Hack 75. Seek Out New Life and New Civilizations

The search for extraterrestrial life is alive and well. You can use statistical sampling and probability to focus the search.

The scientific quest to make contact with life on other worlds requires that decisions be made. First, one must decide if life exists at all beyond on our own planet (mine's Earth, what's yours?). Second, one must determine how and where to look for it. You can apply statistical procedures to make both these decisions.

Estimating the Number of Smart Planets

In 1961, Frank Drake, an astronomer who was interested in looking at the universe from afar by reading radio waves (a bunch of which are bouncing off Earth all the time), decided to estimate how many other technologically advanced civilizations probably exist.

Being a little Milky Way-centric, he was most interested in determining the number of advanced worlds (planets willing and able to talk with us) that are nearby, in our own galaxy. Drake suggested this equation:

Table 6-23 shows the meanings of the abbreviations in Drake's equation.

Table Drake equation components

Term	Meaning
R	Rate at which new stars are produced in the galaxy (per year)
Nh	Average number of planets orbiting each star that can support life
Fl	Proportion of planets (from Nh) on which life does develop
Fi	Proportion of planets (from Fl) on which intelligent life develops
Fc	Proportion of planets (from Fi) on which civilizations develop
L	Average lifetime (in years) of civilizations (from Fc)

The formula is really nothing more than a chain of probabilities. The number of expected positive outcomes is determined by multiplying all the separate likelihoods together. Though a simpler equation without all the different permutations of F would work just as well, the specific different components were included to help scientists identify the important questions that needed to be answered to estimate the probability that we are not alone.

Applying Drake's Equation

To calculate a realistic number of planets in our galaxy that currently have intelligent life, you have to plug in some realistic numbers. Also, we know that the correct answer (the solution) must be at least 1, because there is intelligent life on Earth (insert your own joke here), and must be no more than 250,000,000,000 (the number of stars in the Milky Way) times the average number of planets around stars that could support life.

When the equation was first introduced, only one of the terms could be estimated with any consensus among astronomers. R, the number of new stars produced in our galaxy each year, is believed to be about 10.

If R were known to be 10 in the 1960s, I guess the correct number of stars in our galaxy would be closer to 250 billion + 40.

In 1980, Carl Sagan, popularizer of astronomy, discussed the Drake equation in his television series and book, Cosmos. Because we knew less about the planets in our own solar system then and, more importantly, knew nothing about planets in other solar systems (or even if there were such things), Sagan's estimates for each value and his best-guess solution was somewhat speculative, but his answer was that about six million planets in the Milky Way at any given time have the technology to communicate with us.

Using what we know today, Table 6-24 provides one set of values that produces one possible answer. These values are taken from an essay in an October 2005 edition of Astrobiology Magazine (you probably have a copy on your coffee table) by Dr. Steven Soter of New York University. In some cases, I chose an exact value from Soter's discussion of a range of values.

Table One application of Drake's equation

Term	Estimates	Calculations
R	10 per year	10
Nh	.01 (1 planet out of 100 stars)	10x.01 = .10
Fl	1 (assuming Earth is representative)	.10x.10 = .10
Fi	.001 (Soter suggests "small fraction")	.10x.001 = .0001
Fc	.20	.0001x.20 = .00002
L	100,000 years	.00002x100,000 = 2

With these numbers, the equation estimates a total of two planets in the entire galaxy who could communicate with each other at any given time. Earth is one of those. What is the other?

As Sagan, Soter, and other authors point out, the number of planets in our galaxy that support advanced life at any given time depends on so many arbitrarily estimated factors that any little choice one makes when entering values dramatically changes the result. There is an important difference between six million possible friends and only two possible friends, but both estimates come from reasonable sets of assumptions.

Notice how the solution to the equation changes as you try different estimates for each component. If most groups of intelligent creaturessay, 80 percent, for exampleeventually produce civilizations, the number of smart planets jumps to eight. If the average number of planets around a star that could support life is actually 2 (as Sagan suggests), our 8 would jump to 1,600 planets.

Soter advises that different reasonable estimates could produce an answer between a couple thousand and so few that our own planet's radio capabilities make it a statistical improbability, placing us as the only advanced civilization across many thousands of galaxies.

Finding our Space Chums

One possible outcome of the Drake equation is that there are only two planets in our galaxy with advanced intelligent civilizations capable of sending and receiving radio waves. If we really have only one other potential cosmic pen pal, it will be tough to find him or her or it in such a large haystack of planets. So, what to do?

The current strategy in seeking new life and new civilizations is to scan the skies with microwave receivers. Radio signals have a wide range of spectrums. Some occur naturally, and others are a particularly narrow range that are believed to only be created artificiallysuch as from the transmission of Three's Company TV episodes, or by radar, for example. By paying particular attention to those signals that are within this supposed artificial spectrum, those who search for alien life forms hope to discover and isolate either the random output of an advanced civilization or, perhaps, intentional signals broadcast for the benefit of any interested observer.

If you own your own array of microwave listening stations, you'll want to tune them to the favored frequency for hunting life on other planets: 1.42 gigahertz. It is believed unlikely that any natural source would emit waves at that frequency.

The sky is big, though, and researchers use both targeted and convenience sampling techniques to decide where to look. The search strategy is to focus on a subpopulation of stars that meet two criteria:

• They are suns that share characteristics of our own.

• They are nearby (within a mere 100 light years of Earth).

Data Analysis

If the number of planets that could be emitting these key signals of life is very small (as some of the Drake equation permutations suggest), a search of this sample must be very thorough; otherwise, we might miss it. Statisticians would refer to this situation as a study that needs a great deal of power [Hack #8] because the effect size is so small.

There is so much data being collected as part of systematic efforts to scan the skies, no one person or even one computer can possibly analyze it all. You can help! SETI@home is a Berkeley University-based program that arranges for regular people with regular home or office computers to receive some of this data, so their computers can analyze it when they're not doing something else. SETI is the acronym for Search for Extraterrestrial Intelligence. The program works like a screensaver and can be downloaded for free at http://setiathome.berkely.edu.

The data won't make sense to you when you get it, but your computer will begin to use statistical analyses to sort through the signal information, looking for the telltale nonrandom narrow bandwidths that might mean another planet has reached the level of sophistication to produce something like Gomer Pyle or Melrose Place. You could be the first to discover life on other planets, so get to work!