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Earth has been habitable for billions of years — simulations show it was ‘just luck’

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Earth has been habitable for billions of years — simulations show it was 'just luck' 2

It took evolution 3 or 4 billion years to produce Homo sapiens. If the climate had completely failed just once in that time then evolution would have come to a crashing halt and we would not be here now. So to understand how we came to exist on planet Earth, we’ll need to know how Earth managed to stay fit for life for billions of years.

This is not a trivial problem. Current global warming shows us that the climate can change considerably over the course of even a few centuries. Over geological timescales, it is even easier to change climate. Calculations show that there is the potential for Earth’s climate to deteriorate to temperatures below freezing or above boiling in just a few million years.

We also know that the Sun has become 30% more luminous since life first evolved. In theory, this should have caused the oceans to boil away by now, given that they were not generally frozen on the early Earth – this is known as the “faint young Sun paradox”. Yet, somehow, this habitability puzzle was solved.

Scientists have come up with two main theories. The first is that the Earth could possess something like a thermostat – a feedback mechanism (or mechanisms) that prevents the climate from ever wandering to fatal temperatures.

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The second is that, out of many planets, perhaps some just make it through by luck, and Earth is one of those. This second scenario is made more plausible by the discoveries in recent decades of many planets outside our solar system – so-called exoplanets. Astronomical observations of distant stars tell us that many have planets orbiting them and that some are of a size and density and orbital distance such that temperatures suitable for life are theoretically possible. It has been estimated that there are at least 2 billion such candidate planets in our galaxy alone.