Musings on optics, physics, astronomy, technology and life

Way back when I was a graduate student in astronomy, a schoolboy asked me a question that I never forgot. It took scientists only another 20 years or so to come up with a reasonable answer to it.

For part of my teaching assistantship during my first year of grad school, I was assigned the public observatory program. I had to recruit faculty members and postdocs to give twice-monthly public talks, handle reservations for school and Scout groups who wanted their own private presentations, and corral fellow grad students into running the slide projector (before the days of PowerPoint) and helping with the telescopes. (I used to suggest that all grad students in astronomy should get an automatic master’s degree in slide projectorology.)

One evening a school group showed up for its talk an hour before the public lecture, and for some reason I gave them a presentation. I can’t remember why — perhaps someone had got sick or even forgot to show up. And I don’t even remember what I talked about — maybe the solar system or something really basic like the differences between stars and planets.

Anyway, when I was fielding questions after the talk, one boy — maybe about 10 years old — asked: “How many planets are there in the entire galaxy?”

“Ooh,” I said, trying to stall for my time while my mind raced. This happened to be after the discovery of a planet around a pulsar, but before the teams of Mayer & Queloz and Marcy & Butler had found any planets around “normal” (that is, main-sequence) stars. At the time I was really, really interested in the possibility of finding extrasolar planets.

“That’s a really good question,” I told the kid. “In fact, it’s such a good question that scientists are still trying to answer it!” I explained that astronomers were very busy trying to search for extrasolar planets but the search was really difficult, and maybe, just maybe, in future years they would be able to start answering that question.

Well, the exoplanet discoveries started to roll in a couple of years after that evening’s Q&A session. Currently, astronomers know of more than 1,000 actual extrasolar planets, with thousands more candidates awaiting confirmation.

Finally, just this week, astronomers came up with the first reasonable estimate of just how many Earth-like planets may occupy our galaxy: 40 billion. That’s 40,000,000,000. That’s how many potentially habitable worlds may be out there, in our own little spiral clump of stars, without even crossing intergalactic space to get to the billions and billions of other galaxies out there.

The thought takes my breath away.

And I keep thinking of that boy’s question. I have no idea what this kid’s name was, or where he went to school. Doubtless he is an adult by now, and wherever he is, I hope that he read that news story and realized that, after all these years, he finally got his answer.

You can read the full scientific paper here and New York Times writer Dennis Overbye’s coverage of it here.


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