Do Other Worlds Mean Other Life?

Maybe. How’s that for a definitive answer?

New planets around stars other than our sun (exoplanets) continue to be discovered almost weekly. If we include the 1200 or so from the last Kepler mission data release (see this earlier post), the total number is rapidly approaching 2000. And the longer we look, the more likely we are to find planets similar to Earth—relatively small rocky planets orbiting stable stars at a distance allowing liquid water to exist on the surface. Does that mean that these planets harbor life?

More than 50 years ago, astronomer Frank Drake strung together the information we would need to know how likely communication with extraterrestrial intelligence might be. He listed these factors in decreasing order of how well we know the numbers, from information we know fairly well (how many stars are there in the Milky Way) to things about which we have no clue (how long does a civilization that is capable of and interested in communicating actually survive). The great contribution of recent work to the “Drake Equation” is reducing the uncertainty in some of these factors.

There is more than one version of the Drake equation, but here are the factors:

1. Number of stars in the Milky Way galaxy (stars—and life—in other galaxies would be too distant to allow any meaningful communication).
2. Fraction of these stars that have planets around them.
3. Average number of planets capable of supporting life in each of these systems.
4. Fraction of these planets that actually do develop life.
5. Fraction of the above that develop intelligent life. Intelligence is defined as the ability to employ technology.
6. Fraction of the above that develop the technology necessary to communicate over interstellar distances.
7. Longevity of each such civilization.

What Kepler has done is to give us better insights into factors 2 and 3. Based on the preliminary data (and it is very preliminary), it seems that roughly half of all stars have planets around them, and roughly 1% of those have planets in the “habitable zone”. This is the region of space around a planet’s host star where the planet’s surface temperature is above the freezing point of water but below its boiling point, allowing liquid water to exist. We know of no forms of life (certainly no forms of intelligent life) that can exist in the total absence of liquid water. Perhaps this is a failure of imagination, but a conservative estimate seems most sensible here.

So how many habitable planets does that make for the Milky Way galaxy? The best estimate of the Kepler team is 500 million—half a billion. Within 1000 light years of Earth, a distance we can reach with present-day radio transmitters, there might be 10,000 to 30,000 such worlds, depending on our assumptions.

Does that mean the universe is teeming with life? Before you get too excited, realize that the answer to this question depends as well on factors 4 through 7. How well do we know these numbers? We don’t. We have a single example—life on Earth—and half-educated guesses about how likely are the processes giving rise to life. Intelligence doesn’t seem to be a necessary outcome of evolution, either. The dinosaurs did just fine, dominating the Earth for millions of years, and they didn’t have radio telescopes. Of course, if they had, perhaps they could have done something about that asteroid that wiped them out and allowed those little mammals to eventually supplant them!

The numbers one attaches to these factors seem more a matter of personal belief. In the absence of evidence, human beings generally convince themselves of what must be true, and argue the case with great passion. I recently sat in on a panel discussion where one participant asserted that there were intelligent extraterrestrials almost literally next door, while another stated with equal certainty that we were alone in the universe.

I am often asked whether I believe in extraterrestrial life. Scientists are routinely asked questions that begin “Do you believe…”, and depending on what side of bed we got out of that morning, we answer the question with more or less good grace. The most polite response begins with “The evidence indicates that…” Physical evidence is always the final criterion by which we judge a scientific theory. As I tell my students repeatedly every semester, the universe doesn’t care at all what you would like to be true. The job of a scientist is to follow the evidence wherever it leads. My favorite response to the “Do you believe?” question came from one of the best teachers I ever had. She pondered for a moment, then responded: “I choose to act as though it were true. I suppose you could call that belief.” I don’t think I can improve on that.

So—what truth do I choose to act on? I don’t know what the truth is in this case, but I think the search is well worth the effort. The implications of finding life of any sort, much less intelligent, communicating life, are simply enormous. In my next post, I want to talk about what some of those implications might be.