Searching for Life around Other Sunlike Stars

Project Phoenix's primary targets for signs of extraterrestrials are the 1,000 closest sunlike stars. The list is dynamic, though, and other targets have included the META candidates and stars with known planets.

At first glance, the nearby stars with known planets would seem to be a poor choice of targets for searching for extraterrestrial life. After all, the exoplanets are all massive--at least as massive as Saturn. Moreover, many are close to their host stars, in some cases even closer than Mercury is to the sun. Neither of these characteristics would seem to be particularly favorable for finding ET life on a planet around such a system.

On the other hand, a decade ago had one asked where planets should be found, no sober astronomer would have suggested that it would be possible to find giant planets orbiting at distances less than than 1 astronomical unit (AU, the distance from Earth to the sun) from their host stars. Nature has a habit of surprising us.

The current census of planetary systems is 87; the number of planets in those systems totals just more than 100 (some have more than one planet). Not all of these planetary systems have been observed by Project Phoenix. The most crucial restriction is placed by the telescope used. Project Phoenix has made use of three instruments, the Parkes telescope in Australia, the old 140-foot Green Bank telescope and the Arecibo Observatory in Puerto Rico. Project Phoenix started with the Parkes telescope but has spent much of its recent time on the northern hemisphere telescopes. Some exoplanets simply cannot be observed from the northern hemisphere, like the star tau^1 Gruis, around which an exoplanet was found recently. Currently Project Phoenix is using Arecibo Observatory. This is the world's largest radio telescope, but its size comes at a severe price: it can see only about 30 percent of the northern sky.

A further restriction on the search is imposed by the need to confirm possible signals with a second telescope. Terrestrial radio emissions are so strong and numerous that SETI observers must take great care to ensure that a "detection" is not simply a terrestrial transmitter. Complicating matters, Project Phoenix's goal is to survey the stars at radio frequencies between 1.2 and 3 gigahertz (GHz), but the current confirmation telescope is the Jodrell Bank telescope in England, which can observe only between 1.2 and 1.75 GHz.

Nonetheless, about one-quarter of the stars with known planets can be, and are being, observed from Arecibo, with a small number of stars also having been observed with the Parkes and Green Bank telescopes. As it says in the summary of the project: "So far, no clearly extraterrestrial transmissions have been found. But the faint [signal] that would betray an alien civilization might be heard tomorrow." --Joseph Lazio

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