A group of scientists from Queen's University Belfast and the Max Planck Institute for Solar System Research in Germany have turned exoplanet-hunting on its head, in a study that instead looks at how an alien observer might be able to detect Earth using our own methods. They find that at least nine exoplanets are ideally placed to observe transits of Earth, in a new work published in the journal Monthly Notices of the Royal Astronomical Society.

In the new study, the authors reverse this concept and ask, "How would an alien observer see the Solar System?" They identified parts of the distant sky from where various planets in our Solar System could be seen to pass in front of the Sun -- so-called 'transit zones' -- concluding that the terrestrial planets (Mercury, Venus, Earth, and Mars) are actually much more likely to be spotted than the more distant 'Jovian' planets (Jupiter, Saturn, Uranus, and Neptune), despite their much larger size.

To look for worlds where civilisations would have the best chance of spotting our Solar System, the astronomers looked for parts of the sky from which more than one planet could be seen crossing the face of the Sun. They found that three planets at most could be observed from anywhere outside of the Solar System, and that not all combinations of three planets are possible.

Of the thousands of known exoplanets, the team identified sixty-eight worlds where observers would see one or more of the planets in our Solar System transit the Sun. Nine of these planets are ideally placed to observe transits of Earth, although none of the worlds are deemed to be habitable.

In addition, the team estimate that there should be approximately ten (currently undiscovered) worlds which are favourably located to detect the Earth and are capable of sustaining life as we know it. To date however, no habitable planets have been discovered from which a civilisation could detect the Earth with our current level of technology.

The ongoing K2 mission of NASA's Kepler spacecraft is to continue to hunt for exoplanets in different regions of the sky for a few months at a time. These regions are centred close to the plane of Earth's orbit, which means that there are many target stars located in the transit zones of the Solar System planets. The team's plans for future work include targeting these transit zones to search for exoplanets, hopefully finding some which could be habitable.