Dr. Seth Shostak is a senior astronomer at the SETI Institute. On Wednesday, he and his colleague, Dr. Dan Werthimer, testified before the House Committee on Science, Space, and Technology on the “search for life in the universe.”
From Dr. Shostak’s written testimony:
In the last two decades, astronomers have uncovered one so-called exoplanet after another. The current tally is approximately two thousand, and many more are in the offing thanks to continued analysis of data from NASA’s enormously successful Kepler space telescope.
Estimates are that at least 70 percent of all stars are accompanied by planets, and since the latter can occur in systems rather than as individuals (think of our own solar system), the number of planets in the Milky Way galaxy is of order one trillion. It bears mentioning that the Milky Way is only one of 150 billion galaxies visible to our telescopes – and each of these will have its own complement of planets. This is plentitude beyond easy comprehension.
The Kepler mission’s principal science objective has been to determine what fraction of this planetary harvest consists of worlds that could support life. The usual metric for whether a planet is habitable or not is to ascertain whether liquid water could exist on its surface. Most worlds will either be too cold, too hot, or of a type (like Jupiter) that may have no solid surface and be swaddled in noxious gases. Recent analyses of Kepler data suggest that as many as one star in five will have a habitable, Earth-size planet in orbit around it. This number could be too large by perhaps a factor of two or three, but even so it implies that the Milky Way is home to 10 to 80 billion cousins of Earth.
There is, in other words, more than adequate cosmic real estate for extraterrestrial life, including intelligent life.