<p>The star won't notice.</p>
<p>The planet itself might very slowly (rather slowly, see below) evaporate by the intense insolation and might reduce its orbit slightly (<a href="https://doi.org/10.1051/0004-6361/202244180" rel="nofollow noreferrer">though this seems to not be a big issue either, Temmink and Snellen (2022)</a>)</p>
<p>The duration of this process is more defined by the evolution of the star itself (it doesn't become a red giant instantanously) than the planet itself. As a star does not have a well-defined surface, the closer to the photosphere the planet gets, the more it will be exposed to the atmosphere of the star, slowly heating the planet and slowly degrading its orbit, thus increasing the speed of the planets heat-up evaporation especially of the more volatile elements.</p>
<p>We can do a brief estimate to possibly see which effects must dominate:</p>
<p>Let's assume that the Sun has to evaporate the complete planet Earth. At the red giant phase, the Sun has about <a href="https://faculty.wcas.northwestern.edu/infocom/The%20Website/end.html" rel="nofollow noreferrer">3000x the current luminosity</a>. Let's also assume that the whole Earth consists of iron. Iron has a heat capacity of 0.45J/g/K or 25 J/mol/K and requires an evaporation energy of 350kJ/mol. Earth has a mass of about <span class="math-container">$6\cdot 10^{24}$</span>kg or about <span class="math-container">$10^{26}$</span> mol of iron (again, if its mass were entirely only in iron). Iron evaporates at about 2900°C, so we have to heat the iron to that temperature and then evaporate it.
Thus per mol it needs (assuming our comfy 300K starting temperature):
<span class="math-container">$$ E_{total} = Q_{heatup} + Q_{evaporate} \\ = 2600 K\cdot 25 \frac{J}{mol\cdot K} + 350000 \frac{J}{mol} \\ = 415\frac{kJ}{mol} = 4.15\cdot 10^{5}J/mol$$</span>
So for the whole Earth to evaporate we would thus need about <span class="math-container">$4\cdot 10^{31}J$</span>.
Assuming 3000 solar constants of insolation and an Earth cross-section of <span class="math-container">$\pi\cdot 6371^2 km^2$</span>, Earth then receives about <span class="math-container">$2\cdot 10^{10}$</span>W.
Thus it would take about <span class="math-container">$10^{21}$</span>s to evaporate Earth, and that is about ... <span class="math-container">$10^{13}$</span> years... longer than the age of the universe.</p>
<p>Thus what will have to happen is that the effect of the planet slowly spiraling in due to the interaction with the red giant's outer atmosphere has to play a role (e.g. see the above linked paper). Whether that happens quicker than the red giant phase is another thing one will have to estimate similarily and depends sensitively upon the exact difference in the planet's orbit and the actual size of the red giant... the planet might simply end up as a rocky iron ball with blast-furnace temperatures in a close-in orbit around the red giant.</p>

CC BY-SA 4.0
2024-08-09T12:55:38.643Z
Please expand upon what you mean by "slowly evaporate" and "slowly degrading its orbit". I don't think either is an accurate description.
ProfRob
CC BY-SA 4.0
2024-08-09T13:06:53.660Z
@ProfRob fair comment. I amended a (very rough) guestimate on a possible evaporation time - I'm surprised by the length tbh. I shall ponder how to estimate the orbit decay time given the environment conditions.
planetmaker
CC BY-SA 4.0
2024-08-09T13:15:12.050Z
The more I did, the more I believe we just have to live with hot rocky-iron balls in close-in orbits :)
planetmaker
CC BY-SA 4.0
2024-08-09T13:33:27.787Z
Well, I suspect there are pieces of published research that make reasonable models and calculations of these issues. You need to discuss tidal dissipation in terms of orbital decay - which I suspect is a rather rapid process.
ProfRob
CC BY-SA 4.0
2024-08-09T18:05:43.190Z
I would also have thought that planet-plasma interactions (collisions) would add to the energy deposition (and orbital decay) of the planet.
Jon Custer
CC BY-SA 4.0
2024-08-10T04:53:30.130Z
This analysis seems to assume the planet is close to, but outside, the photosphere. But the question, as I read it, is assuming the photosphere is past the planet. Can a planet orbit for the duration of the red giant phase while engulfed by the star?
Mark Foskey
CC BY-SA 4.0
2024-08-11T02:27:45.933Z
I could be wrong, but you seem to be assuming that the planet is evaporated by the light coming from the star.  The Sun has an astonishingly huge amount of thermal energy already, as does the core of Earth.  A not insignificant fraction of the planet would boil off instantly if it didn't have thousands of kilometers of rock on top, and the energy delivery from the Sun would also include the thermal energy of the plasma itself.  It's tenuous, but there's an awful lot of it, and Earth's corpse would continually be attracting more.
Jason Patterson
CC BY-SA 4.0
2024-08-09T12:37:53.387Z
2024-08-09T13:14:46.530Z
2024-08-09T13:14:46.530Z
planetmaker
<p>You often hear about the Sun going to be a red giant in a few billion years and engulfing Mercury, Venus and finally Earth. But what happens to the planets after they get engulfed?</p>
<p>How long will they stay intact inside the star?
How fast will their orbit decay and sink towards the core?
What depths and pressures will they &quot;survive&quot;?
What mechanisms will destroy the planets? Will they get slowly eroded away by high windspeeds or heated until they melt/vaporize and disintegrate?
What else happens to them or the star?</p>

CC BY-SA 4.0
2024-08-09T15:04:42.817Z
Recently asked: https://astronomy.stackexchange.com/questions/58259/has-anyone-simulated-how-long-the-earth-would-last-if-when-engulfed-by-our-sun-t
Nilay Ghosh
<stellar-evolution><red-giant><stellar-dynamics>
What happens to planets after they get swallowed by red giants?
airace3
# What happens to planets after they get swallowed by red giants?

