<p>The exact fate of Earth as the sun becomes a red giant is somewhat uncertain because of two factors.</p>
<p>The first is that as the sun expands, its surface gravity <span class="math-container">$GM_\odot/R_\odot$</span> will decrease and gas can more easily escape as solar wind. This will reduce the mass of the sun, making Earth's orbit spiral outwards. But how much expansion depends on details of the far future solar wind we do not know yet, and this makes it slightly uncertain whether Earth can get into an orbit beyond the maximal solar radius. (The gas will also produce some drag, but it is likely negligible in this case)</p>
<p>The second factor is tidal interactions. As the sun expands it rotates more slowly, and in particular will rotate slower than Earth's orbital speed. That means that the tidal bulge induced by Earth will always be trailing the planet, creating a backward pull slowing it and dragging it inwards. This effect is small when the surface is far away, but grows rapidly as it approaches. Hence the sun can gobble up Earth even if the orbit is larger than the solar radius - but much depends on somewhat weakly understood tidal interactions.</p>
<p>Once the planet is inside the envelope it will rapidly get dragged in. To get a timescale, consider the time to push aside an Earth mass of gas: <span class="math-container">$\tau = M_\oplus /  \pi R_\oplus^2 \rho v$</span>. For <span class="math-container">$\rho=$</span> 0.1kg/m<span class="math-container">$^3$</span> I get <span class="math-container">$\tau=0.4972
$</span> years. Maybe a bit longer if the orbit got wider and slower. Since the density of Earth is much higher than the gas, <a href="https://academic.oup.com/mnras/article/425/4/2778/1077595" rel="noreferrer">the planet will not be tidally disrupted but instead leave a hot plasma wake</a> (that paper has more detailed calculations of infall timescales etc.).</p>
<p>The shocked gas will form a plasma sheath that erodes the planet at some rate, leaving a trail of metal-enriched material. Eventually the gravitational potential energy is released as heat, but this is mixed up with the rest of the solar luminosity over maybe a million years or so.</p>
<p>If Earth survives the giant phase it will orbit the white dwarf core (at a distance set by mass-loss expansion of the orbit minus gas drag inspiral; about twice the current orbit if drag is small). At this point it will become an airless (due to luminosity-induced atmosphere loss earlier), frozen rock over a few hundred million years or so as it cools from earlier heating.</p>

CC BY-SA 4.0
2020-08-19T22:24:47.790Z
2020-08-19T22:24:47.790Z
Anders Sandberg
<p>Sometime soon (astronomically speaking) the Sun will, as a main sequence star, expand into a red giant and, potentially, &quot;consume the Earth&quot;.</p>
<p>If we take it as read that the surface of Sol will expand past the aphelion of Earth's orbit, what will Earth inside Sol entail, physically speaking, considering Earth as a rock, not an ecosystem, both as the surface of Sol expands past the orbit of Earth, and as the stellar envelope is ejected, leaving Sol as a white dwarf?</p>

CC BY-SA 4.0
2020-08-19T17:26:27.683Z
It will cease to exist as it evaporates completely as it spirals towards Sun due to increased drag with the Solar wind and extended solar atmosphere..
planetmaker
CC BY-SA 4.0
2020-08-19T18:35:01.040Z
There's some relevant info at https://en.wikipedia.org/wiki/Sun#After_core_hydrogen_exhaustion The Sun is about 4.6 Gyr (billion years) old and it won't start being a red giant for another 5 Gyr or so, but in about 1 Gyr it will be too hot for life to exist on Earth.
PM 2Ring
CC BY-SA 4.0
2020-08-19T18:40:12.040Z
Are you asking what it would look like from the POV of the Earth's surface, or from the POV of an observer at a safe distance from the Sun? Bear in mind that the outer layer of a star is rather tenuous, although a red giant's atmosphere is more opaque (and they tend to spew out a lot of dust).
PM 2Ring
CC BY-SA 4.0
2020-08-19T19:11:49.697Z
@PM2Ring I'm asking about what specifically would happen to the Earth, considered solely as a piece of stone floating in the abyss, the effects on various ecosystems and so on being beyond obvious (we all dead).
Williham Totland
CC BY-SA 4.0
2020-08-19T19:43:26.437Z
Oh, ok. But as I said, the ecosystems get trashed long before then. [Wikipedia](https://en.wikipedia.org/wiki/Timeline_of_the_far_future#Earth,_the_Solar_System_and_the_universe) has a rough timeline. The oceans will be gone before the Sun enters the RGB (Red Giant Branch). Earth *probably* won't get swallowed until the Sun is near the tip of the RGB. As planetmaker said, the inner planets will get vapourised, and some of their matter will get spewed out as dust.
PM 2Ring
<astrophysics>
What will "the Earth being consumed by the Sun" look like?
Williham Totland
# What will "the Earth being consumed by the Sun" look like?

