How many times have you looked up at the night sky — or at a stunning image in Astronomy magazine of a single galaxy containing billions of stars — and thought: "There has just got to be life out there"?
The trick is finding it. The SETI program listens for radio-frequency whispers from the vastness of space. Rasmus Bjørk at the Niels Bohr Institute in Copenhagen, Denmark, reaches for his calculator.
Bjørk 's paper, "Exploring the Galaxy using space probes," showed up on my radar as I was perusing a site based at Cornell University that posts pre-publication versions, or "preprints," of scientific papers. This particular paper will be published in the Journal of Astrobiology.
In it, Bjørk asks a simple question: If you were to explore the Milky Way's most habitable zone with space probes, looking for signs of life, how long would it take?
The "galactic habitable zone" is defined as that region of the galaxy where conditions are amenable to terrestrial life. It turns out to be about 4% of the galaxy. As Bjørk envisions the search, each probe, or "host," is a sort of mother ship that travels to a star and dispatches up to 8 "sub-probes" to explore the nearest 40,000 stars, traveling at 1/10 the speed of light. The probes then return to the host for maintenance. Then the host moves on to the next dispatch point.
Bjørk found that using 8 host probes, the search for life would take about 10 billion years. Put another way, it takes half the age of the entire universe just to explore 4% of a single galaxy.
That's clearly too long to wait. Congress would have long since cancelled funding for the program by the next election cycle. So Bjørk increased the number of probes to 200 and got a more reasonable answer: only 400 million years.
The universe is very big. And sending out space probes to search for life is, Bjørk concludes, "horribly slow." And not just for us — for any other intelligent species out there. Bjørk concludes with this thought: "We have not yet been contacted by any extraterrestrial civilizations [simply] because they have not yet had the time to find us."