Step 2: We estimate
how many of the 400 billion stars have planetary systems. Scientists guess that
at least 1 in 4 have planets. Therefore, there are at least 100 billion stars
with planets.
Step 1: We
start with the number of stars in our galaxy, the Milky Way. Most scientists
think there are around 400 billion.
Step 3: We estimate
how many planets have conditions suitable for supporting life. It is estimated
that 2 planets per system can support life (e.g. Earth and Mars in our system
are capable of supporting life). This means there are 200 billion planets in the
Milky Way galaxy that could support life.
Step 4: We estimate
how many planets that can support life might actually have life. Based on our
solar system, we can guess that 1 out of 2 planets with life supporting
conditions will actually have life. This means 100 billion planets with life.
Step 5: We estimate
how many planets with life will also have enough intelligence to communicate
with us. Scientists feel a conservative estimate is 1 in 10. This means there
may be 10 billion planets with intelligent life in our galaxy alone.
Even if the figures are
off by 90%, it still gives us one billion planets to work with here. And
remember, this is only OUR galaxy. There are billions and billions of
other galaxies.