Are we being watched? Tens of other worlds could spot the Earth

September 8, 2017, Royal Astronomical Society
Diagram of a planet (e.g. the Earth, blue) transiting in front of its host star (e.g. the Sun, yellow). Left: The lower black curve shows the brightness of the star noticeably dimming over the transit event, when the planet is blocking some of the light from the star. Right: How the transit zone of a Solar System planet is projected out from the Sun. The observer on the green exoplanet is situated in the transit zone and can therefore see transits of the Earth. Credit: R. Wells

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.

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 ' 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.

"Larger planets would naturally block out more light as they pass in front of their star", commented lead author Robert Wells, a PhD student at Queen's University Belfast. "However the more important factor is actually how close the planet is to its parent star – since the are much closer to the sun than the gas giants, they'll be more likely to be seen in transit."

Image showing where transits of our Solar System planets can be observed. Each line represents where one of the planets could be seen to transit, with the blue line representing Earth; an observer located here could detect us. Credit: 2MASS / A. Mellinger / R. Wells

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.

Katja Poppenhaeger, a co-author of the study, adds, "We estimate that a randomly positioned observer would have roughly a 1 in 40 chance of observing at least one planet. The probability of detecting at least two planets would be about ten times lower, and to detect three would be a further ten times smaller than this."

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 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 located in the transit zones of the solar . The team's plans for future work include targeting these transit zones to search for exoplanets, hopefully finding some which could be habitable.

Explore further: Finding a 'lost' planet, about the size of Neptune

More information: R. Wells et al. Transit Visibility Zones of the Solar System Planets, Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stx2077

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5 / 5 (6) Sep 08, 2017
Tens of planets could mean billions of sophonts observing us. How embarrassing that they could be seeing our wars, environmental destruction, reality tv and the comment section on this site's environment articles!
1 / 5 (4) Sep 08, 2017
Well TB, you succinctly stated a reasonable explanation of why all exo-intelligent beings would studiously avoid approaching or even signalling this cesspool.

I will continue to criticize the concept of 'habitable' planets used in these articles. The only way a planet could be accurately described as 'habitable' if it already has it's own, unique biosphere. Lifeforms evolved over billions of years to inhabit that singular world.

All other speculations are hollybollyhonglywooded imagineered assumptions. Wishful fantasies that will bite you on your assumption!
not rated yet Sep 09, 2017
Tens of planets could mean billions of sophonts observing us. How embarrassing that they could be seeing our wars, environmental destruction, reality tv and the comment section on this site's environment articles!

How do you know they don't have wars, are destroying planets and have trolls in their alien forums. Technologically advanced =/= culturally advanced.

OF course there might be no aliens at all either, at least within our galaxy.
not rated yet Sep 09, 2017
Tens of planets could mean billions of sophonts observing us. How embarrassing that they could be seeing our wars, environmental destruction, reality tv and the comment section on this site's environment articles!
Any species that began to eliminate attentive elements in it's environment with technology would have immediately begun to suffer the results of overpopulation, same as us. This means tribal war and environmental devastation.

And the problem wouldn't have abated until they got their growth rate under control, one way or another, same as us.

But re the article, consider our technology in only a few hundred years. Technosavvy species should be able to image our planet directly without transit, and know where we are and what we are doing.

Google earth so to speak.
3.5 / 5 (8) Sep 09, 2017
"Tens of planets could mean billions of sophonts observing us. How embarrassing that they could be seeing our wars, environmental destruction, reality tv and the comment section on this site's environment articles!"

How do you know they don't have wars, are destroying planets and have trolls in their alien forums. Technologically advanced =/= culturally advanced.

It would still be embarrassing, 'cause we should know better by now.
1 / 5 (5) Sep 09, 2017
This should make us feel better. S. Hawking seems to have gotten it wrong. He has warned that our radio transmissions could be the invitation for space aliens to invade. This he likened to the Colombian Exchange, natives -75 and Europeans +500. Being the advanced aliens can identify the spectral lines of biological life. Then if they haven't come yet it is unlikely they will come any time soon. Considering the light bubble starting at the Devonian is 408 million light-years wide and the galaxy is only 100,000 we should feel safe and maybe very alone. Unless Andromedians attack.

Maybe aliens have adopted the Star Trek policy of non-contact.

What an interesting effect might an alien artifact/monument denoting alien life have had on the development of civilization.
4.5 / 5 (8) Sep 10, 2017
You don't have to fall out of your tree yet ;) once the aliens read a comment from an ntigoracle sockpuppet aka Turdgent, they would realize this primitive thinking troll will not enhance their cause and abrubtly all interest would be concentrated far away from this solar system.
not rated yet Sep 10, 2017
Any species that began to eliminate attentive elements
I meant to say ATTRITIVE elements. Goddamit. I am having unprecedented spell check problems with this new S8.
not rated yet Sep 11, 2017
Doubt aliens will waste their breath on us. Look folks, once we send ships out even among the bodies of our birthright solar system, we will soon find that we CAN live out there among those minable asteroids, gas jetting comets, water bearing bodies whereever found, etc. On or in those places can be found shelter if we scoop out and seal caves, etc and develope bases all over the solar system. We are on hte cusp of going into space in a big way as our corporations and smaller players are getting into the act. We even have one company with a viable modular 'inflatable' that is actually a robust subunit of any structure we would want, such as a ring that is big enough to be spun to generate near earth gravity in shirt sleeve working conditions....and protect from radiation too. Not long before self sufficient colonies arise. No need for a planet if enough of them. So again, many aliens can NOT have NOT found this out too. What they want US for?
1 / 5 (1) Sep 11, 2017
Well O1, please do not take it personally but I do agree with your general presumption that aliens have no reasonable purpose to expend their resources trying to contact us.

I do however disagree with your assertion about Humanity colonizing the Solar System. I sincerely doubt that people, especially women, will be willing to pay the butcher's bill it will cost to achieve space habitation.

For instance, as one disregarded danger, heavy radiation. It don't just bounce off a hull. Cosmic Rays and Gamma Rays are deeply penetrating, even before they shatter into sprays of secondary radiation.

We'll go deep, hiding inside a solid asteroid...Yeah? You do know many of those metals are toxic? And the perfect mass to collect and accumulate random radiation events.

Same reason as once a nuclear reactor is decommissioned and shut down...You won't volunteer to live in the empty structure!

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