How Does a Gaseous Sun Have Gravitational Pull on Solid Planets?

10/09/2017 01:31 pm ET
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How can the sun have gravitational pull on all the other planets if it's not even solid? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Viktor T. Toth, IT pro, part-time physicist, on Quora:

If you think the Earth is solid, please think again.

The diameter of the Earth is about 13,000 kilometers, give or take. The “solid” crust that you stand on? At most it is a few tens of kilometers thick. The rest is, surprise, surprise… in a molten, liquid state*.

Imagine a large beach ball. Fill it with water. That’s actually a surprisingly accurate analogy of what the Earth is like. Except that its skin is not even unbroken. It consists of pieces that slide over and under each other, and break from time to time. Which is why all that molten stuff from underneath gets to the surface all the time (in volcanoes) and which is why the “solid” skin is often not solid at all (think earthquakes).

Meanwhile, take the interior of the Sun. Technically, it is in a gaseous state, but this “gas” is actually many times thicker than concrete; its density far exceeds that of lead or uranium.

Fortunately, none of this has anything to do with gravity. Density, pressure, viscosity and similar factors are irrelevant. (Well, almost; in relativity theory, they do contribute tiny corrections. But let’s not go there.)

The only thing that really matters when it comes to gravity is mass. The gravitational field of the Sun is what it is because the mass of the Sun is roughly 300,000 times the mass of the Earth. The Sun’s gravitational field would not change if you suddenly managed to freeze the Sun solid. It makes no difference.

*Yes, I know about the solid (at least insofar as its ability to conduct shear waves is concerned) inner core and the stiff/rigid mantle that nonetheless behaves as a highly viscous fluid over long timescales. I just didn’t want to start a geology lecture. The bottom line is that the Earth is a near perfect sphere because over geologic timescales, it behaves as a mostly fluid sphere under its self-gravity.

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