Experts are still on the hunt for extra-terrestrial life but so far the search has not provided any positive results.
But scientists remain optimistic that we are not alone in the cosmos, and one ex-Nasa employee says that space boffins may be looking in the wrong place.
Speaking at the 2017 Division for Planetary Sciences, Alan Stern, who now works for the Southwest Research Institute in Texas, said that aliens are bound to exist, but that scientists should focus on water worlds.
When asked about why aliens have not been found, Mr Stern said: “Many possible explanations have been proffered.
“We suggest another — namely that the great majority of worlds with biology and civilisations are water ocean worlds.”
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ETs at the bottom of ocean worlds would be protected from the likes of deadly radiation from space, if their planet does not have a protective atmosphere like it does here on Earth, and exploding stars.
But, Mr Stern explains, ocean worlds are usually freezing cold, and these beings could be living beneath a thick crust of ice that makes it almost impossible to see or contact them.
He continued: “Water worlds are naturally cut off from communication by their interior nature below a thick roof of ice or rock and ice, therefore do not easily reveal themselves.”
However, earlier this year, Dr Fergus Simpson from the Institute of Cosmos Sciences at the University of Barcelona produced a set of mathematical equations called Bayesian probability to conclude that most worlds are more than 90 per cent covered in water.
From this, he claimed that underwater aliens would not be very advanced.
He said: “The main reason I’m sceptical about there being lots of intelligent underwater alien species is that I think it’s harder for aquatic creatures like dolphins to use tools or build a fire.”