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On the number of guns and planets out there, Part 1


Ta-Nehisi Coates recently asked his readership to "talk to him like he's stupid" about gun ownership rates in the US and in other countries. I really like it when he makes these requests. It's how I often feel about stories in the news, which make me feel like I'm walking in on the middle of a grown-up conversation. I need someone to talk to me like I'm stupid about Benghazi or the fiscal cliff. Fortunately, Slate and Salon are good sources for this sort of information, as is Andrew Sullivan.

Anyway, regarding gun ownership rates, the discussion that Ta-Nehisi sparked got a bit muddled over the question of guns per capita (number of guns per person) versus the number of guns per gun-owner. This is an important distinction. There are two ways to get 1 gun per person in a hypothetical town of 100 people. One way is to give a gun to every person in town. The other is to have one person in town with 100 guns.

Of course this whole discussion goes back to the most recent mass shooting. But when I read it, my mind drifted into a much nerdier direction, as it is wont to do. Call it a coping mechanism. Or just call me a nerd.

What I immediately thought about was the question of the number of planets in the Galaxy, which Jon Swift, myself and our collaborators touch on in a soon-to-be posted paper about planets around red dwarf stars in the Galaxy. Long story short, we come to the conclusion that there is one planet per star throughout the Galaxy. Given that the Galaxy has 200 billion stars, and that 70-80% of those stars are red dwarfs, that's a whole lot of planets! Of order 100 billion planets in our Galaxy...as an order-of-magnitude estimate...of the lower limit.

This many: 100,000,000,000+

But the way in which those planets are distributed throughout the Galaxy matters. Is that literally one planet per star, or no planets for most stars with dozens of planets around a few stars? The number we quote in our paper and in our upcoming press release makes for good press: billions and billions of planets! It's also good fodder for Drake Equation discussions (for what those conversations are worth). But from the standpoint of planet-hunting, we're more interested in the fraction of stars with at least one planet (think of it as the fraction of stars with planetary systems), and the number of planets per system.

In analogy to the gun discussion, it's the number of guns per capita, versus the number of gun-owners, versus the number of guns per gun-owner. The correct statistics depends on what question you want to answer. I'll get to the math of the matter in my next post.

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