By Dr David Whitehouse
BBC News Online science editor
There could be an alien spacecraft with a message for us lurking somewhere in our Solar System, say scientists writing in the journal Nature.
The message attached to the Pioneer interstellar probe
Until now, it was generally believed that the best way to find ET is to look for a radio signal from them as such signals can travel vast distances.
But an analysis by US researchers suggests that sending a probe into space would be more efficient.
A recent radio search of 800 stars showed no sign of a signal from ET.
The search for life in space moved into the modern era with a 1959 paper in the journal Nature by Giuseppe Cocconi and Philip Morrison of Cornell University.
They argued that the radio telescopes being constructed to scan the sky for natural radio sources could also look for radio signals from aliens.
By one method of analysis, radio waves offer the best way to pass information over interstellar distances.
There are even certain frequencies that may be logical to use. The so-called "water hole" is a quiet region in the radio spectrum marked by emission from hydrogen atoms that, given the connection between water and life, may provide a spectral rendezvous for species.
Is this the best way to search for ET?
Since 1959 there have been many searches, most around the "water hole", that have drawn a blank. Searches have also been carried out looking for laser flashes as well, because they may in certain circumstances be a good way to pass a message from star to star.
Message in a bottle
Christopher Rose of Rutgers University and Gregory Wright of Antiope Associates, both in the US, present a new analysis of an old topic that may explain why the ET radio searches have been unsuccessful.
They argue that, in many circumstances, it makes more sense to send a space probe carrying a message to another star system, rather than a radio beam.
Unless the radio messages are short, they say, their "package strategy", is more efficient, requiring less energy per bit of information transmitted.
Their work re-opens a line of research that was discounted by ET hunters who thought the energy required to construct and launch a "message in a bottle" was much greater than transmitting a signal on a radio beam.
The possibility of an alien space probe hiding in our Solar System is one of the staple ideas of science fiction.
It was used effectively by Arthur C Clarke in his 1951 story called The Sentinel which describes how an alien artefact is discovered on the Moon. The book was the inspiration for the 1968 film 2001: A Space Odyssey.
Perhaps it is not so science fiction after all.