Astrobiology (from Greek ἄστρον, astron, "constellation, star"; βίος, bios, "life"; and -λογία, -logia) is the interdisciplinary study of life in the Universe, combining aspects of astronomy, biology and geology. It is focused primarily on the study of the origin, distribution and evolution of life. Given the influx of new information about planetary systems around other stars, its mandate has expanded beyond the study of exobiology (from Greek ἔξω, exo, "outside").
Some major astrobiological research topics include: What is life? How did life arise on Earth? What do astrophysical observations tell us about the present and future of life on Earth? What kind of environments can life tolerate? Are microscopic forms of life common in deep space? How can we determine if life exists on other planets? How often can we expect to find complex life? What will life consist of on other planets? Will it be DNA/carbon based or based on something else? What will it look like?
Though once considered outside the mainstream of scientific inquiry, astrobiology has become a formalized field of study. NASA now hosts an Astrobiology Institute. Additionally, a growing number of universities in the United States (e.g., University of Arizona, Penn State University, and University of Washington), Canada, Britain, and Ireland now offer graduate degree programs in astrobiology.
A particular focus of current astrobiology research is the search for life on Mars. There is a growing body of evidence to suggest that Mars has previously had a considerable amount of water on its surface; water is considered to be an essential precursor to the development of life, although this has not been conclusively proven. At the present, the creation of theory to inform and support the exploratory search for life may be considered astrobiology's most concrete practical application.
Missions specifically designed to search for life include the Viking program and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive, and Beagle 2 failed to transmit from the surface and is assumed to have crashed. A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been cancelled. Currently, the Phoenix lander is probing the environment for suitability of microbial life on Mars, and to research the history of water there. In 2009, NASA plans to launch the Mars Science Laboratory Rover which will continue the search for past or present life on Mars using a suite of scientific instruments.
Missions to other planetary bodies, such as the Phoenix lander, Mars Science Laboratory, ExoMars, Beagle 2: Evolution to Mars, the Cassini probe to Saturn's moon Titan, and the "Ice Clipper" mission to Jupiter's moon Europa, hope to further explore the possibilities of life on other planets in our solar system.
Efforts to answer secondary questions, such as the abundance of potentially habitable planets in habitable zones and chemical precursors, have had much success. Numerous extrasolar planets have been detected using the wobble method and transit method, showing that planets around other stars are more numerous than previously postulated. The first Earth-like extrasolar planet to be discovered within its star's habitable zone is Gliese 581 c, which was found using radial velocity.
Most of the planets so far discovered have been hot gas giants, thought to be inhospitable to any life. It is possible that some of these planets -like the gas giant Jupiter in our solar system- may have moons with solid surfaces or oceans that are more hospitable. It is not yet known whether our solar system, with a warm, rocky, metal-rich inner planet such as Earth, is of an aberrant composition. Improved detection methods and increased observing time will undoubtedly discover more planetary systems, and possibly some more like ours. For example, NASA's Kepler Mission seeks to discover Earth-sized planets around other stars by measuring minute changes in the star's light curve as the planet passes between the star and the spacecraft. Research into the environmental limits of life and the workings of extreme ecosystems is also ongoing, enabling researchers to predict what planetary environments might be most likely to harbor life.
Progress in infrared astronomy and submillimeter astronomy has revealed the constituents of other star systems. Infrared searches have detected belts of dust and asteroids around distant stars, underpinning the formation of planets. Some infrared images purportedly contain direct images of planets, though this is disputed. Infrared and submillimeter spectroscopy has identified a growing number of chemicals around stars which underpin the origin or maintenance of life.
In the book Rare Earth: Why Complex Life is Uncommon in the Universe, Peter Ward, a geologist and paleontologist, and Donald Brownlee, an extraterrestrial materials pioneer and astrobiologist, propose that life as we know it, is rare in the Universe. They suggest that microbial life, however, is probably common in the Universe, because of recently discovered extremophile bacteria. The book argues that the chances of all the conditions that occurred to create the Earth occurring again, would be rare; thus intelligent life would be rare. One important factor focused on in the book is planetary habitability (see section below).
