Many of these clouds have a little bit of rotation. As they collapse, the cloud spins faster and faster, like an ice skater pulling in her arms. This causes the cloud to form a flattened disk of gas. At the center, the main star forms. Out further, smaller eddies can form planets. Until recently, we did not have any evidence of planets outside our solar system. In the last couple of years, several teams of astronomers have announced the discovery of planets surrounding nearby stars (see interview with Geoff Marcy and Didier Queloz). This exciting discovery increases the likelihood of other planets around many stars. Let's estimate conservatively that one-half of the stars form planetary systems; the other half form binary star systems, so f(p) = 0.5.

 Geoffrey Marcy
Geoffrey Marcy is a professor at San Francisco State University.
There's more on Marcy and his search for extraterrestrial planets from "What's New" December 1996.

The n(e) factor is a little tricky. Small stars are cool and red. Planets would have to orbit very close to be in the ecosphere. Also, this ecosphere would be very narrow; like the skin on an orange. Not much room for planets. Planets that orbit very close to their parent star are often tidally locked and present one face to the star at all times. The atmosphere of such a planet would freeze on the cool side that faces away from the star; this does not promote life. On the other hand, huge hot blue stars have a farther and wider ecosphere. Of course, judging from our solar system, planets are spaced further apart the farther they are from the star, so the wider ecosphere is cancelled by this effect. These larger stars also burn their fuel faster and don't last very long. They are usually so short-lived that life does not even get a chance to start before the star goes nova or supernova and destroys everything in the system. In our solar system, with our average-sized yellow sun, we have two (Earth and Mars) or maybe three (Venus) planets within the ecosphere. A conservative guess for the number of planets within the "life zone" or ecosphere is one. n(e) = 1.

The next factor, f(l), is where things become a little sticky. The problem is that we only have a few examples of planets where conditions are right for life to evolve. As stated above, Venus, Earth, and Mars all could have had, at one time, proper conditions. We know life evolved on Earth, and there is now tantalizing evidence for primitive life existing on Mars billions of years ago. A conservative guess for this number is 0.2, or one in five planets with proper conditions will evolve life. f(l) = 0.2.

"SERENDIP: The Search for Life"


Bottom Bar

©1997 Exploratorium