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Comment by ciphre | Hacker News Reader

So, instead of 200 billion galaxies, the number would be closer to 2 trillion galaxies. I thought the first number seemed like a lot, wow. I wonder how many stars there are if that is the case. At 100 billion stars per galaxy, the number would be 2×10²³ stars. I believe that's 2000 billion trillion.

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Multiply that with about 5-10 for planets. Mindboggling.

5-10 planets per star seems a little high.

Really? Considering how many bodies orbit our Sun, I would have thought 5-10 would be a good average.

This Quora answer (https://www.quora.com/What-is-the-average-number-of-planets-...) takes a dataset of 2560 stars and comes out with an average of 1.3 planets per star. As the author caveats, this is an extremely small sample and probably not accurate, but it's something to start from.

Caveat+: No one yet knows if we can even reliably count the planets of a distant star, so the figures could be off by some significant amount.

Caveat++: That light reaching earth from far away galaxies was then, this is now.

Caveat+++: We're also interested in moons, which I don't think have been detected yet.

Caveat++++: Factor in rogue planets that don't orbit any stars, or large asteroids drifting through nebulas etc...

I'm sure the number of solid bodies — potential habitats for life or artificial constructs — in our universe is magnitudes larger than the number of stars we've detected, and the latter is already a mind boggling quantity.

Thanks.

The way we detect planets only detect some of them.

The best method is to notice when a planet passes in front of its star, by how it gets slightly dimmer. That only works if its orbit happens to be directly between us and that star.

We can also detect them by a Doppler shift of the star, because the planet (if big enough) will move the star as it orbits. This works when the transit method fails, but provides fewer data on the planet itself.

It also only works for big enough planets close enough to the star.

Doesn't work when the orbit is perpendicular to our sight line either.

this would not show any planets beyond saturn for solar system, for example

I would tend to agree. The effective gravitational influence of a star would be relative to its mass. We can pretty easily determine the (rough) mass of a star, and ours appears(?) to be somewhere in the middle, so it stands to reason that 8-ish decently sized planets is not an unreasonable number.

Extrapolating from a single example? There's an XKCD for that: https://xkcd.com/605/

Then figure in the dwarf planets, asteroids, and comets, and we'll start to get to some big numbers.

And don't forget about invisible «almost stars», Jupiter like systems, which outnumbers visible stars (IMHO), with their own planets (satelites?) and asteroids.

Almost Avogadro's number - i.e., number of water molecules in a small cup of water (18ml/0.6oz).

8oz = small cup. 0.6 oz ~= table spoon (15ml)

That doesn't hold I'm afraid. The newly discovered galaxies are small and dim, so won't change the numbers very significantly. It's not as radical a discovery as it appear at first glance.

2 trillion times 100 billion is 200 billion trillion. Your scientific notation is correct, though.

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