<p>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.</p>
<p>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 <span class="math-container">$\omega$</span>, orbital distance (semi-major axis) <span class="math-container">$a$</span> and mass of the Sun <span class="math-container">$M$</span>, are related by
<span class="math-container">$$\omega \propto M^{1/2} a^{-3/2}\ . $$</span>
The angular momentum of a planet is proportional to <span class="math-container">$\omega a^2$</span> and so the quantity <span class="math-container">$(M a)^{1/2}$</span> is fixed.</p>
<p>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 orbital radii (semi-major axes), barring engulfment, orbital instabilities or tidal effects. This means that the separation between the surviving planets increases too, by the same factor.</p>
<p>About 50% of the solar mass loss
will occur during the ascent of the initial red giant branch in around 7 billion years time. The other 50% occurs during the short asymptotic red giant branch phase after core helium core burning ends.</p>
<p>The only force involved here is the gravitational force between Sun and planet, which gets weaker as the Sun loses mass.</p>

CC BY-SA 4.0
2021-11-01T09:22:50.527Z
Does that mean that the planets will be sucked in closer to smaller orbits than before?
Constantthin
CC BY-SA 4.0
2021-11-01T10:32:42.283Z
@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" ?
ProfRob
CC BY-SA 4.0
2021-11-01T11:03:21.770Z
M= the sun's mass. But what does "a" stand for? And please what does "separation" mean?, Does it mean "orbit"?
Constantthin
CC BY-SA 4.0
2021-11-01T14:04:43.650Z
@Constantthin $a$ is defined in my answer already. Separation means the distance between two objects.
ProfRob
CC BY-SA 4.0
2021-11-01T14:32:11.523Z
Ok. So if I am understanding you correctly then the orbits of the planets will increase in direct relation to decreasing solar mass. Right?
Constantthin
CC BY-SA 4.0
2021-11-01T21:14:43.073Z
@Constantthin Mercury, Venus, and Earth may be engulfed and destroyed in the red giant phase, but the other planets will orbit further away.
Ryan_L
CC BY-SA 4.0
2024-09-23T00:32:00.730Z
Ok. So the outer planets will orbit at an increased distance from the sun, but will the distance between the outer planets increase as well?
Constantthin
CC BY-SA 4.0
2024-09-23T08:21:38.773Z
@Constantthin Yes. Answer clarified.
ProfRob
CC BY-SA 4.0
2021-11-01T08:04:33.847Z
2024-09-23T06:47:34.320Z
2024-09-23T06:47:34.320Z
ProfRob
<p>When the Sun becomes a red giant will the planets be pushed outwards towards the periphery of the Solar System?</p>
<p>If so, will this happen simultaneously for all planets or could, for example, a planet like Uranus move closer to Neptune so decreasing the distance between the two?</p>
<p>If not, why would the orbits of the planets not be changed? What cosmic forces are involved keeping them in place?</p>

CC BY-SA 4.0
2021-11-01T05:30:28.093Z
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.
Justin T
CC BY-SA 4.0
2021-11-01T05:43:51.757Z
@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.
Constantthin
CC BY-SA 4.0
2021-11-01T05:45:04.830Z
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
Justin T
CC BY-SA 4.0
2021-11-01T06:06:49.443Z
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.
Constantthin
CC BY-SA 4.0
2021-11-01T06:31:08.770Z
But *where* is the claim made? If there is various, it should be easy to supply source info?
planetmaker
CC BY-SA 4.0
2021-11-01T06:34:32.163Z
@Planetmaker. I am trying to find it as we speak. I think it could have been in a comment.
Constantthin
CC BY-SA 4.0
2021-11-01T07:39:57.947Z
@Planetmaker. Couldn’t find it. Rewrote the question instead.
Constantthin
CC BY-SA 4.0
2024-09-10T16:22:15.807Z
Not quite: https://astronomy.stackexchange.com/questions/58287/what-happens-to-planets-after-they-get-swallowed-by-red-giants
Nilay Ghosh
<the-sun><solar-system><red-giant><neptune><uranus>
Will the planets be pushed outwards when the Sun becomes a red giant?
Constantthin
# Will the planets be pushed outwards when the Sun becomes a red giant?

(Asked by: username_0 on 2021-11-01)

When the Sun becomes a red giant will the planets be pushed outwards towards the periphery of the Solar System?

If so, will this happen simultaneously for all planets or could, for example, a planet like Uranus move closer to Neptune so decreasing 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?

**Tags:** the-sun, solar-system, red-giant, neptune, uranus

> **Comments:**
> **username_1**: 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.  
> **username_0**: @username_1ackett. 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.  
> **username_1**: 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  
> **username_0**: 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.  
> **username_2**: But *where* is the claim made? If there is various, it should be easy to supply source info?  
> **username_0**: @Planetmaker. I am trying to find it as we speak. I think it could have been in a comment.  
> **username_0**: @Planetmaker. Couldn’t find it. Rewrote the question instead.  
> **username_3**: Not quite: https://astronomy.stackexchange.com/questions/58287/what-happens-to-planets-after-they-get-swallowed-by-red-giants  

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## 1 answers

### Answer by username_4 on 2021-11-01. Score: 5

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 (semi-major axis) $a$ 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 orbital radii (semi-major axes), barring engulfment, orbital instabilities or tidal effects. This means that the separation between the surviving planets increases too, by the same factor.

About 50% of the solar mass loss
will occur during the ascent of the initial red giant branch in around 7 billion years time. The other 50% occurs during the short asymptotic red giant branch phase after core helium core burning ends.

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

> **Comments:**
> **username_0**: Does that mean that the planets will be sucked in closer to smaller orbits than before?  
> **username_4**: @username_0 $M$ 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" ?  
> **username_0**: M= the sun's mass. But what does "a" stand for? And please what does "separation" mean?, Does it mean "orbit"?  
> **username_4**: @username_0 $a$ is defined in my answer already. Separation means the distance between two objects.  
> **username_0**: Ok. So if I am understanding you correctly then the orbits of the planets will increase in direct relation to decreasing solar mass. Right?  
> **username_5**: @username_0 Mercury, Venus, and Earth may be engulfed and destroyed in the red giant phase, but the other planets will orbit further away.  
> **username_0**: Ok. So the outer planets will orbit at an increased distance from the sun, but will the distance between the outer planets increase as well?  
> **username_4**: @username_0 Yes. Answer clarified.  

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