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When the sun goes red giant will the planets be pushed outwards towards the periphery of the solar system?

If so, will this happen simultaneously to all planets or could, for example, a planet like Uranus move closer to Neptune so minimising the distance between the two?

If not, why would the orbits of the planets not be changed? What cosmic forces are involved keeping them in place?

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  • $\begingroup$ Could you reference the post that says this? Maybe I'm missing something here, but I believe the current understanding is that the orbits of the planets will be unaffected and that those within the radius of the red giant will simply be consumed (and thus destroyed); since the mass and center of mass of the sun would not change, I don't see any reason for orbits to be affected. $\endgroup$
    – Justin T
    Nov 1, 2021 at 5:30
  • $\begingroup$ @Justin Tackett. I have come across both the two views. But couldn’t they both be true? That the planets will be pushed outwards, but the inner planets will be outrun by the expanding sun going red giant. Remember seeing Sherlock Holmes and his friend trying to outrun an imminent explosion on tv the other day. Although avoiding annihilation they were suddenly knocked to the ground from behind, suffering non-lifethreatening burns. $\endgroup$
    – Constantthin
    Nov 1, 2021 at 5:43
  • $\begingroup$ I guess my main concern is the mechanism by which they would be pushed out; surely the flux of the Sun would decrease during this expansion (since this is radiation per square meter) and even if it didn't I think it would be far fetched to say radiation pressure could affect orbits, and an expanding, isolated body like the sun would not have any repulsive dynamical effects that I can think of. I just don't see how, other than a rise in temperature, outer planets would be affected at all $\endgroup$
    – Justin T
    Nov 1, 2021 at 5:45
  • $\begingroup$ Fair enough. // My reference is probably a casual one. Not a peer reviewed scientific paper. One couldn’t rely on it and I think I would have difficulty finding it anyway. But I have come across the same thought on various websites when surfing around during the years. $\endgroup$
    – Constantthin
    Nov 1, 2021 at 6:06
  • $\begingroup$ But where is the claim made? If there is various, it should be easy to supply source info? $\endgroup$
    – planetmaker
    Nov 1, 2021 at 6:31

1 Answer 1

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The Sun loses mass all the time. That mass loss will accelerate as it ascends the red giant branch and ultimately the Sun will end up as a white dwarf of about half a solar mass.

Since the solar wind exerts almost no torque on the planets, their angular momentum is a fixed quantity. From Kepler's third law, we know that the angular velocity $\omega$, orbital distance $a$ and mass of the Sun $M$, are related by $$\omega \propto M^{1/2} a^{-3/2}\ . $$ The angular momentum of a planet is proportional to $\omega a^2$ and so the quantity $(M a)^{1/2}$ is fixed.

That means that the orbital radii of all the planets are inversely proportional to the mass of the Sun. Thus all planets will orbit a future white dwarf Sun at about twice their current separation, barring engulfment, orbital instabilities or tidal effects.

The only force involved here is the gravitational force between Sun and planet, which gets weaker as the Sun loses mass.

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  • $\begingroup$ Does that mean that the planets will be sucked in closer to smaller orbits than before? $\endgroup$
    – Constantthin
    Nov 1, 2021 at 9:22
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    $\begingroup$ @Constantthin $M$ goes down, so $a$ increases to keep $(Ma)^{1/2}$ constant. "Thus all planets will orbit a future white dwarf Sun at about twice their current separation" ? $\endgroup$
    – ProfRob
    Nov 1, 2021 at 10:32
  • $\begingroup$ M= the sun's mass. But what does "a" stand for? And please what does "separation" mean?, Does it mean "orbit"? $\endgroup$
    – Constantthin
    Nov 1, 2021 at 11:03
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    $\begingroup$ @Constantthin $a$ is defined in my answer already. Separation means the distance between two objects. $\endgroup$
    – ProfRob
    Nov 1, 2021 at 14:04
  • $\begingroup$ Ok. So if I am understanding you correctly then the orbits of the planets will increase in direct relation to decreasing solar mass. Right? $\endgroup$
    – Constantthin
    Nov 1, 2021 at 14:32

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