Peter Ward, one of the authors, said the following:
When looking for life on other planets, some simplifying assumptions are useful to reduce the size of the task of astrobiologists. One is to assume that the vast majority of life forms in our galaxy are based on carbon chemistries, as are all life forms on Earth. While it is possible that non-carbon-based life exists, carbon is well known for the unusually wide variety of molecules that can be formed around it. However, it should be noted that astrobiology concerns itself with an interpretation of existing scientific data; that is, given more detailed and reliable data from other parts of the Universe (perhaps obtainable only by physical space exploration), the roots of astrobiology itself —biology, physics, chemistry— may have their theoretical bases challenged. Much speculation is entertained in the field to give context, but astrobiology concerns itself primarily with hypotheses that fit firmly into existing theories.
The presence of liquid water is also a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life. Some researchers posit environments of ammonia, or more likely water-ammonia mixtures. These environments are considered suitable for carbon or noncarbon life, while opening more temperature ranges (and thus worlds) for life.
A third assumption is to focus on sun-like stars. This comes from the idea of planetary habitability. Very big stars have relatively short lifetimes, meaning that life would not likely have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star. Without a thick atmosphere, one side of the planet would be perpetually baked and the other perpetually frozen. In 2005, the question was brought back to the attention of the scientific community, as the long lifetimes of red dwarfs could allow some biology on planets with thick atmospheres. This is significant, as red dwarfs are extremely common. (See Habitability of red dwarf systems).
About 10% of the stars in our galaxy are sun-like, and there are about a thousand such stars within 100 light-years of our Sun. These stars would be useful primary targets for interstellar listening. Since Earth is the only planet known to harbour life, there is no way to know if any of the simplifying assumptions are correct.
Most astronomy-related astrobiological research falls into the category of extrasolar planet (exoplanet) detection, the hypothesis being that if life arose on Earth, then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect Earth-like exoplanets are under development, most notably NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin programs. Additionally, NASA plans to launch the Kepler mission in 2008, and the French Space Agency has already launched the COROT space mission. There are also several less ambitious ground-based efforts underway. (See exoplanet).
The goal of these missions is not only to detect Earth-sized planets, but also to directly detect light from the planet so that it may be studied spectroscopically. By examining planetary spectra, it would be possible to determine the basic composition of an extrasolar planet's atmosphere and/or surface; given this knowledge, it may be possible to assess the likelihood of life being found on that planet. A NASA research group, the Virtual Planet Laboratory (VPL), is using computer modelling to generate a wide variety of virtual planets to see what they would look like if viewed by TPF or Darwin. It is hoped that once these missions come online, their spectra can be cross-checked with these virtual planetary spectra for features that might indicate the presence of life. The photometry temporal variability of extrasolar planets may also provide clues to their surface and atmospheric properties. One mission was planned to Jupiter's moon, Europa, before recent cuts by NASA; this mission would have searched for life in the ocean of this moon.
An estimate for the number of planets with intelligent extraterrestrial life can be gleaned from the Drake equation, essentially an equation expressing the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise:
Another active research area in astrobiology is solar system formation. It has been suggested that the peculiarities of our solar system (for example, the contestable presence of Jupiter as a protective shield or the planetary collision which created the moon) may have greatly increased the probability of intelligent life arising on our planet. No firm conclusions have been reached so far.
Extremophiles (organisms able to survive in extreme environments) are a core research element for astrobiologists. Such organisms include biota able to survive kilometers below the ocean's surface near hydrothermal vents and microbes that thrive in highly acidic environments. Characterization of these organisms—their environments and their evolutionary pathways—is considered a crucial component to understanding how life might evolve elsewhere in the Universe. Recently, a number of astrobiologists have teamed up with marine biologists and geologists to search for extremophiles and other organisms living around hydrothermal vents on the floors of our own oceans. Scientists hope to use their findings to help them create hypotheses on whether life could potentially exist on certain moons in our own solar system, such as Europa.
