Earth has proven unique in its ability to host life in the universe so far, leading us to question if were truly alone.

Maybe were not.

Scientists have calculated that there could be a minimum of 36 active, communicating intelligent civilisations in our Milky Way galaxy, according to a new study published in the The Astrophysical Journal.

In the video below: Is there intelligent life beyond Earth?

However, due to time and distance, we may never actually know if they exist or ever existed.

Previous calculations along these lines have been based on the Drake equation, which was written by astronomer and astrophysicist Frank Drake in 1961.

Drake developed an equation which in principle can be used to calculate how many Communicating Extra-Terrestrial Intelligent (CETI) civilizations there may be in the Galaxy, the authors wrote in their study.

However, many of its terms are unknowable and other methods must be used to calculate the likely number of communicating civilizations.

So scientists at the University of Nottingham developed their own approach.

The key difference between our calculation and previous ones based on the Drake equation is that we make very simple assumptions about how life developed, said study coauthor Christopher Conselice, a professor of astrophysics at the University of Nottingham, in an email to CNN.

One of them is that life forms in a scientific way that is if the right conditions are met then life will form. This avoids impossible to answer questions such as what fraction of planets in a habitable zone of a star will form life? and what fraction of life will evolve into intelligent life? as these are not answerable until we actually detect life, which we have not yet done.

They developed what they call the Astrobiological Copernican Principle to establish weak and strong limits on life in the galaxy.

These equations include the history of star formation in our galaxy and the ages of stars, the metal content of the stars and the likelihood of stars hosting Earth-like planets in their habitable zones where life could form.

The habitable zone is the right distance from a star, not too hot or too cold, where liquid water and life as we know it may be possible on the surface of a planet.

Of these factors, habitable zones are critical, but orbiting a quiet, stable star for billions of years may be the most critical, Conselice said.

The Astrobiological Copernican Strong limit is that life must form between 4.5 to 5.5 billion years, as on Earth, while the weak limit is that a planet takes at least 4 billion years to form life, but it can form anytime after that, the researchers said.

Based on their calculations using the Astrobiological Copernican Strong limit, they determined that there are likely 36 active and communicating intelligent civilisations across our galaxy.

This assumes that life forms the way it does on Earth which is our only understanding of it at the moment. It also assumes that the metal content of the stars hosting these planets are equal to that of our sun, which is rich in metals, Westby said.

The researchers believed the strong limit is the most likely because it still allows intelligent life to form within a billion years after it did on Earth, which seems like plenty of time, Conselice said.

Another assumption of these potential civilisations is that theyre making their presence known in some way via signals.

Currently, weve only been producing signals like radio transmissions from satellites and televisions for a short time. Our technological civilisation is about a hundred years old. So imagine about 36 others doing the same thing across the galaxy.

The researchers were surprised that the number was so small but not zero. That is fairly remarkable, Conselice said.

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