A recent study claims that intelligent aliens would only be interested in contacting the most technologically advanced planets. Earth, however, is not successful in this regard as it may be seen as unattractive or not smart enough by the aliens.
A preprint paper published in the arXiv database suggests that intelligent extraterrestrials may not find planets hosting life particularly attractive.
"If life has evolved on many planets in the galaxy, then aliens may be more interested in planets that have not only biological but also technological signatures," wrote study author Amri Wandel, an astrophysicist at The Hebrew University of Jerusalem.
The study explores the Fermi paradox, which argues that given the age of the universe, it is more likely that intelligent aliens will develop long-distance space travel by now, and thus are more likely to visit Earth. The fact that they have not (as far as we know) made contact, may be proof that there is no other intelligent life in the Milky Way galaxy.
But experts have provided other explanations regarding the mystery of alien existence. Perhaps they have visited Earth in the past, before humans evolved or were able to record such visits.
Or, perhaps long-distance space travel is more difficult than we have been led to believe. It could be that aliens developed an advanced civilization that was too new to come to Earth. Alternatively, they deliberately decided not to explore the cosmos. Maybe they even disappeared.
In this new paper, as quoted from Live Science, Wandel offers another explanation: that life is actually very common in the Milky Way. If lots of rocky planets orbiting the star's habitable zone host life, aliens probably won't waste their resources sending signals to every planet. They might try to communicate with alien algae or amoebas.
If life is something common in our galaxy, intelligent aliens are likely more interested in signs of technology. But technology signals can be difficult to detect. Earth has only emitted detectable signals from space (in the form of radio waves) since the 1930s.
In theory, these signals have now swept about 15,000 stars and planets orbiting them, but even then only a fraction of the 400 billion stars in the Milky Way.
Furthermore, wrote Wandel, it takes time for any reply messages from aliens to travel back, so only stars within 50 light years have had time to respond from the moment Earth begins to air beyond the planet.
"Worse still, the oldest radio signal on Earth was accidentally broadcast into space, so it is likely so messed up after about a light year that aliens can't tell the difference," the researchers said.
Earthlings sent the first intentional high-power broadcast to aliens via the Arecibo Observatory in 1974. At that time, messages from Arecibo were directed to the globular star cluster M13. Some scientists think it's time to send another message.
"Unless intelligent civilizations are extremely abundant, with more than 100 million technologically advanced planets in the Milky Way, it is unlikely that Earth's signals have yet reached other forms of intelligent life. However, as time passes, and as our planet emits more and more radio signals, it is more likely that Earth's technology signal will find listeners," Wandel wrote.
The findings suggest there may not be an intelligent civilization within about 50 light years of our planet. But intelligent life may still be out there, they are just waiting for our call.