The beginnings of life on other worlds
Ok, so the universe is utterly enormous and we have an understanding of how important stars are to life. But what is life? Well, we're not exactly sure what life is.
Although we have a good understanding of how living systems function, multiply and interact with each other and the environment, it is difficult to distinguish life from non-life. Life elsewhere in the universe might be based on the same carbon-chemistry as life on Earth, or it might be based on something completely different.
Here is one attempt at a definition (it's a work in progress).
Life: a growing, reproducing, responsive and adaptable set of complex, dynamic chemical reactions. Life is poised on the balancing point between order and chaos, requiring a regular energy source to persist and create order in a thermodynamically open system. If any of these conditions aren't met, we are not talking about life.
See, that's not complicated at all(!), and as we said, it's a work in progress. You can try to come up with your own definition of life whether your definition is able to classify life and non-life.
So if we can agree on what life is (it may turn out to be just a human categorisation) and what environmental conditions are required for life to begin, then we can have a pretty good idea of where in the universe to look for life.
We haven't really answered the question 'What is life?' though, have we?
Check out this video for some more questions we can ask to help us get closer to an answer:
The study of how life began is know as 'abiogenesis' - it is the study of the boundary between chemistry and biology.
Note: Abiogenesis is not the same as Evolution. The biological Theory of Evolution as proposed by Darwin and Wallace, explains only how life develops once there already is life. Evolution does not explain the origins of life itself, either here on Earth or elsewhere in the universe.
We know that life started some 3.8 billion years ago. Some of the oldest fossils in the world come from rocks around this age which can be found in Western Australia. As Australia is such an ancient landscape, a lot of astrobiologists come here to look for clues about life's origins on Earth.
The early Earth was a violent and inhospitable environment. There was no ozone to protect complex molecules like DNA and RNA from UV damage so it is unlikely that life started out exposed to the atmosphere. Early lifeforms had to be basic cells with simple metabolic processes for harnessing the energy available in thermal or chemical gradients.
Life probably began somewhere more sheltered where conditions were stable. One place life could have began is around 'black smokers' or hydrothermal vents at the bottom of the ocean, or inside warm rocks. This is not to say that life was easy - some of the microbes we know about on Earth today are still capable of growing under extreme conditions by human standards. These we call the extremophiles ('extreme-lovers').
There are many hypotheses that try to explain how life got started, however many of them don't really answer the question; they only push the question back a step: For example 'panspermia', which proposes that life was brought to Earth on rocks from other planets like Mars (for a science-fiction version of panspermia where advanced alien civilisations got involved, watch the movie Prometheus).
One of the most famous examples of an experiment designed to answer the question of how the building blocks of life could have possibly formed in Earth's early atmosphere is the Miller-Urey experiment. Some people have criticised the results of this experiment because it didn't accurately model the atmosphere and the level of electrical energy present in the form of lightning. However, these critics are missing the point - the experiment demonstrates the POSSIBILITY for complex organic (carbon) molecules to form from inorganic components! Consequently it has been known since the 1950's that there are paths available for non-living, inrganic compunds to arrange into complex, self-replicating living systems.
It seems that the most plausible way to find out how life began is to start with the things we know life needs (for example a way of separating its internal processes from the external environment i.e. a cell wall) and see if we can replicate those features with non-living, chemical components: in this case, oil-droplets with a 'mind' of their own...