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Are We Being Watched? Tens of Other Worlds Could Spot the Earth (eurekalert.org)
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.
The aliens would see our earth at the same time we see their exoplanet.
By the time the light from the alien planet has reached our Earth to let us determine it's habitable, the light from our earth has reached them.
They are looking for worlds that could see us using our current technology. Its an interesting study, but 100 years ago we would have tried (and failed) with a very different technique (direct telescopic examination). 100 years from now we may be using a completely different technology (solar system sized interferometers?, Some new trick to drastically reduce stray light in an image? X-ray telescopes? I have no idea) )
We know nothing about alien technology (if it even exists). It may have followed similar paths to ours, but it might be wildly different.
Considering that we detect planets that transit the parent star, and that we've detected thousands of them, AND that such an occultation is visible ONLY when the Earth is precisely on the plane of that system's ecliptic ... the only viable conclusion must be that planets around other star systems are LITERALLY as "common as dirt". It's likely that there are planets around MOST stars.
TFA does not mention which exoplanets, but they will need to be relatively close to detect our transits. By "relatively close" we would be talking 10-20 light years - so us, not our ancestors (apart from the kids here).
Anyway they would not need to look for transits when we are blasting radar beams and TV broadcasts in all directions, the latter of which which might be shit but nevertheless has strong self-correlation, such as we ourselves look for in the SETI project.
Anyway they would not need to look for transits when we are blasting radar beams and TV broadcasts in all directions, the latter of which which might be shit but nevertheless has strong self-correlation, such as we ourselves look for in the SETI project.
With the distance and dispersion they'd still have to have an helluva big antenna pointing in our direction. So much so that AFAIK we're assuming they have better technology than we have, we couldn't pick up a TV broadcast from an alien world. If we point a radio telescope at them and "ping" them they'd get the message with current technology - assuming the antenna is pointing in the right direction, but that's quite different from an accidental pick-up of a signal intended for Earth.
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