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. From a report: 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. Thanks to facilities and missions such as SuperWASP and Kepler, we have now discovered thousands of planets orbiting stars other than our Sun, worlds known as 'exoplanets.' The vast majority of these are found when the planets cross in front of their host stars in what are known as 'transits,' which allow astronomers to see light from the host star dim slightly at regular intervals every time the planet passes between us and the distant star. 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.
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