Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Tuesday, January 25, 2011

Alone again, naturally

Today's London Telegraph features a story in which Dr. Howard Smith, senior astrophysicist at Harvard University, has stated that alien life is almost certainly impossible based upon the conditions on the exoplanets so far discovered.

"We have found that most other planets and solar systems are wildly different than our own," Smith was quoted as saying.  "They are very hostile to life as we know it."

So according to Smith, we're... all alone.  *cue sad music*

Hang on a second.  I think, Dr. Smith, that we may not want to resign ourselves to being Lost in Space quite yet.

We have, at last check, found about five hundred exoplanets.  Most of them are of the "hot Jupiter" variety -- large, probably gaseous planets, orbiting very close to their sun.  The reason that we've preferentially found those has nothing to do with their being likely to be more common in the universe; it's that they're easier to find, because they create a greater gravitational perturbation of the star they orbit.  Small, rocky worlds, such as the Earth, are harder to detect, although we're beginning to be quite good at that, too.  When the data from NASA's Kepler satellite is released in a few weeks, it is expected to include information about hundreds of additional, newly-discovered exoplanets.

However, there's a far bigger problem with Dr. Smith's statement than this.

I must say that I would not have expected a prominent scientist to make quite such a catastrophically faulty inference.  In order to make an inference, you're supposed to take into account a very important factor -- what your sample size is.  Dr. Smith's mistake is analogous to someone going through my house, and finding that I own twelve flutes, recorders, tinwhistles, and so on, and concluding that that there must be 80 billion wind instruments on the planet Earth.

"Wait," you might be saying.  "That's a bad comparison -- no one would be so foolish as to take a sample size of One Person and extrapolate it to all 6.7 billion people on Earth."

Okay, fine.  Point made.  Let's see how Dr. Smith's inference compares to that one.

There are something on the order of a hundred billion galaxies in the universe, and each of those has maybe a billion stars.  (This information is from Cornell's astrophysics faq website, if you're wondering about my sources.)  This means that there are potentially one hundred billion billion -- that's ten followed by twenty zeroes -- stars in the observable universe...

... and we've surveyed about a thousand of them.

So Dr. Smith's sample size is a thousand, out of a hundred billion billion.  This comes to not one out of 6.7 billion, which was the sample size in my ridiculous inference about wind instruments, but one out of one hundred million billion.

There are so many stars in the universe that if we surveyed one star system every second of every day; 365 days a year; no time off for holidays; no potty breaks, for cryin' out loud -- it would take one hundred trillion years to finish.

And Dr. Smith thinks a thousand stars is an adequate sample size to conclude that we're alone in the universe?  Oh, the pain, the pain...!

It's a little like my climbing on to my roof, and looking around, and saying, "No wombats in my back yard!  No wombats in my neighbor's yard!  In fact, no wombats anywhere to be seen!  I guess wombats don't exist."

I know it's tempting to draw conclusions quickly; patience is not a notable human trait.  However, a mark of a skeptical mind is the willingness to suspend belief and disbelief -- to be completely comfortable with saying, perhaps indefinitely, "the jury's still out on this one."  I am frequently asked by students if I "believe" in various things -- UFOs, bigfoot, ghosts, the Loch Ness Monster, god...  and my usual answer is, "I neither believe in, nor disbelieve in, anything for which I have no concrete evidence of any kind.  If you want, however, we can discuss how likely I think those things are."

All of which brings me to a comment I've made before; the world would be a far better place if people had more facts and fewer beliefs.

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