The astronomers could hardly believe what they were seeing. For months, China's giant Sky Eye telescopeThe world's largest radio telescope. It is extremely sensitive to low-frequency radio bands.  had searched for signs of life beyond our planet. Now, it had detected a suspicious signal from its exoplanetA planet that orbits a star outside of our solar system. The radio frequency detected was too precise to be made by nature alone.  target. 

But for some, the news was just too good to be true. Not long after the report was published in the Science and Technology Daily newspaper, it vanished. Sceptics around the world began to dismiss the reports as radio pollution from humans. 

Even Chinese astronomers poured cold water on the findings. "The possibility that the suspicious signal is some kind of radio interference is also very high, and it needs to be further confirmed or ruled out," declared Zhang Tong-jie, chief scientist of China ET Civilisation Research Group. "This may be a long process." 

For now, the search continues. Yet many are convinced that it is only a matter of time before aliens make their presence known. The proof, they say, is in the maths. 

In 2020, two scientists at Nottingham University, Christopher Conselice and Tom Westby, made a set of extraordinary calculations. They concluded that the most likely number of contactable civilisations in the universe is 36. 

The calculations are based on the assumption - called the AstrobiologicalAstrobiology is the study of life in the Universe, including how it originated and evolved, and whether aliens exist. Copernican Principle - that, if intelligent life on Earth is the product of the natural law of evolution, rather than something that appeared at random, then there is no reason why the same thing should not have happened on similar planets 

The question, if we want to contact those intelligent lives, is how far in the evolutionary process they have got, and how far away they are.

In 1961, the astronomer Frank DrakeAn American scientist who has pioneered the search for extraterrestrial intelligence.  argued that there were seven things we needed to know in order to count these civilisations, including the average number of stars formed in the galaxy each year and how long the aliens could be expected to send out signals for.

If the Astrobiological CopernicanNicolaus Copernicus (1473 - 1543) was a Polish astronomer who discovered that the Earth goes around the Sun. Because this contradicted the Church's teaching that the Earth was the centre of the Universe, his book on the subject was not published in full until the year of his death. Principle is added, it narrows down the possible answers considerably.

Conselice and Westby have worked out that, if life emerges between 4.5 and 5.5 billion years after a star is formed - as it did on Earth - then there are between 4 and 211 civilisations in the Milky WayThe galaxy that contains our solar system. It has the structure of a disc, but looks to us like a band because we are inside it. that could communicate with us, with 36 as the most likely total.

However, as the nearest ones could be 17,000 light years away, we will not be able to exchange signals with them for at least 6,000 years.

These conclusions could help answer a fundamental question. Are values simply a human invention, or would things like the difference between right and wrong exist even if we did not? Discovering aliens, the philosopher Tim Mulgan believes, would support the latter view.

Mulgan argues that, if we find one alien civilisation, there is reason to suppose that many more exist. But why have they not made contact? He suggests that they have chosen not to because they know better than we do what is best for the Universe

Yes: If there are many alien civilisations, it is highly unlikely that none can reach us. So, if all have decided to stay quiet, it is evidence of shared values - values that can exist without humans.

No: We should not assume that aliens have kept to themselves as a matter of principle. They may not be as intelligent as us or as advanced. Or they may have already destroyed their civilisations.

Or... We should not get ahead of ourselves. Most scientists agree: the suspicious signals were likely influenced by humans, not extraterrestrialsBeyond the Earth. The film ET the Extraterrestrial is about an alien who makes friends with a group of children.. We have no clear proof that alien life exists at all.