What is the chances that there are beings light-years away on other worlds wondering if we exist? In 1961, Frank Drake , now Emeritus Professor of Astronomy and Astrophysics at the University of California, Santa Cruz, developed an equation that bears his name, designed to give a working estimate of the probability of intelligent life on other worlds in our galaxy. It goes like this::

N* represents the number of stars in our Milky Way Galaxy. Current estimates place the figure at 100 billion.

fp is the fraction of stars that have planets around them. New “extrasolar” planets are being discovered very rapidly of late. Current estimates range from 20% to 50%.

ne is the number of planets per star that are capable of sustaining life. Scientists believe that to average from 1 to 5.

fl is the fraction of planets in ne where life develops

that’s the most uncertain – it could be anywhere from all of them to none of them.

fi is the fraction of fl where intelligent life develops.

Again, this can be all, some, or none.

fc is the fraction of fi that communicate.

That’s thought to be 10% to 20%.

fL is fraction of the planet's life during which the communicating civilizations live.

This is also very uncertain. After all, we don’t even know how long we Earthlings will survive – whether we will avoid nuclear or ecological catastrophe, or will persevere to advance in extra-terrestrial communication and exploration.

If all of these variables are multiplied together, the result is N, the number of communicating civilizations in the galaxy. (Obviously, if the third, fourth, fifth, or sixth variable is placed at 0, there is no one out there trying to talk to us!)

If, though, we plug in estimated fractions -- for example, estimating that each star that has planets has at least one that could support life – we can come up with a figure of, say, 1,000 stars in the Milky Way that have communicating civilizations on them (see box).

If that’s the case, we can only hope that they send out the note in the bottle – and that we can find it.

N* = the number of stars in the Milky Way galaxy: 100 billion

fp = fraction of stars with planets around them: 50 %

ne = number of planets per star ecologically able to sustain life: 1

fl = fraction of those planets where life actually develops: 50%

fi = the fraction of fl that develops intelligent life: 20%

fc = the fraction of fi that communicates: 20%

fL = Portion of the planet's life during which the communicating civilizations survives: 10,000 years

N = the number of communicating civilizations in the galaxy: 1,000

The Planetary Society

The name sounds like something out of Star Trek, but it’s a product of the 20th century, not the 23rd.

It was founded 25 years ago by the late, populist astronomer Carl Sagan and others to, in the words of its literature, “demonstrate — simply by its existence — that the public strongly supported planetary exploration and the search for extraterrestrial life.”

“But it quickly became clear,” its statement continues, “that we were going to have to do more than just exist. To achieve our goals of keeping spacecraft exploring, seeing humans walk on other worlds, and searching for signs of life in the galaxy, we added sponsoring research projects, publishing a magazine and a website, holding events to celebrate exploration, and preparing for the future by educating the next generation of space explorers.”

The Society's website is www.planetary.org.