Why haven't we found alien life yet? It's a question that's proving to be one of humanity's most trying endeavours. Indeed for some, it seems to have become so difficult that there might actually be a reason behind it.
Earth 2.0 might exist, but the problem is it won't stay Earth-like for long enough.
While it's not disputed that simplistic organisms could well exist within our universe, the fact that we have yet to find complex, intelligent extraterrestrial life of any kind is proving to be something of a conundrum.
Two scientists in Australia believe they have a theory which could finally wipe the confused looks off our faces, although you're not going to like what they've got to say.
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Aditya Chopra and Charles H. Lineweaver, both academics who work at the Planetary Science Institute, The Australian National University believe that the reason we haven't found alien life yet is because, quite simply, there isn't any.
It's a new theory which the pair are calling the Gaian Bottleneck and it suggests that while the percentage of planets needed to create a habitable atmosphere are high, they simply don't remain that way for long enough.
The paper's abstract explains further saying, "In the Gaian bottleneck model, the maintenance of planetary habitability is a property more associated with an unusually rapid evolution of biological regulation of surface volatiles than with the luminosity and distance to the host star."
Kepler claims to have found over 4,706 exoplanets with 12 believed to be in the 'habitable' zone.
So what does that mean for aliens? Well it doesn't mean good news that's for sure as the paper goes on to predict:
"Such a Gaian bottleneck suggests that (i) extinction is the cosmic default for most life that has ever emerged on the surfaces of wet rocky planets in the Universe and (ii) rocky planets need to be inhabited to remain habitable."
So essentially the reason we haven't found any aliens yet is that quite simply while the percentage of life-sustaining environments could be high enough, they're not around long enough for them to evolve from the pools of primordial life.
Arguing the point of course is NASA's Kepler mission which has, in partnership with SETI, been looking for 'exoplanets' which could in theory support habitable life.
Kepler has so far proven to be a success, finding a grand total of 4,706 exoplanet candidates with 12 believed to be in the 'habitable zone'.
Most recently of course was Kepler's discovery of 'Earth 2.0' otherwise known as Kepler-452b.
It has a mass that's five times Earth's, with a gravity that's twice as strong. Don't panic though, you'll still be able to walk on the surface once you've 'lost a few pounds'.
John Jenkins, Kepler data analysis lead at NASA's Ames Research Center in Moffett Field, California, points out that while life would be tough at first, after a few generations, humans would actually adapt to conditions on the planet.
In terms of its geology Kepler-452b probably has an atmosphere of thick cloud and will likely be potholed with volcanoes.
Is There Alien Life On Kepler-452b?
What makes this planet so important is the amount of energy that it receives. Kepler-452b's size means that proportionately it receives the same amount of energy that Earth does needed to foster a habitable planet.
John Jenkins, Kepler data analysis lead at NASA's Ames Research Center in Moffett said:
"If you travel to this star with an arkful of plants, we'd expect based on our understanding of planetary system formations there would be a lot of raw materials for you to use, but more importantly for plants the sunshine from this star is very similar to the sunshine from our star, so the plants would photosynthesise just like on Earth."
So there's good news to be had, and while the Gaian Bottleneck is still only a theory, it does leave us with a rather important message which is that we should all consider ourselves extremely lucky,