The company my Dad used to work for, Indian Detonators Limited, acquired their first computer in the 70's. During one summer Alu spent some time programming on it (my parents deemed me too young, too stupid or both - so I spent that summer dismantling more alarm clocks and radios). I remember the first program Alu returned home with was for the Drake Equation. He had one of those old-school accordion-folded printouts and explained to me that he knew how many communicative civilization there were in our galaxy. I was impressed. He had asked the Great Computer a question. And the Great Computer had replied. And then I was disappointed when I realized that he had plugged in estimates for seven variables in a simple equation. It felt like a parlour game, not real usable knowledge. The answer, as it turns out, is between one thousand and a hundred million depending on what values you use.
A decade or so later I discovered one of my favorite Calvin and Hobbes strips.
Since the 1960's when radio telescope technology became precise enough, SETI has been listening for aliens. They tune their radio telescopes to the radio frequency that corresponds to the radio emission of natural hydrogen, the most plentiful element in the universe, and point the telescope towards different stars. This assumes that the aliens would choose to transmit at that frequency, that they would and could transmit radio waves, and that they exist or at least existed in the light cone that we are now viewing on earth. SETI even has a protocol in place for what to do when they detect such a signal (tell other astronomers first, then the UN and other scientific bodies, don't send a response signal, etc.)
In 2009 NASA launched Kepler. It has a photometer - a very sensitive camera that detects the brightness of objects, in this case stars. When a planet passes in front of it's star the brightness of the star is slightly diminished. When viewed from deep space, earth causes the brightness of the sun to decrease by just 84 parts per million when summed up over the 13 hours it takes to cross the center of our sun. So the instrument is very sensitive. Further, to cancel out noise, natural variations in the brightness of a star, and non-center crossings, Kepler needs to record multiple crossings. Giant planets close to a star can orbit around the star quickly - Kepler found some that do so in just a few hours. But rocky planets the size of earth and about the same distance from a sun-like star pass in front of their star about once a year. So that's one observation per year.
Kepler has stared open-eyed and unblinkingly at the same 150,000 stars for four years and noted every change in their brightness. From these measurements scientists are beginning to get a much better idea about the number of rocky earth sized planets that are the right distance from their star - in a habitable zone for life as we know it. Planets that are not too hot and not too cold. Goldilocks planets.
Humans have wondered whether we are alone for as long as we have looked up at the night skies. Till recently, we had no basis for believing that we weren't. But there were those who thought that there is nothing exceptional about the earth. Copernicus convinced us that the earth isn't the center of the universe and Kepler (the person) worked out the laws of planetary motion. Then a mere 15 years ago astronomers using earth-based instruments spotted the first planet outside our solar system ever. Till then we had no proof that the trillions of stars out there had planets like our sun does. The first exoplanet was a huge jupiter like giant. Eventually this observation led to the Kepler spacecraft. In December 2011 NASA confirmed that Kepler had spotted two earth-sized planets. They weren't in the Goldilocks zone but it was hailed as a "watershed moment in human history".
Kepler had equipment problems this summer and can no longer gather useful data. But using data already collected, ten days ago scientists concluded that there may be approximately 40 billion earth-sized habitable planets in our galaxy alone.
This more or less nails down two factors out of the seven in Drake's Equation: what fraction of stars have planets, and what fraction of them could be habitable. Another factor relating to the rate of new star formation in our galaxy is known. That leaves four factors unknown. These are: the fraction of habitable planets where life actually develops, the fraction where intelligent life develops, the fraction where intelligent life emits signals detectible from space, and the number of years that an intelligent civilization emitting detectible signals does so.
Till we have more data, the first of these fractions is a complete guess. Life could develop in every planet that is habitable or in none. On one hand we could be the exception in the entire universe. On the other there's no reason to believe that. Once life develops, how often does it become intelligent? In our case, life on earth started 3.5 billion years ago. Primates have only been around for the last 75 million years. Stable species like the dinosaurs did very well for 150 million years (far longer than the 200,000 years that anatomically moderns humans have been around) but they didn't show signs of intelligence (in retrospect, neither may we, but that's a different factor). And as spectroscopy gets more precise, we may not need radio signals from aliens to indicate their presence. By studying starlight that passes through planetary atmospheres we may be able to search for telltale gases. So the third fraction - detection of signals from intelligent life - may be going up.
The last factor is another crapshoot. Once a life form gets intelligent, how long before it destroy its habitat? Are there cosmic events that cause mass extinctions in every world? Evan & Vivian, will you or anyone be around to ponder this in a couple of decades?
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