NASA picked Columbus Day of this year to turn on the most advanced radio receivers ever built for SETI--the Search for Extraterrestrial Intelligence. Why radio? Because space travel is very expensive, and the distances between stars are vast. Radio waves travel at the speed of light and are far cheaper than spaceships. So on the 500th anniversary of Columbus’s voyage, NASA is embarking on a radio exploration of the galaxy, looking for faint unnatural signals from the Milky Way.
The scientists involved in the search are pretty sure something is out there. A lot of numbers, some high and some low, are thrown around to express the probability of intelligent life. Here are some figures that are middle-of-the-road: There are an estimated 400 billion stars in the Milky Way. Planets may be fairly common, so you can figure one out of every ten of these stars has planets, which equals 40 billion stars with planets. If every such star has ten planets, that’s 400 billion planets. But how many of these places might be suitable for life? If life elsewhere is similar to our own form of life, then we need a planet that’s not too hot or too cold, with an atmosphere and with water. In our solar system only Earth qualifies, though Mars and Venus come close. So let’s be conservative and estimate that only one planet in each stellar system will do. That’s 40 billion habitable planets.
