How Is Ground Coffee Like a Planet? Just Add Zero Gravity.

Bad Astronomy
The entire universe in blog form
Nov. 24 2013 8:00 AM

Planet Espresso

protoplanetary disk
A solar system's baby portrait: A disk of material around a star.

Artwork by Karen L. Teramura / UH IfA

A long time ago — 4.56 billion years ago, give or take an eon or two — the Earth upon which you currently live wasn’t here. Well, not all in one place, at least: It was actually scattered over trillions of cubic kilometers of space, occupying part of a dust-laden disk orbiting the just-forming Sun.

This disk was huge, hundreds of thousands of kilometers thick, and stretching for billions of kilometers out from the Sun. But it wouldn’t stay that way: Over time, pieces of it stuck together, getting bigger, aggregating more and more material. Eventually, those clumps stuck together to form ever-bigger lumps called planetesimals. Once they got big enough that gravity could start actively drawing material in, they grew into proper planets.


Describing this process in general isn’t hard, but nothing beats a good demo. I’ve often wondered if it were possible to do such a thing, and now I know the answer is yes. And of all places, it comes from my old friend Dan Durda, asteroid scientist, who performed a lovely and elegant demonstration of this on the TV show “Richard Hammond Builds a Planet”. The necessary ingredient that never occurred to me? Gravity. Or really, the removal thereof:

Nice. The coffee grounds have a slight charge to them, which makes them stick together into clumps, much like when you rub a balloon on your head and can stick it to a wall. This is actually an annoyance in some coffee grinders, because the electrostatically charged bits stick to the plastic walls of the grinder bucket, making it hard to get them out (and they get all over your hands and clothes).

On the other hand, that same irritating principle is what makes planets, or at least gives them a good start. I’d say that’s good grounds for dismissing the charge.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  



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