If aliens do exist, one of the chief concerns is that they could be far more advanced than us — and a lot less friendly.
Scientists have tended to pour cold water on this suggestion because they say it is a lot more likely that extraterrestrial life will be primitive and more akin to bacterial microbes.
But what if there was a world that not only had liquid water and oxygen, but also land masses which emerged five billion years before the continents on Earth?
With such a lengthy head start, one astronomer thinks it 'could potentially host life more evolved than us', meaning such a civilisation probably would have the capacity to wipe us out if it wanted to.
Cardiff University's Professor Jane Greaves has identified at least two such worlds that might exist in our own Milky Way galaxy, hypothetically orbiting stars 70 and 110 light-years away.
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