Space boffs, UFO hunters and New Age spiritualists are convinced the Solar System's fabled ninth planet – dubbed "Planet X" – exists and it is just like Earth.
But we cannot see it as it is locked in exactly the same orbit as Earth – 180 degrees around, on the other side of the Sun.
The mind-blowing theory dates back to an ancient Greek wiseman called Philolaus – who called the planet "Counter Earth".
Scientists have struggled to disprove the seemingly incredible claim as believers say the Sun – which is more than 100 times the size of Earth – would always keep it hidden.
Amazingly, the US Air Force has funded research into the existence of Counter Earth.
A planet with the same mass as ours and the same distance from the Sun could share Earth's orbit, according to Counter Earth theory.
It would have the same orbit speed as Earth and a similar surface environment – with the same gravity, atmosphere and temperature.
This could make it habitable by beings similar to humans.
Some Counter Earth proponents suggest the planet is like Earth before the continents split.
The more religious claim it is an Eden or heavenly paradise "devoid of sin".
Others believe it is an alien homeland and have warned caution against angering our extraterrestrial near-neighbours.
The idea of another planet sharing Earth's orbit is so compelling that it has inspired a huge amount of sci-fi.
The Cybermen in Doctor Who come from a counter Earth called Mondas – which left the shared orbit when the Moon arrived from space, upsetting the gravitational field.
But scientists insist Counter Earth is just impossible fantasy.
(The Daily Star)
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