Aliens on at least nine planets beyond the solar system would be ideally placed to spot and spy on Earth, scientists say.
Their research has echoes of classic sci-fi novel War Of The Worlds in which HG Wells writes about “intellects vast cool and unsympathetic” watching the human race.
The scientists identified parts of the sky from where various planets could be seen passing in front of the sun as they undertake so called “transits”.
They calculated that the smaller rocky planets including Earth would be far more easy to spot than the gas and ice giants Jupiter, Saturn, Uranus and Neptune that are further from the sun.
Robert Wells, from Queen’s University Belfast, said: “Larger planets would naturally block out more light as they pass in front of their star.
However, the more important factor is actually how close the planet is to its parent star.
Since the terrestrial planets are much closer to the sun than the gas giants, they’ll be more likely to be seen in transit.
Of the thousands of known exoplanets, the team identified 68 worlds where observers would see one or more planets in our solar system transiting the sun.
Nine of these planets were “ideally” placed to observe transits of the Earth, although none are thought to be capable of hosting Earth-type life.