Once upon a time, in a galaxy far, far away, an alien race may have been watching and listening to humans on Earth, a study has claimed.

The findings state there are 29 planets that are “potentially habitable” – and therefore may harbour life – and are close enough to our planet to have received human radio waves.

The researchers were unable to prove that aliens did spy on humans, or indeed if aliens exist at all, but they sought to find out whether it was possible that we were being watched by beings on another planet.

Astronomers from Cornell University, New York, set out some simple criteria to do so.

First, the distant planets must be deemed “potentially habitable”; second, they must be within range.

To meet the first aspect, academics trawled through the potentially habitable planet database created by Gaia, the two-telescope mission run by the European Space Agency.

Researchers looked for planets that had been observed passing in front of their star, and then assumed that about a quarter of these are rocky worlds.

It is believed that it is essential for a planet to be rocky for life to exist, and a gaseous planet could not sustain life.

Next, the results were narrowed down to determine how close the planets were to Earth and if this was near enough to detect human activity.

Human beings have existed for more than 300,000 years, but only formed large, complex civilisations about 5,000 years ago, shortly before the advent of the Mesoptamian and the Egyptian empire.

As a result, alien astronomers would need to be within 5,000 light years of the Earth to have a realistic chance of seeing human activity en masse because light from Earth, which travels at the speed of light, needs to reach them.

Therefore, someone who is 5,000 light years away would see Earth as it was in 3,000BC.

A stargazer 1,000 light years away would see it as it was during the Battle of Hastings, which took place in 1066 between English and Norman forces.

The researchers found 1,715 stars within this range but say that the radius will continue to grow, and in another 5,000 years it will include a further 319 stars.

A prime candidate of a potentially habitable planet that is close enough to Earth for its inhabitants to have seen humans at work is called Ross-128b, which orbits the star Ross-128.

The researchers said that this planet would have been able to see Earth against a backdrop of the Sun between 3,057 and 900 years ago.

“We assume that any nominal civilisation on an exoplanet would have astronomical instrumentation comparable to what we have now,” the authors write in their study, published yesterday in the journal Nature. Of the 1,715 stars within the 5,000-year range, only 75 were found to be within 100 light years of our home world.

This makes these planets close enough to pick up radio waves from Earth.

Radio waves were first produced in the 1880s and they were commonplace after the First World War. These waves also travel at the speed of light, and therefore a planet 55 light years away would hear radio waves from 1966.

Of the 75 stars in this 100-year range, seven are known to have rocky planets orbiting them. In these seven star systems, there are 29 planets that could host life.

The researchers added that they expected a further 42 exoplanets would move to within this range within the next 5,000 years.