Billions and Billions of Earths
Date: February 15, 2009 | Author: Steven NovellaCategory: General Science | Comments: 2 » | Tags: drake equation, life
At the recent AAAS (American Associations for the Advancement of Science) meeting in Chicago, Dr. Alan Boss of the Carnegie Institution of Science gave a talk in which he claimed that there are probably 100 billion earth-like planets in the Milky Way Galaxy. He arrives at this number by extrapolating from the number of stars in the Milky Way – 200-400 billion is the current estimate, but some argue this figure could be higher. And from the number of exoplanets discovered (340 so far). I don’t know the specific calculations he used, but he estimates that there is one earth-like planet per stellar system.
This kind of speculation is part of what is called the Drake Equation, after Dr. Frank Drake, which is the formula for calculating the number of technological civilizations in the galaxy or the universe. Some have criticized the Drake Equation of being pseudoscience, but I think this misses the point. The equation simply lays out those variables that would need to be known in order to make such a calculation – number of stars, percentage with earth-like planets, percentage of those planets with life, etc. Using the equation to calculate a specific estimate of the number of technological civilizations is mostly speculation, not hard science – but the equation itself is legitimate.
In the AAAS presentation Boss was focusing on one variable in that string of variables – the percent of stars with earth-like planets. However he has taken on the entire Drake Equation in the past – he is the author of The Crowded Universe, a book in which he argues that we will soon find many earth-like planets and we will discover that life is common in the universe. Although, he argues, it is probably mostly simple life.
He may be right – I hope he is. The universe will be eve more interesting if it is replete with life. I do not think, however, that we currently have enough information to speculate as confidently as he has.
How many earth-like planets are out there is somethin that we will soon have much more information about. In March the Kepler mission will launch. This is a satellite that has the capacity to fin earth-sized planets around nearby stars. Once we start actually conducting a survey with equipment sensitive enough to do this, we will be able to say with much more confidence how many earths are out there. It seems that in a few years we will start finding them, and in about a decade we will be able to say within reasonable error bars how many earths there probably are in the Milky Way.
Plugging this more acurate figure into the Drake Equation will certainly help, but is still a long way away from knowing how much life there is out there. I always took for granted that our solar system is probably fairly representative. This has proven to be a reasonable assumption. It could be wrong, we could be the victims of a statistical fluke and just happen to inhabit an unusual system. Or perhaps life can only exist in an unusual system. Maybe life by definition exists in rare systems. Even still, one earth per stellar system has long been a default assumption, so if this turns out to be correct no one will be surprised and this will not have rocked the Drake Equation.
The other variables in the Drake Equation always seemed much more speculative to me. How likely is it for life to form in a suitable environment. Are we the result of an incredibly unlikely sequence of events, or does life rapidly and inevitably appear. Or – does life already seed the universe (frozen in chunks of ice) and rain down on all newly forming worlds? Life did appear within millions of years of earth’s surface cooling. While this is a long time, it is fairly rapid on planetary time scales. This suggests life is common – but still there is great uncertainty. It’s just hard to extrapolate from one point of datum.
That is why it would be so fascinating to find life on Mars or perhaps on Europa or another moon of Jupiter or Saturn. If life sprung up in even one more location in our solar system that means it is probably not exceedingly rare or unlikely (unless of course it arose in one location and then seeded the other).
Even if life is common, we have no idea how often life will evolve a complex central nervous system which further evolves the collection of skills that leads to the kind of intelligence required for advanced technology. Again – trying to extrapolate from our one source of information, earth, it seems like this is very unlikely. Only one twig of the vast evolutionary bush (ours) managed to do this over several billion years of evolution.
This is why Boss speculates that life is common, but probably mostly simple life and not technological, or even multicellular. Bacteria may rule the universe.
I find such speculation interesting, and it is a completely legitimate scientific endeavor, as long as one does not pretend the error bars are smaller than they are. Also, at least in the case of the number of earth-like planets in the galaxy, it can be tested. In the next decade we will put Boss’s predictions to the test. I do look forward to the day I can blog about the discovery of the first truly earth-like exoplanet being discovered.
2 Responses to “Billions and Billions of Earths”
By irishjazz on Feb 17, 2009 | Reply
Cool story.
While 25-50% of stars having earth-like planets- whatever that means (size? water? location? Certainly not single-star systems…)- seems optimistic, even a few hundred million hypothetically life-supporting planets would be significant.
An interesting question, inherent in your article, is whether or not there is a natural directionality to evolution. Once life exists, is there a tendency to perfect environmental responses, which lead to anticipatory behaviors and eventually to environmental modification, tools, technology? Or is this bias toward “progress” just a cultural illusion?
Of course this is a question that wildly expands the error bars.
By Brian on Feb 18, 2009 | Reply
100 billion earth-like planets in a galaxy of 200-400 billion stars (give or take) seems high to me. I can’t put my finger on it, and have no data or evidence to back this up, but I will be surprised if we eventually find it is that high. If we find I am wrong, I will be the first to admit it and “WHOOP” with joy! I want to be wrong. I hope that any form of life is common…though I do doubt it is intelligent in it’s most common form(s).
@irishjazz: I would argue that your are correct, and that the aspects of evolution that rely on natural selection should indeed tend toward more “perfect” environmental responses. I say that in the sense that more “perfect” responses or traits are those which lend themselves to survival of the individual; which should translate directly to better survival of those in the species that exhibit (and ultimately inherit) that response/trait. I could drag this out, but will attempt not to do that: I don’t think it is a cultural illusion. I think it is simply … evolution. I think it more likely our culture is, in a sense, more a product of our evolution than the reverse. “Tools” gave a survival advantage, thus we continued to survive since they helped make us one of the “fittest”, thus we developed better tools. I would argue that this, in a sense, equates to scientific progress. The sientific method is the best survival tool we have come across to date (IMHO).
[Put error bars to that! LOL; they probably would have to be the size of the "bar" at the center of the Milky Way...;P]