habitable planets puts the nearest one (to Earth) at about 12 light-years away. This makes such planets superb targets for the James Webb Space Telescope and for future optical-ultraviolet telescopes to search for biosignatures—signs of life—in their atmospheres. Still, intelligent civilizations may be rare, which would make the average distance among such civilizations in the Milky Way quite large. What might we expect the characteristics of extrasolar life to be?
Astrobiology is a rapidly evolving, interdisciplinary field of research that concerns the origin, frequency, and evolution of life in the universe. Given, however, that so far we only know of one example of life—the one on Earth—astrobiology generally proceeds with the assumption that, in terms of its basic requirements, extraterrestrial life should resemble the terrestrial template.
Ingredients that appear to have been crucial for life on Earth to emerge were: a certain level of environmental stability (e.g., not too many impacts by asteroids); the presence of liquid water; temperatures and levels of radiation that are not too extreme; a reliable energy source (the Sun); and the availability of certain elements such as oxygen, carbon, and phosphorus. It is not unreasonable to assume as a first guess that many or maybe even all of these ingredients are essential for life anywhere (after all, carbon, for instance, is quite unique in its ability to form complex molecules), but until we find alien life we wouldn’t know for sure which of these are absolutely necessary.
All life on Earth, for instance, relies on DNA (Figure 3) and RNA for replication, the issuing of instructions at the molecular level, and heredity. Does that mean that our Earthly DNA is universal throughout the cosmos? That’s actually hard to believe, since studies show that even our DNA could continue to function after the insertion of laboratory-created bases into its molecular structure. Similarly, different genetic codes can be used to create amino acids, which are the building blocks of proteins. Furthermore, one of the pillars of Darwin’s theory of evolution by means of natural selection is the concept of a common ancestor—in his words, “all the organic beings which have ever lived on Earth have descended from some one primordial form.” This means that the fact that all life forms on Earth use the same DNA is not that surprising, and it certainly does not necessarily imply that this is the only way for life to evolve. The biochemistry involved in the emergence of life may not be unique either. While we normally discuss processes based on carbon and oxygen, some researchers have suggested sulfur and iron as potential alternatives (e.g., in the environments of ocean floor hydrothermal vents; Figure 4). The existence of extremophiles—life forms that survive and even multiply in conditions that to us appear extreme (such as very cold or very hot temperatures; high degree of salinity)—also suggests that life can surprise us. The bottom line is simple. With only one known form of life, our conception of what it takes for life to emerge is necessarily biased, and no definitive conclusions can be reached. The lesson is also clear: seek, and ye shall find.
What are the near-future steps that astrobiologists are taking to discover biosignatures in extrasolar planets?While it is hard to believe that extrasolar life does not exist in our Milky Way galaxy, even the nearest life-harboring planet may be tens of light-years away. This means that our best shot at detecting such life is through remote observations by large telescopes. In particular, future telescopes will be examining exoplanet atmospheres for biosignatures—characteristics that are produced (ideally uniquely) by life processes.
You may wonder what another civilization might regard as relatively reliable biosignatures of Earth, were this civilization to observe Earth from a distance of tens of light-years. One of the telltales could be the relatively high abundance of oxygen (about 21% by volume) and the presence of ozone (a byproduct of oxygen; composed of three oxygen atoms). While small amounts of oxygen were initially released into the Earth’s atmosphere through the dissociation of water by the Sun’s radiation, the vast majority was contributed photosynthetically by plants and bacteria. The ozone layer probably played a crucial role in blocking ultraviolet radiation, thereby allowing more complex molecules to form. Water vapor in the Earth’s atmosphere (and the associated inference of liquid water on the surface) would have been another positive indicator for the potential existence of life on Earth for a remote observer. Most importantly, however, the most telling biosignature for life is an atmosphere that is out of thermochemical equilibrium. In other words, astrobiologists observing exoplanet atmospheres would be looking for gases whose abundances are absolutely discrepant when considering expectations from equilibrium chemical processes alone.
Two of the most promising telescopes for this type of quest for the near future are the Transiting Exoplanet Survey Satellite (TESS; Figure 5), scheduled for launch in 2017, and the James Webb Space Telescope (JWST; Figure 6), scheduled for launch in 2018. While TESS will not be able to detect Earth-size planets around Sun-like stars, it will most probably find at least a few Earth-size planets orbiting (and transiting) smaller M-dwarf stars, in the Habitable Zone around those stars. JWST will be able to study in detail the composition of the atmospheres of those candidates for life-bearing planets.
However, even the powerful pairing of TESS and JWST working in tandem (TESS leading to detections and JWST to atmospheric follow-up characterizations) will be unlikely to find biosignatures. Nevertheless, the probability of finding life is not zero, either. In particular, if given the right conditions life always emerges, then JWST could perhaps find life on suitable TESS candidates.
To do the life-finding search properly will ultimately require a large optical telescope in space, such as the proposed Advanced Technology Large-Aperture Space Telescope (ATLAST; Figure 7). Such a telescope would take us to the next step—the characterization of up to 50 or more habitable zone exoplanets with the goal of finding an Earth analog with life on its surface. What a huge discovery that would be!