Econintersect: It has been estimated there are more than 100,000,000,000,000,000,000,000 or 100 sextillion stars in the universe. Of these, approximately 1 out of every 500 billion (or fewer) are expected to be found in our galaxy, the Milky_Way. Only one out of how many??? It seems to be a vanishingly small number, but that tiny ratio actually produces estimates of 200-400 billion stars in our galaxy. An article published in early 2011 at Discovery News summarizes recent estimates that about 50 billion planets are orbiting these stars and about 50 million exist within the habitable zones of their parent stars.
The numbers estimated are subject to great uncertainty because they are extrapolated from a much smaller (fair to say miniscule) number of actual observations. From Discovery News (19 February 2011):
This announcement was made on Saturday by Kepler science chief William Borucki at the American Association for the Advancement of Science in Washington D.C. However, Kepler didn’t actually count 50 billion exoplanets, this number comes from extrapolations of the data taken so far by the exoplanet-hunting space telescope.
For example, as Kepler has spotted 1,235 exoplanet candidates so far — 53 of which orbit stars in their habitable zones — knowing approximately how many stars there are in our galaxy (there are thought to be around 300 billion stars in the Milky Way), an estimate can be made of how many worlds are orbiting these stars.
This is not quite obtaining one sample and describing the universe, but it is close to that.
Note: Exoplanet refers to extrasolar planets which are located outside our solar system.
If we extrapolate the Milky Way numbers to the entire universe (may be highly speculative?) the number of potential earths in the measureable universe would be of the order of a quintillion (10 to the 18th power). And, of course, no one knows just how large the universe is compared to what we can see. A theologian might say it is infinite. Can physicists prove that is wrong?
This article could have used the headline “More than a Quintillion Earths in the Universe?” That might have seemed just not comparable to reality for many. Now 50 million more earths? That is much more in the realm of conceptualization for mere mortals.
The bottom line is that the odds are so overwhelming that life forms similar to earth must exist elsewhere in the universe that trying to calculate them seems pointless. How do you calculate the probability of a certainty? The more we learn the less we know.
Meanwhile there are efforts underway in often unpublicized corners. A story this week by Diane Smith of McClatchy Newspapers (Raleigh News Observer) reported on research from the University of Austin, Arlington by physics professor Zdzislaw Musielak, associate professor Manfred Cuntz and doctoral student Billy Quarles. The trio is looking at specific planetary situations that might be conducive to support life within the Kepler-16 system. Data comes from the NASA Keppler Mission (mentioned previously) which is designed to explore the nearby region of the Milky Way looking for Earth-size (and smaller) planets that might be in the habitable zone which could support life forms such as we know here on earth, such as simple organisms such as bacteria as well as plants.
Hat tip to Naked Capitalism