SETI and Beyond
A technologically advanced extraterrestrial civilization could likely detect life on Earth, if such beings exist. Life on Earth could be detectable in our planet’s atmospheric spectral lines for over a billion years. Most of our atmospheric oxygen is due to life, and can be observed over interstellar distances — across thousands of light-years.
Over this long time, many stars have swept near our solar system and Earth. If extraterrestrial intelligence (ETI) on such "nearby” planets did send probes to remotely observe our planet, where in the Solar System should we look to find evidence of their past visitation?
The Moon is the obvious, closest place. Another option would be a newly discovered class of co-orbital objects, an equally logical place to locate for observing Earth. These objects approach Earth very closely every year at distances much closer than any large body besides our Moon. They are an ideal way for ET's to watch our world from a secure natural object that provides resources an ET life form might need: materials, a firm anchor, and concealment. They might likely be robotic probes, like our own Voyager and New Horizons probes, remaining on site after exhausting their energy supply.
Studying the Moon and co-orbitals could be termed "extraterrestrial archeology". For the Moon, we can use the photographic mapping of the Moon’s surface by the Lunar Reconnaissance Orbiter. Co-orbitals have been little studied by astronomy and not at all by SETI or planetary radar observations.
This discussion describes a strategy of looking for ETI artifacts. It proposes both passive and active observations by optical and radio listening, radar imaging and launching probes. We might even broadcast to them. But what if we find nothing there? That would be a profound result: suggesting that, perhaps, no ET intelligence has yet come to look at Earth, on that other hand, perhaps other civilizations are simply not as curious as we are or are better at concealing their activities than we are.
Such speculation forms the basis of this lively conversation between astrophysicist and associate director of the Arthur C. Clarke Center for Human Imagination, Dr Brian Keating, Prof. Paul Davies, Dr. James Benford and Mat Kaplan (Planetary Society).