Saturday, September 16, 2017

stars and planets

I looked up some numbers for Aubrey. From the amazing Wikipedia...

The Milky Way galaxy has a diameter between 100 000 and 180 000 light-years. I didn't realise it was so indeterminate. The number 100 000 had been in my brain last night. But it could be double. That affects my distance remark. Last night I had said that all the stars we could see would be a fraction of 100 000 light years away. I have to revise that to 180 000. All the stars we can see are 180 000 light years or less. And that we are about 25 000 light years from galactic centre, then the (current) furthest extent is 115 000 light years.

Our galaxy may contain 100 to 400 billion stars. Wow.

Then I found the note: "There are probably at least 100 billion planets in the Milky Way." That's worded weakly, I think. And it's a number I see as changeable. As our exoplanet research evolves and our instruments get better, we're going to find more planets around stars. As a thought experiment, let's consider the 200 billion stars harbour solar systems and the average number of planets in a solar system is 10, then that means we're in the 2 trillion range.

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