What do we mean by Earth 2.0?

Over the past few years we’ve started to make some truly remarkable discoveries. Thanks to the Kepler Space Telescope we’re starting to discover the first Earth-sized planets orbiting other stars in the galaxy. We must tread forward carefully though, calling some of these planets Earth’s twin is dangerous, and we’re at risk of not realising the importance when we find the real thing.

The problems started last year with the announcement of the first Earth-like planet, Gliese 581g. The unfortunate thing is is that this planet probably doesn’t exist. More recently Kepler has discovered Kepler-22b, a super-Earth orbiting a slightly smaller star than our own in the habitable zone, the area where liquid water can exist. We don’t yet know though, given this planets larger size, whether it’s rocky or gaseous like Neptune. Yet the media hype this up and only make a small reference to the uncertainties. Just the other day NASA announced the discovery of the Kepler-20 system. Two of these planets are the same size as the Earth, does this qualify them to be our twin?

Kepler-22b – Rocky or Gaseous? We don’t know yet!

Venus is about the same size as the Earth but yet most scientists would avoid calling it our twin. Firstly there’s no water, then there’s the crushing atmosphere, runaway greenhouse effect creating temperatures of 460°C, it rains sulphuric acid. Twin? I think not.

When we do find a real Earth-twin though what do we really mean? A planet the same size as ours in the habitable zone? No, I think we need to go further, much further. An Earth-twin should also have a similar mass to ours. Too much mass and plate tectonics may not operate on the hypothetical world, meaning the surface doesn’t get recycled and there probably won’t be any life. We need to analyse the atmosphere. The James Webb Space Telescope will be able to provide this when she launches, hopefully in 2018. The gases in the atmosphere should be in a similar abundance to ours. We don’t want another Venus! Does it have a Moon? The Moon stabilises the Earth’s axis so it doesn’t wildly fluctuate that would cause catastrophic climatic changes (new evidence suggests a moon may not be necessary however). Only when these criterion have been met can we even start to think of announcing the discovery of Earths-twin.

Space agencies really should define what we really mean by an ‘Earth-twin’. We must be careful with upcoming reports of new planets, there are going to be hundreds more from Kepler over the next few years. We don’t want to become normalised to these discoveries because we’ll miss the significance when we do find a truly habitable Earth 2.0. That day will be one of the greatest days for science in history, let’s make sure we realise it.


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