Only certain parts of the universe would have a good vantage point to watch the planets in our solar system pass in front of the sun, called a transit. The lines in this image represent those vantage points, with the blue line showing the places from where one could spot Earth transiting the sun. 2MASS/A. Mellinger/R. Wells

Several planets in other solar systems are in an excellent position to view Earth crossing in front of the sun during its orbit, meaning if those exoplanets contain extraterrestrial intelligence, the aliens there could have discovered Earth in their skies much like we discover exoplanets in our own.

Telescope-wielding scientists here use the transit method to detect planets in faraway solar system — when a planet passes in front of its host star from the viewpoint of the observer, known as transiting the star, scientists can spot it because a portion of the star is obscured and some of its light is blocked. In this way, experts have found and catalogued thousands of planets outside our solar system, although the same method can be used to detect objects other than exoplanets.

A group of astronomers wondered whether planets in the greater universe could find Earth in this way. According to a study in the journal Monthly Notices of the Royal Astronomical Society, the team investigated in what portions of the sky one could spot planets within our solar system transiting the sun. The analysis showed that outside the solar system, other worlds would be able to observe a maximum of three of our eight planets.

Overall only 2.5 percent of all the vantage points would be in a good position to see even one of our planets. To observe the maximum three planets, the chances drop to 0.027 percent.

According to the researchers, there are 68 known exoplanets that would be able to see transits in our solar system, although that number may increase as astronomers continue discovering exoplanets. Of those 68 exoplanets, about nine “temperate, Earth-sized planets” in orbit around the coolest types of stars are located where they could view Earth transits.

They are not believed to be habitable but as organizations like NASA, with its alien-hunting Kepler spacecraft, search the sky for signs of exoplanets and extraterrestrial life, things may change.

“The team estimate that there should be approximately 10 (currently undiscovered) worlds which are favorably located to detect the Earth and are capable of sustaining life as we know it,” the Royal Astronomical Society said in a statement about the research. “To date however, no habitable planets have been discovered from which a civilization could detect the Earth with our current level of technology.”

Even though the planets in the outer solar system are much larger, their distance from the sun would make Jupiter, Saturn, Uranus and Neptune more difficult to spot than the terrestrial planets in the inner solar system: Mercury, Venus, our Earth and Mars.

“Larger planets would naturally block out more light as they pass in front of their star,” lead study author Robert Wells, from Queen’s University Belfast, said in the statement. “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.”