If there were aliens sitting on at least nine exoplanets in other solar systems, they would be ideally placed to spy on what we’re doing on Earth, a new study has found.
Researchers 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’.
In addition to this, the team estimate there should be approximately ten (currently undiscovered) worlds which are favourably located to detect the Earth and could sustain life as we know it.
Image shows 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 the transit of Earth. If an observer were located along this line they could detect us
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’.
These 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.
Of the thousands of known exoplanets, the team identified 68 worlds where observers would see one or more of the planets in our solar system using methods that are available on Earth.
Researchers concluded the terrestrial planets (Mercury, Venus, Earth, and Mars) are 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 terrestrial planets are much closer to the sun than the gas giants, they’ll be more likely to be seen in transit’, he said.
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.
‘We estimate that a randomly positioned observer would have roughly a one in 40 chance of observing at least one planet’, said Katja Poppenhaeger, a co-author of the study from the Max Planck Institute.
Diagram of a planet (for example the Earth pictured in blue) is shown transitioning in front of its host star (the sun, in yellow). The observer on the green exoplanet is situated in the transit zone and can therefore see transits of the Earth
‘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.’
Experts say they do not currently believe the exoplanets they are aware of to be habitable.
To date, no habitable planets have been discovered from which a civilisation could detect the Earth with our current level of technology.
Of the thousands of known exoplanets, the team identified 68 where observers would see one or more of the planets in our Solar System using methods available on Earth (stock image)
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.