The belief that mankind is unique in our universe has not been proven otherwise. So what if one day we discover we as humans are not one of a kind, and not only is there maybe other life out there, but it is a world with life identical to our own. So identical in fact they are another us, another Earth, and living on this other Earth is another you.

Already in cinemas in the US and opening in the UK in December (Ireland in October) is the sci-fi/drama film Another Earth. This film looks at the life of a young student who after causing a terrible car accident resulting in the death of a mother and young child leads to four years imprisonment.  Trying to right her wrongs, an opportunity to visit Earth II presents itself, giving her the chance to see if her life on the other planet has taken the same twists and turns.

A Second Earth is an idea which has captured the imagination for many films. Another sci-fi movie based on this second Earth was Gerry Anderson’s Doppelganger released in 1969 (also known as Journey to the Far Side of the Sun). In this film Europe and NASA work together to send astronauts to a newly-discovered planet before rivals in the Eastern Bloc achieve the feat. Faced with spacecraft malfunctions after three weeks in orbit, the astronauts crash land on what they think is home but discover they have actually travelled to a second Earth. On this other Earth they realise that it is actually a mirror image of Earth so everything is the same but reversed, including books, their home and even their internal organs! (Editor’s note: Other science fictional treatments of this idea include the planet Mondas, original home of the Cybermen in the long-running BBC series Doctor Who and the dismal Gor novels of John Norman.)

The idea of another Earth is not an idea developed for the realms of science-fiction, instead dates back to the classic Greek philosophy put forward by Philolaus (c. 470 – c. 385 BC). Philolaus suggested that the Sun, Moon, Earth and planets moved around a central fire, with the Sun taking a year to rotate completely around the central fire.

Image of Antichthon orbits

Philolaus’s idea of our universe’s 10 bodies orbiting a central fire. (Image credit:


He also suggested the idea that a ‘Counter-Earth’ existed in a collinear orbit with the Earth, so it was always hidden from the Earth by the Sun.  Philolaus concluded since that up and down do not exist in space but there would be a tendency for objects to fall to the centre of the Universe, hence something must be in existence to maintain the balance and stop things falling off the Earth. As he believed that the planets were not very dense apart from the Earth, he surmised that another planet identical to our Earth must exist to act as a counter-weight.

Another phenomenon in the Pythagorean world was the concept of ten being the perfect number, which fitted nicely with the counter-earth.  The visible planets (Mercury, Venus, Mars, Jupiter and Saturn), the Earth, Sun and Moon and the stars as the fixed backdrop along which all the celestial bodies moved accounted to 9, therefore a tenth earth- like planet completed this sequence. This counter-Earth was considered to be of equal size and density to the Earth and moved at the same orbital speed, maintaining its invisibility to people here on Earth.

This concept of another Earth represents the classic Greek philosophy, this other planet called Antichthon was believed to have been placed between the heavens and Earth to prevent man from looking directly at God. With the theory of gravitation explaining why things do not fall off the Earth; the Counter-Earth remains mainly in the land of science fiction. However research does exist to confirm that our planet is unique.

Just imagine there was a twin of the Earth on exactly the opposite side of the Sun from us. At first glance it would seem impossible to disprove this idea. Such a hypothetical planet would be always concealed by the Sun from our gaze…or would it? Periodic wobbles in the Earth’s orbit over thousands of years would result in sightings of this twin particularly at times around dusk, dawn and during total solar eclipses. Also another Earth identical to our planet would start to gravitationally influence the other planets in our Solar System. Mathematical research has been conducted to determine the motions of the planets over 150 years, with the premise that any additional planet would not remain undetected in that time frame. Even using predictions assuming that Earth II may have little or zero mass, it would still not remain undetected all of the time making small movements over a long time period.  Also as many satellites have been sent to the Sun such as NASA’s Solar Terrestrial Probes program (STP), the most recent of which it the pair of STEREO probes (Solar Terrestrial Relations Observatory) launched in 2006, there is no mysterious planet hiding there.

Image of AnotherEarth

Another Earth. To visit or not visit? (Image Credit:


Despite being alone in the Solar system, the idea of another Earth identical to our own is something exciting. The idea of being able to have a second chance at life and as a place of renewal is all very romantic but in reality is the grass always greener? The movie Another Earth opens on 7 October in Ireland and 2 December in the UK and is sure to offer an interesting insight into one person’s journey of self-discovery.

(Article by Martina Redpath)

Image of_Martina Redpath

Martina Redpath, ESO (Image credit: Armagh Planetarium)


admin · October 10, 2011 at 10:47

This would be the “Many Worlds” interpretation of quantum mechanics? If it’s correct there are an infinite number of Earths “somewhere”. Some are to all intents identical to our world (maybe one radium nucleus decayed a milli-second later on a planet in M31), others are radically different from the history we know (Tom Selleck played Indiana Jones, 20 April is the Hitler Day bank holiday or a really smart dinosaur was the first person on the Moon in 1969).

Alas most physicists do not take it seriously but it’s inspired some great science fiction.

Paul Evans · October 9, 2011 at 16:42

Thanks for a thought provoking article Martina. Have you heard about the theory that there are many Earths all slightly different? This is the parallel universe theory in which for every event that can happen two duplicate universes are created, one in which the event happens and one in which it doesn’t, so there could be billions upon billions of Earths on that basis!


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