Earth 2.0 has been a popular term in the media for many years now, despite the fact that – to date – no planet beyond our solar system has yet been found that can be confidently described as a copy of our world (let alone a superior, more advanced version of Earth, as the postpositive 2.0 tends to imply). This has led to some prevalent misconceptions of the planets we have been finding recently. This handful of fascinating rocky worlds orbit their stars at just the right distances to have surface temperatures that have a good chance of being not-too-hot and not-too-cold for liquid water, which could mean oceans. An abundance of liquid water could mean life.
However, wet planets with average temperatures somewhere between 0ºC and 100ºC are not all going to be like Earth. Earth has an average global temperature of around 15ºC thanks to an insulating blanket of an atmosphere. Around two thirds of its surface is covered in oceans, with the remaining one third being land. Overall, a perfect environment for life on Earth as we know it today. Yet, to assume that all the other habitable worlds we will find will be like this is a little naive.
This is where the Great British weather analogy comes in. The British weather is both notoriously talked about and notoriously mild (most of the time). It is fairly consistent in its grey dampishness, remaining, if not necessarily comfortable, at least tolerably not-too-hot and not-too-cold. Perfectly habitable. Yet it is not the only example of a habitable climate on Earth. This is just one example of the many climate zones that exist on Earth.
If we were to extrapolate ideas about what environments and biology on Earth were like based purely on knowledge of the habitats, flora and fauna of the British Isles, we might be more-or-less right about some parts of northern Europe and a smattering of other places across the globe. However, we would be very wrong about life on the majority of the Earth’s surface. These comparatively extreme regions are still habitable and inhabited, just with slightly different rules and appearances.
The habitability of any given planet – its suitability for life as we know it – can be thought of as scale ranging from “very habitable” to “barely habitable”. There is no universally agreed scale for planetary habitability yet, because we are still working with only one example of a habitable planet: our own. However, given our tendency to measure things against what we are already familiar with, it it likely that any future such scale would place Earth as the mild not-perfect-but-not-terrible benchmark against which all other worlds could be compared. (It is worth noting here that this refers to Earth as it is today; in fact, our planet has jumped about the habitability scale over the course of its 4 and half billion year lifetime).
Therefore, this assumption, or expectation, that other habitable worlds will be just like Earth excludes other possible possibilities, and would be just as bad as assuming Earth in its entirety would resemble the green and pleasant lands of the British Isles. The wide variety of habitable environments that we are just beginning to uncover will no doubt – if inhabited – require some un-Earthly expressions of biology that would ultimately enrich our knowledge of the biological sciences, but would far from resemble a second Earth.