A Nasa scientist has theorised intelligent alien life may have already been on Earth and believes that current thinking about extraterrestrial life is far too narrow.

Silvano P. Colombano, who works in Nasa's Intelligent Systems Division, believes we may have missed a visit from an alien life form, claiming it could be very different from the traditional carbon based life humanity is used to, the Daily Mail reported.

"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 be produced by carbon based organisms like us," Colombano wrote in a paper first presented at a Decoding Alien Intelligence Workshop in March organised by the Search for Extraterrestiral Intelligence.

Alien life may have gone far beyond human technology, Colombano said, and may even have mastered interstellar travel.

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"We need to re-visit even our most cherished assumptions," he wrote.

"How might that change the above assumptions about interstellar travel?" he wrote.

"Our typical life-spans would no longer be a limitation (although even these could be dealt with multi-generational missions or suspended animation), and the size of the 'explorer' might be that of an extremely tiny super-intelligent entity."

"Considering further that technological development in our civilization started only about 10K years ago and has seen the rise of scientific methodologies only in the past 500 years, we can surmise that we might have a real problem in predicting technological evolution even for the next thousand years, let alone 6 Million times that amount!"

He also warned that radio waves, could be out of date.

He called for physicists to take part in "speculative physics", grounded in our most solid theories but "with some willingness to stretch possibilities as to the nature of space-time and energy" and to "consider the UFO phenomenon worthy of study".