In his 1960 article for Physics Today, Frank Donald Drake (1930 – Now) discusses the rationale for searching for extra-terrestrial (ET) intelligent civilizations using radio surveys, and after doing so describes Project Ozma. Further, he lays the groundwork to quantify the probability of finding intelligent life, which was later formalized as the ‘Drake Equation’.
Project Ozma conducted at Green Bank using the 26 m (85 ft) diameter radio telescope, was one of the first SETI (Search for Extra-Terrestrial Intelligence) experiments to search for intelligent transmissions of ET origin. It included observations of Tau Ceti and Epsilon Eridani, two stars spectrally similar to the Sun. With the exception of a false alarm due to a secret military project, the project did not yield any significant signal from these two stars.
Drake starts off by discussing how later generation stars contain not only Hydrogen and Helium but also metals. These metals (heavier elements) are required to form solid bodies like planets. Further, the formation of planets assuages the angular momentum problem in a cloud of condensing gas. Sun and other stars like the Sun have relatively slow rotational periods. This rotational period does not conserve the initial angular momentum and hence leads to a discrepancy. This can be solved by the introduction of secondary bodies like planets or binary stars, to which the gas cloud transfers angular momentum as it slows down. Drake suggests that as high as 60 percent of stars should harbour planetary systems.
Establishing heuristic arguments for their existence, Drake goes on to hypothesize whether life can arise on these extra solar planetary systems. He then cites the Urey – Miller experiment, which managed to successfully create amino acids in the laboratory using gases like ammonia, methane, hydrogen and water vapour and an electric discharge (simulating the early atmosphere and a lightning discharge). Amino acids are the building blocks of proteins which are the key ingredients for life. Therefore, the oceans were the harbinger of early life, which after about 5 billion years of evolution led to intelligent civilization. Drawing parallels to the origin and evolution of life on Earth, he postulates the fact that since life would take so long (5 Gyrs) to develop and achieve intelligent civilization one can discount non main – sequence stars and those which have relatively short life spans (stars much larger than the Sun).
Another consequence of the comparison to life on Earth is the hypothesis that life needs liquid water to develop, due to which the planet (if it has water on it), cannot be too cold or too hot. This leads to existence of a narrow band around the star a planet can orbit – The Habitable zone. Being much closer, or much farther would lead to the vapourisation or freezing of water, respectively.
To search for such life on Earth – like planets around Sun – like stars, the use of narrow – band transmission in the radio is suggested. Discovery, and subsequent contact with such a civilization would likely be in the vicinity of the 1420 MHz region of the radio spectrum. This would be because it corresponds to the 21 cm Hydrogen line spin transition in neutral Hydrogen, a spectral feature that should be known to an intelligent life form. Also, in this region the cosmic noise signal is negligible making it easier to transfer signal at cosmic distances. On the other hand, even if we want to actively seek out ET intelligence this would be the appropriate EM region to seek communication in, since there is a greater possibility of such civilization having radio telescopes tuned and actively searching in this region of the spectrum.
Thus Drake lays the justification for Project Ozma where he searches in this radio band around two Sun -like stars (for princess Ozma?) . He concludes by stating the goal (of finding ET intelligence) justifies the amount of effort required to carry out this work, and with the hope that in the near future, the search will be successful.