Are we being watched? Aliens on at least nine planets beyond solar system could see Earth

Launched in 2009, the Kepler space telescope is our first mission capable of identifying Earth-size planets around other stars. Source: NASA's Ames Research Center

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.

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", the MailOnline reported.

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.

"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."

Scientists found that smaller rocky planets including Earth would be far more easy to spot than the gas and ice giants Jupiter, Saturn, Uranus and Neptune. Photo / Getty

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.

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.

IS THE UNIVERSE TOO HOT FOR ALIENS TO MAKE CONTACT?

In June research suggested that intelligent aliens haven't made contact yet because they are hibernating until conditions in the universe become more habitable.

Living life as a machine would have many advantages, including being able to process information faster and being less vulnerable to disease and death, they argue.

But supercomputers capable of processing vast amounts of information are known to be more efficient at lower temperatures.

That's because a large amount of energy is required to cool down the processing activity of a super-fast machine.

Because of this, intelligent machine-based lifeforms have chosen to go into hibernation until the climatic of the universe starts to cool down, according to the researchers.

Scientists from the University of Oxford call this theory the 'aestivation hypothesis.'

"If a civilisation wants to maximise computation it appears rational to aestivate until the far future in order to exploit the low temperature environment," the scientists, led by Dr Anders Sandberg, said in the Journal of British Interplanetary Society.

"We hence suggest the 'aestivation hypothesis': the reason we are not observing manifestations of alien civilisations is that they are currently (mostly) inactive, patiently waiting for future cosmic eras."

The level of background radiation of the universe is currently three degrees Kelvin above zero, which could be intolerably hot for machine-based life, the scientists said.

As the universe expands and stars die out over the course of millions of years, it's likely that the temperature will drop back to absolute zero.