This may shock some people, but the concept of life on other planets predates Steven Spielberg’s movie E.T. It also predates Kenneth Arnold’s coining of the term “flying saucer” in 1947 and even H. G. Wells’s late-19th-century novel War of the Worlds.
Percival Lowell, in the mid-19th century, claimed to see canals on Mars. Immanuel Kant talked about life on other planets in the century before that, and way back in 1634 Johannes Kepler imagined life on the Moon. But even he was a latecomer to the idea, which was touched on by the ancient Greeks five centuries before Christ.
Through most of those years, though, we’ve only been able to speculate on the possibility of life on other planets, since there seemed to be no way to find out for sure.
That doesn’t mean the speculation has been without scientific merit. For example, it seems clear a planet needs three conditions to support life: a decent temperature, water, and an atmosphere. (You might be able to base life on a completely different chemistry that wouldn’t require these things, but then we probably wouldn’t even recognize it.)
Every star has an “ecosphere,” the narrow range around it in which temperatures are suitable for life, water can exist as a liquid, and an atmosphere won’t boil off into space.
The ecosphere of our own sun includes us, the Moon, and Mars, but the Moon is utterly dry and airless and ranges in temperature from 117 Celsius to -190. Mars has small amounts of water and at the equator gets as warm as 17 C, but its atmosphere is mostly carbon dioxide at an average pressure only about one-half of one percent of Earth’s. That doesn’t entirely rule out life, but the Viking Mars landers turned up nothing. If life exists on Mars, we may have to wait for a manned expedition to find it, and it likely won’t be anything Spielberg will be interested in, since lichens make lousy movie stars.
As for the rest of the planets in the Solar System–forget it. Mercury and Venus are way too hot, at 430 and 485 C, respectively, and all the other planets are too cold, starting at -140 in the upper cloud layers of Jupiter and going down from there.
Beyond the solar system things look brighter, simply because there are so many stars in the universe it’s inconceivable that our very ordinary sun is the only one with planets, and that only this single planet has life on it. Most scientists (with some exceptions) are confident there are lots of planets out there nurturing not only life, but intelligent life.
There’s even an equation, designed by radio astronomer Frank Drake, to tell you how many. You need to know is the rate of star formation, the fraction of stars with planetary systems, the number of planets in each system with conditions favorable to life, the fraction of those planets on which life develops, the fraction of those planets on which life develops intelligence, the fraction of those planets on which intelligent life develops a technical civilization capable of interstellar communication, and the lifetime of such a civilization.
The reason Drake included that bit about being capable of interstellar communication is because that’s what he’s most interested in. Drake, you see, is the father of SETI–the Search for Extraterrestrial Intelligence.
Drake’s equation suffers from a lack of accurate figures for any of its factors. Nevertheless, even conservative guesses point to huge numbers of extraterrestrial civilizations capable of interstellar communication. That being the case, some of them should be communicating–and we may be able to hear them.
Drake tried to do just that in 1960 with Project Ozma at the National Radio Astronomy Observatory in West Virginia. He focused on just two sun-like stars, Epsilon Eridani and Tau Ceti, listening on a specific frequency, 1,420 megahertz, thought to be the most likely communication channel because it is the frequency emitted when an electron reverses its spin in an atom of hydrogen, the most abundant element in the universe.
Project Ozma heard nothing, but since that time at least eight other similar searches have been carried out, all focusing on a few sun-like stars, and mostly at that same frequency. There have been a few false alarms, but no extraterrestrial “Eureka!”
However, all these searches have been miniscule compared with the one NASA launched on October 12, 1992, 500 years after Columbus “found” America (although the people already living here weren’t aware it was lost). The decade-long project will consist of two parts, a targeted search and a sky survey. The targeted search will use the largest available radio telescopes from around the world to listen in on sun-like stars within 80 light years over a relatively narrow frequency range that includes the popular 1,420 mHz. The sky survey will use NASA’s Deep Space Network radio telescopes to scan the entire sky over a much larger frequency range. (After all, just because 1,420 mHz seems like the likeliest frequency to us doesn’t mean everyone else feels the same way.)
NASA’s project is so far beyond anything done before that within its first few minutes more searching was accomplished than in all the previous attempts put together.
Even if we don’t hear anybody, that doesn’t mean they’re not there. They could be hiding, they could be using unknown technology, or maybe they’re just not interested.
One thing’s for certain, though. Anybody within about 50 light years who’s conducting a similar program knows WE exist. Heck, with all the radio, television and radar signals we’re sending out, they probably think we’re shouting at them.
Let’s just hope they enjoy Gilligan’s Island.