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If the sun were to go red giant wouldn't it cause all planets to move as well?


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17 minutes ago, inbreeding said:

I realize that the space is a vaccum but the laws of physics still are there with gravitational waves so in theory for every action there is an opposite reaction so that means that all planets will shift won't they?

The orbits of planets depend only on the total mass of the central body. If you change only its radius, all other things remaining equal, the planetary orbits will not be affected. This is a direct consequence of Birkhoff’s Theorem (using a simple Schwarzschild model).

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7 minutes ago, Markus Hanke said:

The orbits of planets depend only on the total mass of the central body. If you change only its radius, all other things remaining equal, the planetary orbits will not be affected. This is a direct consequence of Birkhoff’s Theorem (using a simple Schwarzschild model).

So the sun will absorb the planets like mercury and venus?

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6 minutes ago, inbreeding said:

So the sun will absorb the planets like mercury and venus?

Yes, exactly. Do note that at this point the total mass of the sun does change, so absorbing these planets will have a slight effect on all the other planets. I’m not sure whether the effect is large enough to really destabilise any other orbits - probably not.

Edited by Markus Hanke
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3 minutes ago, Markus Hanke said:

Yes, exactly. Do note that at this point the total mass of the sun does change, so absorbing these planets will have a slight effect on all the other planets.

Then explain why the planets are currently moving slightly more and more farther apart from the sun?

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6 minutes ago, inbreeding said:

Then explain why the planets are currently moving slightly more and more farther apart from the sun?

That’s because the sun slowly looses mass and angular momentum through radiation and emission of particles (‘solar wind’), so it’s total mass decreases over time, making planetary orbits larger. The effect is really small though.

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1 hour ago, Markus Hanke said:

The orbits of planets depend only on the total mass of the central body. If you change only its radius, all other things remaining equal, the planetary orbits will not be affected. This is a direct consequence of Birkhoff’s Theorem (using a simple Schwarzschild model).

strictly speaking, the motion in a circular orbit is determined by the sum of the masses of the planets and the central body

The speed (or the magnitude of velocity) relative to the central object is constant:[1]:30

{\displaystyle v={\sqrt {GM\! \over {r}}}={\sqrt {\mu \over {r}}}}v = \sqrt{ GM\! \over{r}} = \sqrt{\mu\over{r}}

where:

1 hour ago, inbreeding said:

So the sun will absorb the planets like mercury and venus?

Perhaps the Earth too

36 minutes ago, Markus Hanke said:

That’s because the sun slowly looses mass and angular momentum through radiation and emission of particles (‘solar wind’), so it’s total mass decreases over time, making planetary orbits larger. The effect is really small though.

To this effect, you can also add the influence of tides. It is not a solid body. Due to the tides, it gradually loses the angular momentum, which should be compensated by the removal of the planets. In addition, the rotation of the planets around their axis is gradually synchronized with their rotation around the Sun, and this effect also leads to an increase in the radii of the orbits of the planets. But the impact of these effects is also insignificant.

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13 minutes ago, SergUpstart said:

Perhaps the Earth too

 

Depending on the size that the sun gets to and the new orbit of the earth? Is the Earth situated in such a position that it could go either way, or is it more likely to get engulfed like Mercury and Venus?

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14 hours ago, SergUpstart said:

strictly speaking, the motion in a circular orbit is determined by the sum of the masses of the planets and the central body

Yes you are right - but I disregarded this (and other effects) here, for clarity and simplicity.

14 hours ago, Intoscience said:

Depending on the size that the sun gets to and the new orbit of the earth? Is the Earth situated in such a position that it could go either way, or is it more likely to get engulfed like Mercury and Venus?

I think current models suggest it will be Mercury and Venus only.

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