Ever feel like somebody’s watching you? They could be, from millions, if not billions, of miles away.
While we’ve been exoplanet-obsessed in the ongoing search for habitable zones and possibly aliens, scientists from Queen’s University Belfast and Germany’s Max Planck Institute for Solar System Research have literally reversed their point of view to see how said hypothetical aliens might be peering at Earth, using methods that are eerily close to ours. Most exoplanets have given themselves away to telescopes like SuperWASP and Kepler when in transit of their host stars. When a planet passes between Earth and its star, astronomers are able to identify it when starlight dims at regular intervals with each transit.
The scientists temporarily put themselves in extraterrestrial brains as they figured out locations in distant space that would be ideal to identify transiting planets in our solar system. You would think that the gas and ice giants would be the most visible—but behemoth planets like Jupiter, Saturn, Uranus, and Neptune are actually harder to observe from a distance, because they block out more sunlight than terrestrial planets such as Mercury, Venus, Earth, and Mars. Not that aliens would want to end up anywhere as inhospitable as Neptune.
"Larger planets would naturally block out more light as they pass in front of their star," explained Robert Wells, who led the team that recently published a study in Monthly Notices of the Royal Astronomical Society. "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."
Exoplanets where (again, hypothetical) intelligent civilizations would best be able to spot our solar system exist in the parts of space where alien telescopes would be able to observe more than one planet in transit of the sun. There is approximately a 1 in 40 chance of eyeing one planet transiting, which plummets ten times when you factor in a second planet, and an additional ten when you add a third. The probability of that is likely still much higher than finding extraterrestrials who have gotten far enough technologically to build their own telescopes.
Of the thousands of exoplanets floating around, there are only 68 where someone or something could see a planet from our solar system in transit of the sun. The nine from which Earth transits could be observed aren’t even habitable. Ten of them may or may not have been able to spawn life as we know it, but even if something was creeping around over there, it wouldn’t be able to see Earth. We don’t know of any habitable planets from which a transiting Earth would be seen that also happen to be teeming with intelligent life. Yet.
Meanwhile, NASA’s Kepler keeps on scanning the sky…