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When a 32-metre radio dish north of the Arctic Circle in Norway began a series of transmissions last month, it marked the first attempt to directly signal our existence to aliens on a known Earth-like exoplanet.

The target was , the closest known potentially habitable planet visible from that site. It orbits Luyten鈥檚 star, a red dwarf 12.4 light years from Earth. GJ 273b is one of found in the past two decades, thanks to advances in astronomy.

The transmissions were sent from the European Incoherent Scatter Scientific Association site near Troms酶 in Norway. METI, the San Francisco-based research organisation dedicated to messaging extraterrestrial intelligence, which I head, contributed a mathematical and scientific tutorial to the broadcast, sent with samples of music from Spain鈥檚 S贸nar festival, which initiated the project.

Alien invasion

After the transmission was announced last week, criticisms followed. Some scientists warn against sending such intentional signals to other stars, for fear of provoking an alien invasion. But they overlook one critical point. Any extraterrestrials on GJ 273b capable of interstellar travel would also be advanced enough to know of our existence already.

For nearly a century, we have been transmitting evidence of our technology through radio and television signals leaking into space, which a civilisation only slightly more advanced than ours could readily detect. And for two and a half billion years before that, Earth鈥檚 atmosphere has been giving off signs of life, through the oxygen in our air, that could be detected from afar. Sending intentional signals does not increase the chances of being detected by any civilisation advanced enough to threaten us.

Even if there is a chance ET already knows we are here, there are still good reasons for transmitting. It lets us test one explanation for why we haven鈥檛 yet discovered aliens, even though more than a half-century has passed since SETI began by listening for their signals.

According to the , advanced civilisations may be much more widespread than we imagine, perhaps populating planets around nearby stars. But they are watching us, the hypothesis suggests, much as we watch animals in a zoo.

Life everywhere?

How then to get a response? Consider how we would feel about a similar situation here on Earth. What would happen if we went to a zoo and suddenly a zebra turned toward us, looked us in the eye, and started pounding out prime numbers with its hoof? That would establish a radically different relationship, one that we would surely try to respond to. This is the scenario we are testing by signalling Luyten鈥檚 star.

Are we likely to get a reply? Only if the galaxy is chock-full of intelligent life. Perhaps more likely, we will need to repeat this process with 100 stars, or 1000, or a million, before we detect a reply 鈥� if one ever comes.

And why try directed messaging again when earlier efforts have failed? METI鈥檚 message included several novel features. It was sent three times each day, over three successive days, giving astronomers on GJ 273b a chance to confirm our signal, assuming they follow protocols like those used by SETI scientists on Earth.

Past messages have been one-off affairs that would fail this follow-up check. By repeating our message, any recipients could also correct errors that will inevitably occur as the signal passes through the interstellar medium. But perhaps the most novel feature of METI鈥檚 tutorial is the only part that changes with each iteration: a 鈥渃osmic clock鈥� that marks the passage of time throughout the transmissions.

When we return to Norway for a second round of transmissions in April 2018, six months will have passed on our clock. The final time we will encode in our message, at the end of the third and final day鈥檚 transmission next year, is the date 25 years into the future 鈥� 聽21 June, 2043 鈥� when we will be listening for a response.

Humankind at the beginning of the 21st century is not especially good at thinking long term. If we can start to conceive of science as an inherently multigenerational enterprise, if we can commit now to listen to Luyten鈥檚 star a quarter-century hence and then follow through with observations, I will consider our project a success. Whether we receive a reply or not.

Read more: Aliens slumbering for billions of years are out there 鈥� really?, The exoplanet zoo 鈥� a whistle-stop tour, Where is Everybody? Fifty solutions to Fermi鈥檚 paradox, Stephen Hawking says Earth should not phone ET, Earth calling: A short history of radio messages to ET