What if ET has been phoning home for years, but nobody has answered?
The search for alien life in the universe often focuses on finding as many planets as possible that are similar to ours, capable of hosting life.
But in a paper published today, scientists argue that we should narrow our search down to those that would be able to see us too.
The search for alien life in the universe often focuses on finding as many planets as possible that are similar to ours, capable of hosting life. But in a paper published today, scientists argue we should narrow our search down to those that would be able to see us too in the so-called Earth's transit zone
The research, published in the journal Astrobiology, from Dr Rene Heller and Professor Ralph Pudritz discusses ways to ensure that aliens trying to detect life on Earth are successful.
In the search for alien life, astrobiologists focus most of their search efforts on planets and moons that are too far away to see directly with telescopes, so they study them by tracking their shadows as they pass in front of their own host stars.
'The approach we utilize is the so-called transit method of detecting planets as they traverse their host stars,' Pudritz told MailOnline.
'This is arguably the simplest and most direct method of actually discovering that a planet is present and what its size is.'
If a planet crosses in front of its parent star's disk, then the observed visual brightness of the star drops a small amount.
By measuring the dimming of starlight as a planet crosses the face of its star during orbit, scientists can collect a wealth of information, even without ever seeing those worlds directly.
'To prove that life is present on that planet is a much more difficult and involved task, one that is only now being planned by using the latest telescopes,' the pair continued.
'But once again, one starts with candidates that have been detected to be earth-like, for example rocky planets like ours, with an interesting atmosphere that you would get by doing these transit type of observations.'
To identify a potential target, first scientists would need to understand whether or not the planet could be capable of hosting life as we know it.
'You would analyse the light of a host star that has passed through the planet's atmosphere to see if it contained finger prints of molecules associated with life,' he added.
Heller and Pudritz said that we should be focusing our search on the Earth's transit zone - a thin slice of space that if aliens were living in, they'd be able to see the earth passing in front of the sun.
The Earth's transit zone (ETZ) offers around 100,000 potential targets, each potentially orbited by habitable planets and moons, the scientists say. Geometrical construction of ETZ shown. The yellow circle represents the sun, the blue circle is Earth, not to scale
The Earth's transit zone (ETZ) offers around 100,000 potential targets, each potentially orbited by habitable planets and moons, the scientists said - and that's just the number we can see with today's radio telescope technologies.
'The bottom line is that if you are capable of doing careful observations of stars, it would be difficult to miss the transit method,' continued the researchers.
'So it would be hard to miss our earth if you were an observer in the ETZ - within a few thousand light years from us.'
If any planets in the ETZ host intelligent observers, they could have identified Earth as a habitable or even as a living world long ago
'We obviously cannot predict what other possible extra-terrestrial observers are capable of in terms of detecting earth like planets with intelligent life,' Pudritz said.
'But this search procedure is based on the method that has proven itself as simple and effective and based on basic physical ideas, like blocking the light of the star, that really could not be missed.
Dr Heller added 'if any of these planets host intelligent observers, they could have identified earth as a habitable, even as a living world long ago and we could be receiving their broadcasts today.'
Several projects are already under way, both to send signals from earth and to search for signals that have been sent directly or have 'leaked' around obstacles, possibly travelling for thousands of years.
Heller, a post-doctoral fellow, worked with Pudritz, while at McMaster University in Canada. He is now working in a project at the Max Planck Institute, Germany, that will be heavily involved in a new space observatory dedicated to planet searches (Plato).
'We have also focused our paper at the search for extra-terrestrial intelligence (SETI) community that uses radio telescope searches,' Pudritz said.
'As theorists, we are interested to gauge whether or not surveys might be mounted to sample the large number of sources in the earth transit zone (ETZ) that we have proposed - of the order of 100,000,' Pruditz told MailOnline.
'A full survey would require a huge amount of observation time.'
Heller and Pudritz propose that the Breakthrough Listen Initiative, part of the most comprehensive search for extraterrestrial life ever conducted, can maximise its chances of success by concentrating its search on Earth's transit zone.
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