In the past few decades, the search for extra-solar planets has turned up a wealth of discoveries. Between the many direct and indirect methods used by exoplanet-hunters, thousands of gas giants, rocky planets and other bodies have been found orbiting distant stars. Aside from learning more about the Universe we inhabit, one of the main driving forces behind these efforts has been the desire to find evidence of Extra-Terrestrial Intelligence (ETI).
But suppose there are ETIs out there that are are also looking for signs of intelligence other than their own? How likely would they be to spot Earth? According to a new study by a team of astrophysicists from Queen's University Belfast and the Max Planck Institute for Solar System Research in Germany, Earth would be detectable (using existing technology) from several star systems in our galaxy.
This study, titled "Transit Visibility Zones of the Solar System Planet", was recently published in the Monthly Notices of the Royal Astronomical Society. Led by Robert Wells, a PhD student at the Astrophysics Research Center at Queen's University Belfast, the team considered whether or not Earth would be detectable from other star systems using the Transit Method.
Diagram of a planet (e.g. the Earth, blue) transiting in front of its host star (e.g. the Sun, yellow). 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. Credit: R. Wells.
This method consists of astronomers observing stars for periodic dips in brightness, which are attributed to planets passing (i.e. transiting) between them and the observer. For the sake of their study, Wells and his colleagues reversed the concept in order to determine if Earth would be visible to any species conducting observations from vantage points beyond our Solar System.
To answer this question, the team looked for parts of the sky from which one planet would be visible crossing the face of the Sun - aka. "transit zones". Interestingly enough, they determined that the terrestrial planets that are closer to the Sun (Mercury, Venus, Earth and Mars) would easier to detect than the gas and ice giants - i.e. Jupiter, Saturn, Uranus and Neptune.
While considerably larger, the gas/ice giants would be more difficult to detect using the transit method because of their long-period orbits. From Jupiter to Neptune, these planets take about 12 to 165 years to complete a single orbit! But more important than that is the fact that they orbit the Sun at much greater distances than the terrestrial planets. As Robert Wells indicated in a Royal Astronomical Society press statement:
"Larger planets would naturally block out more light as they pass in front of their star. 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."
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
Ultimately, what the team found was that at most, three planets could be observed from anywhere outside of the Solar System, and that not all combinations of these three planets was possible. For the most part, an observer would see only planet making a transit, and it would most likely be a rocky one. As Katja Poppenhaeger, a lecturer at the School of Mathematics and Physics at Queen's University Belfast and a co-author of the study, explained:
"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."
What's more, the team identified sixty-eight worlds where observers would be able to see one or more of the Solar planets making transits in front of the Sun. Nine of these planets are ideally situated to observe transits of the Earth, though none of them have been deemed to be habitable. These planets include HATS-11 b, 1RXS 1609 b, LKCA 15 b, WASP-68 b, WD 1145+017 b, and four planets in the WASP-47 system (b, c, d, e).
On top of that, they estimated (based on statistical analysis) that there could be as many as ten undiscovered and potentially habitable worlds in our galaxy which would be favorably located to detect Earth using our current level of technology. This last part is encouraging since, to date, not a single potentially habitable planet has been discovered where Earth could be seen making transits in front of the Sun.
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.
The team also indicated that further discoveries made by the Kepler and K2 missions will reveal additional exoplanets that have "a favorable geometric perspective to allow transit detections in the Solar System". In the future, Wells and his team plan to study these transit zones to search for exoplanets, which will hopefully reveal some that could also be habitable.
One of the defining characteristics in the Search for Extra-Terrestrial Intelligence (SETI) has been the act of guessing about what we don't know based on what we do. In this respect, scientists are forced to consider what extra-terrestrial civilizations would be capable of based on what humans are currently capable of. This is similar to how our search for potentially habitable planets is limited since we know of only one where life exists (i.e. Earth).
While it might seem a bit anthropocentric, it's actually in keeping with our current frame of reference. Assuming that intelligent species could be looking at Earth using the same methods we do is like looking for planets that orbit within their star's habitable zones, have atmospheres and liquid water on the surfaces.
In other words, it's the "low-hanging fruit" approach. But thanks to ongoing studies and new discoveries, our reach is slowly extending further!
Further Reading: RAS, MNRAS
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