The "water hole" in the electromagnetic spectrum. Lower background noise makes this a natural choice for sending or detecting interstellar messages. (Courtesy: Astronomica)
Now that we have discussed the prospects of finding primitive life on other planets, what about the prospect of finding intelligent life? What to look for? What kinds of “biomarkers” exist that could allow scientists to scan the heavens for evidence of such life? It has been widely agreed since a pioneering 1959 paper by Cocconi and Morrison that the most favorable part of the electromagnetic spectrum to search is at microwave frequencies (1000-10,000 MHz), corresponding to wavelengths in the range 3-30 cm, where atmospheric and interstellar noise are at a minimum. In particular we find here as gatekeeping "markers" the emission lines of neutral hydrogen (at the famous 21-cm or 1420 MHz line) and of the hydroxyl radical -OH. Together these form the "water hole", around which life might be expected to gather. This region of the spectrum is however shared with powerful radar and TV emitters, with the expected consequences (entertainment is trampling on research). Laser frequencies have also been long advocated and are beginning to be used.
A detectable, repeatable radio or laser signal would make an excellent biomarker! But what if our neighbors are not noisy? They can be intelligent and still not use radios or lasers. After all, this perfectly describes our great-grandparents. What other biomarkers can we think of? We are beginning to detect exoplanets, planets around other stars. By 2016 or so we should be able to detect Earth-like planets in our vicinity, within thirty or so light-years. At first we will only just be able to tell that they are there, and it will be a very long time before we have photos. What will tip us off to the presence of life; what biomarker will be present if there is life and absent otherwise?
Certainly the presence of water or water vapor in a planetary atmosphere spectrum would be encouraging, but remember, there was water on Earth before life. Likewise CO2 seems to be present where there is no life (Venus, Mars). Our experience on Earth would indicate that molecular oxygen, O2 and ozone, O3 would be good biomarkers. Recent calculations indicate that a Sun-Earth system could detect all of these molecules by thermal emission or by reflected light in 1 week's observations from about 30 light-years away with an 8-meter aperturecoronagraph in space. So in a couple of decades we can reasonably expect to be examining nearby planetary systems for life, even microscopic oxygen-generating life such as that on Earth starting some 2 billion years ago!