One of the Important Numbers in Determining How Many Extraterrestrial

Multiple Choice

One of the important numbers in determining how many extraterrestrial civilizations there may be at the present time in the Milky Way Galaxy is the following: Out of the stars that have planets,what fraction,fs,has planets suitable for life? (This number is similar to the number nₑ in the Drake equation.) Based on the extrasolar planets already found by astronomers (see Chapter 5 of Comins,Discovering the Essential Universe,6th ed.) ,what is a good guess for fs? (Note: If all stars with planets have at least one planet suitable for life,then fs = 1; if no such star anywhere in the Milky Way has a planet suitable for life,then fs = 0.)

A) fs = 0 since no star has yet been found to have an inhabitable planet
B) close to fs = 1 since most stars so far found to have planets have high-mass planets (similar to Jupiter or larger) orbiting far from the star, indicating that the solar system is probably typical of planetary systems in general
C) close to fs = 1/2 since about half the stars so far found to have planets have planets similar in mass to Earth orbiting the star at about Earth's distance from the Sun
D) fs less than about 1/10 since most stars so far found to have planets have high-mass planets (similar to Jupiter or larger) orbiting close to the star, which would disrupt the orbit of an inhabitable planet