(Asked by: username_0 on 2024-08-09)

You often hear about the Sun going to be a red giant in a few billion years and engulfing Mercury, Venus and finally Earth. But what happens to the planets after they get engulfed?

How long will they stay intact inside the star?
How fast will their orbit decay and sink towards the core?
What depths and pressures will they "survive"?
What mechanisms will destroy the planets? Will they get slowly eroded away by high windspeeds or heated until they melt/vaporize and disintegrate?
What else happens to them or the star?

**Tags:** stellar-evolution, red-giant, stellar-dynamics
> **Comments:**
> **username_1**: Recently asked: https://astronomy.stackexchange.com/questions/58259/has-anyone-simulated-how-long-the-earth-would-last-if-when-engulfed-by-our-sun-t  

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## 1 answers

### Answer by username_2 on 2024-08-09. Score: 0

The star won't notice.

The planet itself might very slowly (rather slowly, see below) evaporate by the intense insolation and might reduce its orbit slightly ([though this seems to not be a big issue either, Temmink and Snellen (2022)](https://doi.org/10.1051/0004-6361/202244180))

The duration of this process is more defined by the evolution of the star itself (it doesn't become a red giant instantanously) than the planet itself. As a star does not have a well-defined surface, the closer to the photosphere the planet gets, the more it will be exposed to the atmosphere of the star, slowly heating the planet and slowly degrading its orbit, thus increasing the speed of the planets heat-up evaporation especially of the more volatile elements.

We can do a brief estimate to possibly see which effects must dominate:

Let's assume that the Sun has to evaporate the complete planet Earth. At the red giant phase, the Sun has about [3000x the current luminosity](https://faculty.wcas.northwestern.edu/infocom/The%20Website/end.html). Let's also assume that the whole Earth consists of iron. Iron has a heat capacity of 0.45J/g/K or 25 J/mol/K and requires an evaporation energy of 350kJ/mol. Earth has a mass of about $6\cdot 10^{24}$kg or about $10^{26}$ mol of iron (again, if its mass were entirely only in iron). Iron evaporates at about 2900°C, so we have to heat the iron to that temperature and then evaporate it.
Thus per mol it needs (assuming our comfy 300K starting temperature):
$$ E\_{total} = Q\_{heatup} + Q\_{evaporate} \\ = 2600 K\cdot 25 \frac{J}{mol\cdot K} + 350000 \frac{J}{mol} \\ = 415\frac{kJ}{mol} = 4.15\cdot 10^{5}J/mol$$
So for the whole Earth to evaporate we would thus need about $4\cdot 10^{31}J$.
Assuming 3000 solar constants of insolation and an Earth cross-section of $\pi\cdot 6371^2 km^2$, Earth then receives about $2\cdot 10^{10}$W.
Thus it would take about $10^{21}$s to evaporate Earth, and that is about ... $10^{13}$ years... longer than the age of the universe.

Thus what will have to happen is that the effect of the planet slowly spiraling in due to the interaction with the red giant's outer atmosphere has to play a role (e.g. see the above linked paper). Whether that happens quicker than the red giant phase is another thing one will have to estimate similarily and depends sensitively upon the exact difference in the planet's orbit and the actual size of the red giant... the planet might simply end up as a rocky iron ball with blast-furnace temperatures in a close-in orbit around the red giant.

> **Comments:**
> **username_3**: Please expand upon what you mean by "slowly evaporate" and "slowly degrading its orbit". I don't think either is an accurate description.  
> **username_2**: @ProfRob fair comment. I amended a (very rough) guestimate on a possible evaporation time - I'm surprised by the length tbh. I shall ponder how to estimate the orbit decay time given the environment conditions.  
> **username_2**: The more I did, the more I believe we just have to live with hot rocky-iron balls in close-in orbits :)  
> **username_3**: Well, I suspect there are pieces of published research that make reasonable models and calculations of these issues. You need to discuss tidal dissipation in terms of orbital decay - which I suspect is a rather rapid process.  
> **username_4**: I would also have thought that planet-plasma interactions (collisions) would add to the energy deposition (and orbital decay) of the planet.  
> **username_5**: This analysis seems to assume the planet is close to, but outside, the photosphere. But the question, as I read it, is assuming the photosphere is past the planet. Can a planet orbit for the duration of the red giant phase while engulfed by the star?  
> **username_6**: I could be wrong, but you seem to be assuming that the planet is evaporated by the light coming from the star.  The Sun has an astonishingly huge amount of thermal energy already, as does the core of Earth.  A not insignificant fraction of the planet would boil off instantly if it didn't have thousands of kilometers of rock on top, and the energy delivery from the Sun would also include the thermal energy of the plasma itself.  It's tenuous, but there's an awful lot of it, and Earth's corpse would continually be attracting more.  

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