The astronomers’ observations of Orion’s central region of more than 250 known stars showed that only about 10 percent emit 1.3-millimeter wavelength radiation typically emitted by a warm disk of dust. Even fewer – less than 8 percent of stars surveyed – were judged to have dust disks with masses greater than one-hundredth the mass of the sun, a mass thought to be the lower limit for formation of Jupiter-sized planets. The average mass of a protoplanetary disk in the region was only one-thousandth of a solar mass, the researchers calculated.
So let’s say that the formation of Jupiter-sized planets is a prerequisite to the formation of earth-sized planets that aren’t constantly being sterilized by asteroid bombardment, and let’s say that 1% of systems with Jupiter-sized planets have rocky planets within the habitable liquid-water zone. With a lower bound of 100 billion stars in this galaxy (might be four times that many, but only if a whole lot of them are itty-bitty) that’s about 80 million planets potentially habitable by life like us. Oh, the rarity.
Oh, and something north of 100 billion galaxies in the observable universe.