Read more: 鈥�Instant Expert: Astrobiology鈥�
In 1990, NASA鈥檚 Galileo spacecraft flew by Earth en route to Jupiter. Carl Sagan and colleagues seized this opportunity to train its instruments on Earth to see whether they could detect the signs of life. This was a critical demonstration of how the presence of life on other planets might be revealed: for instance, by finding oxygen and methane in the atmosphere, and hints of surface pigmentation. Picking up reflected light from the parent star would hint at liquid water on the surface. The clincher might be narrow-band radio emissions, modulated to carry information 鈥� radio and TV broadcasts, in other words. Galileo was able to pick up all these signs. But while the NASA probe came within 960 kilometres of Earth, astrobiologists have to find ways to test for life hundreds of light years away.
Spotting the signs
Observations with NASA鈥檚 Kepler telescope indicate that at least 6 per cent of sun-like stars harbour small rocky worlds. In our galaxy alone that means there may be millions of Earth-sized planets in the habitable zone around their parent stars. Training our biggest telescopes on the nearest examples may be the key to finding life.
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In particular, astrobiologists will need to sniff for the chemical signs of a well-entrenched biosphere. In the 3.5 billion years or more that microbial life has been around on Earth it has completely changed the chemical composition of land, sea and atmosphere. Oxygen, carbon dioxide and nitrogen participate in global cycles mediated by life and intertwined with geophysical and climatic processes. But will it be possible to detect such cycles on other, distant planets?
Starlight shining through a planet鈥檚 atmosphere can reveal its structure and composition. Astronomers have started to exploit this phenomenon to perform rudimentary temperature and chemical analyses of giant and super-Earth planets as they transit their parent stars. This technique should work on even smaller worlds.
Our abilities to study distant worlds will improve significantly with the launch of the James Webb Space Telescope. This successor to Hubble will be able to probe planets around nearby lower-mass stars. By taking the temperature of these worlds and looking for signs of water, oxygen, ozone and even methane in their atmospheres we will begin to piece together their environment. We may even be able to spot signs of light reflecting from surface oceans. Over time we can monitor seasonal and other changes that might reveal the chemical 鈥渂reathing鈥� of a biosphere.
Large moons around giant exoplanets may also be prime targets. While Europa or Ganymede 鈥� two moons rich in water-ice 鈥� lie well outside the habitable zone, it is likely that there are exomoons comfortably inside the habitable zone of other stars. Ganymede and Titan are bigger than Mercury, and it seems plausible that Mars-sized or even Earth-sized moons could exist around giant exoplanets. With atmospheres and the possibility of geophysical processes boosted by strong gravitational forces these could also provide a habitable environment.
Life in the void
Could life exist, or even originate, off-world in the cold dust and gas of nebulae or the swirling material surrounding young stars? Certainly, the gas between stars seems to contain a rich mix of molecules. Since the first molecules in interstellar space were identified in 1937, more than 150 different compounds have been found there, and astrochemists expect to find many more. Back on Earth, lab experiments that are recreating the extreme conditions of interstellar space have generated astonishingly complex organic compounds as well as cell-like vesicle structures.
So could this mean that some of life鈥檚 ingredients come from outer space? Might we find signs of these chemical precursors, or even simple forms of life, in interplanetary or interstellar space? What鈥檚 certain is that in our solar system, meteorites and comets often carry a chemical smorgasbord that includes amino acids and other fragments of extensive carbon-based molecules. Carbon chemistry seems to pervade the universe: this same chemical richness is also seen in the thick planet-forming discs of gas and dust around newborn stars, where material experiences extreme temperatures and is pummelled by radiation. Under these conditions, opportunities abound for chemical reactions and for the production of carbon-based molecules.
Astronomical observations indicate that some young stars have cyanide and similar molecules in the material around them. Cyanide is a key ingredient in the production of more complex molecules that are involved in biochemistry, often called prebiotic compounds. Perhaps these molecules could act as a starter mix of organic substances, giving rocky planets the ingredients that could set life in motion.
Tools of the trade
New observatories such as the space-based Herschel telescope which was launched in 2009, and the Atacama Large Millimeter Array, due for completion in 2011, are tuned to probe radiation emitted and absorbed by distant clouds of dust and gas. By examining the rich chemistry around young stars we will be able to see how, when and where prebiotic carbon-based molecules form, and learn about what may have happened in our own solar system (see diagram).
Lab-based experiments are revealing the fundamentals of how molecules form in such environments. The chemical pathways that combine atoms like hydrogen and carbon to make simple molecules can involve dozens of steps and different routes. By measuring which pathways are most efficient, these experiments should help reveal how the universe is cooking up biomolecules.
For a complete picture, these measurements must be combined with data from direct sampling missions. For example, in 2014, the European Space Agency鈥檚 Rosetta probe will place a small lander on comet 67P/Churyumov-Gerasimenko to analyse its composition. This will help reveal the biochemical menu that existed in our own solar system when it was still young.
Read next article: 鈥�Astrobiology: Are we unique?鈥�
