A Nasa scientist says Earth may have already been visited by extraterrestrials – but that humans may not have noticed.

In a research paper, Silvano P Colombano, a computer scientist at NASA Ames Research Centre and a professor, suggests that aliens may look vastly different than the expectations that humans have of them – such as being carbon-based organisms – which would allow them to remain undetected.

“I simply want to point out the fact that the intelligence we might find and that might choose to find us (if it hasn’t already) might not be at all produced by carbon-based organisms like us,” Colombano wrote in the paper.

According to Colombano, to advance in the search for extraterrestrial life, scientists must “revisit our most cherished assumptions” and consider the idea of different characteristics - as well as the possibility that interstellar travel is already feasible for extraterrestrials.

“The size of the ‘explorer’ might be that of an extremely tiny super-intelligent entity,” he hypothesised. “If we adopt a new set of assumptions about what forms of higher intelligence and technology we might find, some of those phenomena might fit specific hypotheses, and we could start some serious enquiry.”

Colombano also suggests that it is worth re-considering what civilisation may actually look like when scanning the universe and “considering further that technological development in our civilisation started only about 10,000 years ago and has seen the rise of scientific methodologies only in the past 500 years” when doing so - as humans may have a “real problem predicting technological evolution even for the next thousand years, let alone six million times that amount.”

Aliens could resemble jellyfish the size of a football field says government advisor Show all 3 1 /3 Aliens could resemble jellyfish the size of a football field says government advisor Aliens could resemble jellyfish the size of a football field says government advisor AN5579997Alien_05_large.jpg The alien jellyfish - which Dr Aderin-Pocock imagines could be the size of a football field and have an orange underbelly - would be generated from silicon as opposed to carbon, which is the basis of all life as we know it. PA Aliens could resemble jellyfish the size of a football field says government advisor aliens2-.jpg The aliens are the product of what evolution might create on a world such as Saturn's moon Titan. PA Aliens could resemble jellyfish the size of a football field says government advisor aliens3.jpg Dr Aderin-Pocock (pictured) who was describing her alien as part of Science Month on TV channel Eden, says much of her inspiration comes from the bizarre life-forms discovered deep beneath the ocean.

Additionally, Colombano suggests the reasons UFO phenomena may have gone unnoticed or overlooked in the search for extraterrestrial intelligence are because, in addition to assuming a low likelihood of extraterrestrial travel and a high likelihood of hoaxes, the subject is generally avoided by the scientific community.

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