Peter Shakes, of Wellington, asks :-

Has there been any computer modelling done that can predict the chances of a life-harbouring planet similar to ours, especially one that's been allowed to flourish without their planet's inhabitants being wiped out every 500 million years or so by asteroids?

Duncan Steel, of the Centre for Space Science Technology in Alexandra, responded.

First, let’s leave computers out of this! Any model would likely be an example of GIGO: garbage in, garbage out.

My reason for saying this is we do not know what the inputs should be. Scientists working on questions of the origin of life and the broader field of astrobiology simply do not know how life got started. Many of us have ideas, and expertise on different aspects of the problem, but no-one has a complete answer. Any ‘output’ from a model depends upon the underlying assumptions, which may or may not be valid.

People often ask questions such as: “What is the chance of there being extraterrestrial life?” The answer to that has to be: “We do not know because we lack the necessary information.” The follow-up question then is: “How could you know?” And the answer is that there are two broad possibilities. (a) We actually identify some form of extraterrestrial life, whether it be radio signals from a distant alien civilisation, or just microbes found thriving on another planet; or (b) We develop a complete understanding of how life got started here on Earth. In either case we would then have some information on which to base a scientific evaluation of whether life is rare or common in the cosmos.

Are there planets in our galaxy similar to Earth in terms of size and composition? This is a question we can now answer, with numerous planets orbiting others stars having been identified over the past 20 years. We do not know much about them yet, but some do seem to be Earth-like.

Turning to asteroid impacts (and other mechanisms whereby the evolution of life on Earth has been affected, such as huge volcanic eruptions), it is true that abrupt mass extinction events have occurred, although more often than you suggest. There is an idea that waves of comets may have prompted a cycle of sudden extinctions spaced by around 30 million years. Whilst many (or most) forms of life on or near Earth’s surface may have been wiped out, it does not seem feasible that the planet could be sterilised, because there are microbes living deep below ground, some several kilometres down. In fact it has been suggested that half the total biomass exists down there, essentially obtaining its energy and nutrients by eating rock.

Finally, asteroid impacts are not necessarily a bad thing from our perspective! The death of the dinosaurs 65 million years ago made possible the rise of the mammals, and eventually humans.

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