A plan to beam powerful radio messages to possible alien civilisations on distant planets has caused consternation among some scientists who believe it could spell doom for humanity on Earth.

Leading figures behind the 50-year-old Search for Extraterrestrial Intelligence (Seti), which uses radio telescopes to listen passively for unnatural signals from space, have now proposed an “active” form of search known as Meti – Messages to Extraterrestrial Intelligence. These signals will be aimed at the parts of the galaxy where Earth-like planets and life may exist, based on the growing number of “exoplanets” beyond our solar system identified by the Kepler space telescope, which include many in the so-called “habitable zone” of a star.

However, critics say the proposal betrays the founding principles of Seti, which were about listening rather than transmitting. They fear that sending signals of our existence could lead to visits from malign extraterrestrials intent on doing harm to humans. They also argue that the plans are being formulated by a close-knit group of Seti enthusiasts who have not consulted the wider public and who are personally frustrated that half a century of trying passively to detect signals of extraterrestrial intelligence has failed to bear fruit.

“A small cadre of Seti radio astronomers has resisted the notion of international consultation before humanity takes the brash and irreversible step into Meti, shouting our presence into the cosmos,” said David Brin, a space scientist and author. “That’s all very well if the only one you’re putting at risk is yourself. But when that risk is imposed upon our children and all of humanity on the planet, is it too much to ask that we discuss it first?”

He is not alone in his concerns. Professor Stephen Hawking, the Cambridge cosmologist, warned in 2010 that humans should keep as silent as possible because alien civilisations may be attracted to Earth and have the technology to travel here and exploit its resources. “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans,” he said.

Today, at the annual meeting of the American Association for the Advancement of Science, Dr Brin will share a platform with Seth Shostak and Douglas Vokoch, of the Seti Institute in California, who are leading proponents of beaming messages to alien civilisations with technology sophisticated enough to detect and decipher the signals.

Dr Vokoch said: “With recent detection of Earth-like planets in the habitable zones of other stars, we have natural targets for such transmission projects. Some would argue that we should avoid powerful transmissions at all costs for fear of an alien invasion. If this mindset became entrenched, it would signal a guarded vision for humankind as isolationist, avoiding exploration, trying to minimise risk at any cost.”

The vast distances of space mean that it will take about nine years to send and receive a message from an alien civilisation even if one exists in the closest star system, Alpha Centauri, which is 4.4 light years away.

Out of this world: What we might say

The most popular choices for messages to be broadcast to aliens, as selected in a Seti Institute survey in 2009:

“We are humans on the planet Earth.”

“You are alien to us, but you have know-how.”

“Hello and welcome.”

“Please help.”

“Peace, love and friendship.”

“Transmitting mathematical ideas and binary expressions.”

“We feel alone and are fearful, primarily because of our own propensity for violence.”

“Our gods and religions are influential in our lives.”