The origin of life, as distinct from the evolution of life, is another ongoing field of research. Oparin and Haldane postulated that the conditions on the early Earth were conducive to the formation of organic compounds from inorganic precursors and thus to the formation of many of the chemicals common to all forms of life we see today. The study of this process, known as prebiotic chemistry, has made some progress, but it is still unclear whether or not life could have formed in such a manner on Earth. The alternative theory of panspermia is that the first elements of life may have formed on another planet with even more favourable conditions (or even in interstellar space, asteroids, etc.) and then have been carried over to Earth by a variety of means.
The fossil record provides the oldest known evidence for life on Earth. By examining this evidence, geologists are able to understand better the types of organisms that arose on the early Earth. Some regions on Earth, such as the Pilbara in Western Australia and the McMurdo Dry Valleys of Antarctica, are also considered to be geological analogs to regions of Mars and as such might be able to provide clues to possible Martian life.
The three most likely candidates for life in the solar system (besides Earth) are the planet Mars, the Jovian moon Europa, and Saturn's moon Titan. This speculation is primarily based on the fact that (in the cases of Mars and Europa) the planetary bodies may have liquid water, a molecule essential for life as we know it for its use as a solvent in cells. Water on Mars is found in its polar ice caps, and newly carved gullies recently observed on Mars suggest that liquid water may exist, at least transiently, on the planet's surface, and possibly in subsurface environments such as hydrothermal springs as well. At the Martian low temperatures and low pressure, such liquid water is likely to be highly saline. As for Europa, liquid water likely exists beneath the moon's icy outer crust. This water may be warmed to a liquid state by volcanic vents on the ocean floor (an especially intriguing theory considering the various types of extremophiles that live near Earth's volcanic vents), but the primary source of heat is probably tidal heating.
Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan. Titan has been described as having conditions similar to those of early Earth; according to bbc.co.uk, "The atmosphere on Titan could be identical to that of the early Earth when life began". On Titan, scientists have discovered the first liquid lakes outside of Earth, but they are made of ethane and methane, not water. On March 20, 2008, it was reported that Titan may have an underground ocean of water and ammonia after Cassini data was studied. Additionally, Saturn's moon Enceladus may have an ocean below its icy surface.
Because astrobiology relies mostly on scientific extrapolations over solid, factual evidence, the authenticity of astrobiology as a science can be questioned. While other branches of science remain heavily hypothetical, there is a greater degree of mathematical, pragmatic and/or observational evidence supporting the theories. Neither clear observational evidence nor any definite formal framework exists for astrobiology, save for an asteroid segment which is believed to have fossilized Martian microbes. Although some have thought a formal degree program in astrobiology unlikely, the University of Glamorgan, UK, started just such a degree in 2006.
Characterization of non-Earth life is extraordinarily unsettled; hypotheses and predictions as to its existence and origin vary wildly; true astrobiological experiments (with modest exceptions such as the study of the ALH84001 meteorite and searches for indications of life in Earthshine) simply cannot occur at present. Finally, astrobiology has been criticized for being unimaginative in the tacit assumption that Earth-like life presents the most likely template for life elsewhere. For example, Michael Crow, the president of Arizona State University, said the following:
Biologist Jack Cohen and mathematician Ian Stewart, amongst others, consider xenobiology separate from astrobiology for this reason. Cohen and Stewart stipulate that astrobiology is the search for Earth-like life outside of our solar system and say that xenobiologists are concerned with the possibilities open to us once we consider that life need not be carbon-based or oxygen-breathing, so long as it has the defining characteristics of life. (See carbon chauvinism).
As with all space exploration, there is the classic argument that there is still a lot more scientists have to learn about Earth. Critics of astrobiology may prefer that public funding remain dedicated towards searching for unknown species in our own terrestrial biosphere. They feel that Earth is the most plausible and practical region to search for and study life.