General Astronomy/The Search for Extraterrestrial Intelligence
There is intelligent life other than us humans in the universe: whales, dolphins, elephants, pigs, seals, great apes, and surprisingly octopuses. But none of these is ever likely to build spaceships or transmit signals into space, and we have difficulty understanding the other intelligent species on our planet.
We have an intense curiosity: are we alone in the universe as a technological civilization? Some people would like to believe that other civilizations, perhaps with creatures even more different from us than elephants or even octopuses are far away in space, and that those creatures have something that we can learn.
To find such intelligent civilizations deep in space we need first to find signals from deep in space. That won't be easy; even radio has been broadcast for a short time (about 85 years as of 2008), a short time compared to human existence. We might also look for planets that have conditions similar to those of Earth -- similar in size and temperature. Extraterrestrial civilizations are unlikely to be found on a gas giant planet or a planet of the Moon's size. We also have to be lucky enough to find a congenial planet with a civilization that has been sending signals of its own sophistication for a long enough time to reach us. If some sophisticated civilization has been sending signals to us for forty years but is fifty light-years away from us, then we on Earth will not get them for another ten years, and then only if we are tuned in. A civilization so sophisticated as ours was in 1875 would have given no evidence of its existence (such as electric lighting) visible even from the Moon -- even if we then had some fine art, literature, and music.
What about alien visitors? Some people believe that some UFOs, or Unidentified Flying Objects are visitors from space. Almost all prove to have simpler explanations such as mirages, optical illusions, aircraft, and weather balloons. Even the planet Venus, bright enough to be visible by day is often seen as a UFO. Most scientists discount stories of visitors from outer space even if they think visits from distant space possible. Not until we actually see a creature like "ET" or, worse, the alien visitors of the movie Independence Day can we claim to have met alien visitors. Space travel is difficult and time-consuming. Species with lifespans similar to ours are unlikely to travel to and from "promising" planets such as their and ours.