April nights at Arecibo find Orion the Hunter high overhead. Red Betelgeuse (the eastern shoulder) and blue-white Rigel (the western knee), plus the three stars of Orions belt form an easily recognized and spectacular constellation from any latitude. Here, away from bright city lights, Orion and the Milky Ways hazy band of star clouds stand out against a black velvet sky. So many stars! So little telescope time!
Unlike other SETI programs, Project Phoenix targets individual stars rather than scanning the sky, which is mostly "blank," from our point of view. In a vast sky full of stars, only a fraction are likely to have life-supporting planets and be near enough for us to detect radio waves transmitted at a reasonable power. Therefore, the SETI Institute conducts a "targeted" search of selected stars. This strategy has many advantages over scanning the sky. By choosing particular, relatively close stars and observing each star for a long time, we are sensitive to lower power transmitters over a wider range of frequency channels for more types of signals. Because of these advantages, Project Phoenix scientists have worked from a list of target stars* since inception. So, how exactly do we choose our target stars? We consider a number of characteristics in our star selectionexplaining the process requires a bit of basic astrobiology.
Life as we know it arose on a small, rocky planet orbiting an average star that we call the Sun. Life started very early in the history of the Earth, but intelligent life didnt evolve for several billion years. The Sun has been shining for nearly five billion years and it is only about halfway through its stable lifetime. So, stars like the Sun, with long stable lifetimes that give evolution enough time, are obvious candidates for a search. But how Sun-like must a star be to qualify?
Stars that are much more massive than the Sun exhaust their hydrogen fuel quickly, in less than a billion years, then expand in size one hundred-fold or more with a corresponding increase in energy output. The blistering heat would sterilizeor even vaporizeany planet orbiting such a star. Orions massive stars will shine for brief few million years and do not qualify for the target list.
Stars less massive than the Sun will outlive Earths star, but they are also cooler. Because any planet must be relatively close to the star in order for liquid water to exist (a requirement for life as we know it), for very low mass stars, the planet would need to be very close. As a result, the planet could be "tidally locked" to its star, i.e., one side of the planet would always face the star, while the other side remains in perpetual darkness. (The Moon, for example, is tidally locked to the Earth.) Common wisdom deems these locked planets unlikely to support life, so very low mass stars have been excluded from the list.
Fortunately, thousands of Sun-like stars are catalogued and can be found within a few hundred light years distance. Still, not all of these should be on the list. Most stars orbit other companion stars and most of these systems have dynamic gravitational forces that would disrupt the orbit of any planet. In some cases, however, the stars are sufficiently separated to allow a stable planetary orbit at a distance where liquid water could exist. Those few star systems make the list.
Sifting and sorting catalogs, and excluding unsuitable star systems still leaves us with a list of thousands of Sun-like stars. Which ones are the best to observe? Like any good manager faced with a daunting task list, Phoenix scientists prioritize. We assign a priority to each star based primarily on its distance, with closer stars receiving higher priority. The closer the star, the less power a transmitter needs to produce a detectable signal.
Do we observe the stars known to have planets? Absolutely. This is a "no-brainer," however most of these stars have always been on our target list, which is very similar to the list used by the planet hunters. Once a planet is detected, the stars priority moves up. When we observe, the scheduling software sorts through our star database to find the highest priority stars that are just rising and will be within the view of the telescope. It then checks which of those stars have been observed and over what fraction of the frequency range. If we can complete a search through all frequencies for that star, it receives a higher priority.
When Project Phoenix is finished in early 2004, we will have searched for signals from about 750 stars out to a distance of roughly 200 light years. For each of those stars, nearly 1.7 billion channels will have been searched for both continuous and pulsed signals with unprecedented sensitivity. Perhaps in our remaining telescope time at Arecibo well hear a faint alien whisper across the interstellar void. Searching fewer than a thousand stars out of the few hundred billion in our galaxy, however, may not be enough.
We have plans to accelerate the search with the Allen Telescope Array. The list for the new search will be over 100 times longer than the Phoenix list, including a few hundred thousand stars. We will include lower mass stars and peer further into the galaxy. Even this is but the beginning, for a few hundred thousand stars represent only a small fraction of the galaxy. The path before us is long, but paved with stars.