(Asked by: username_0 on 2020-08-19)

Sometime soon (astronomically speaking) the Sun will, as a main sequence star, expand into a red giant and, potentially, "consume the Earth".

If we take it as read that the surface of Sol will expand past the aphelion of Earth's orbit, what will Earth inside Sol entail, physically speaking, considering Earth as a rock, not an ecosystem, both as the surface of Sol expands past the orbit of Earth, and as the stellar envelope is ejected, leaving Sol as a white dwarf?

**Tags:** astrophysics
> **Comments:**
> **username_1**: It will cease to exist as it evaporates completely as it spirals towards Sun due to increased drag with the Solar wind and extended solar atmosphere..  
> **username_2**: There's some relevant info at https://en.wikipedia.org/wiki/Sun#After_core_hydrogen_exhaustion The Sun is about 4.6 Gyr (billion years) old and it won't start being a red giant for another 5 Gyr or so, but in about 1 Gyr it will be too hot for life to exist on Earth.  
> **username_2**: Are you asking what it would look like from the POV of the Earth's surface, or from the POV of an observer at a safe distance from the Sun? Bear in mind that the outer layer of a star is rather tenuous, although a red giant's atmosphere is more opaque (and they tend to spew out a lot of dust).  
> **username_0**: @PM2Ring I'm asking about what specifically would happen to the Earth, considered solely as a piece of stone floating in the abyss, the effects on various ecosystems and so on being beyond obvious (we all dead).  
> **username_2**: Oh, ok. But as I said, the ecosystems get trashed long before then. [Wikipedia](https://en.wikipedia.org/wiki/Timeline_of_the_far_future#Earth,_the_Solar_System_and_the_universe) has a rough timeline. The oceans will be gone before the Sun enters the RGB (Red Giant Branch). Earth *probably* won't get swallowed until the Sun is near the tip of the RGB. As planetmaker said, the inner planets will get vapourised, and some of their matter will get spewed out as dust.  

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## 1 answers

### Answer by username_3 on 2020-08-19. Score: 7 | Accepted answer

The exact fate of Earth as the sun becomes a red giant is somewhat uncertain because of two factors.

The first is that as the sun expands, its surface gravity $GM\_\odot/R\_\odot$ will decrease and gas can more easily escape as solar wind. This will reduce the mass of the sun, making Earth's orbit spiral outwards. But how much expansion depends on details of the far future solar wind we do not know yet, and this makes it slightly uncertain whether Earth can get into an orbit beyond the maximal solar radius. (The gas will also produce some drag, but it is likely negligible in this case)

The second factor is tidal interactions. As the sun expands it rotates more slowly, and in particular will rotate slower than Earth's orbital speed. That means that the tidal bulge induced by Earth will always be trailing the planet, creating a backward pull slowing it and dragging it inwards. This effect is small when the surface is far away, but grows rapidly as it approaches. Hence the sun can gobble up Earth even if the orbit is larger than the solar radius - but much depends on somewhat weakly understood tidal interactions.

Once the planet is inside the envelope it will rapidly get dragged in. To get a timescale, consider the time to push aside an Earth mass of gas: $\tau = M\_\oplus / \pi R\_\oplus^2 \rho v$. For $\rho=$ 0.1kg/m$^3$ I get $\tau=0.4972
$ years. Maybe a bit longer if the orbit got wider and slower. Since the density of Earth is much higher than the gas, [the planet will not be tidally disrupted but instead leave a hot plasma wake](https://academic.oup.com/mnras/article/425/4/2778/1077595) (that paper has more detailed calculations of infall timescales etc.).

The shocked gas will form a plasma sheath that erodes the planet at some rate, leaving a trail of metal-enriched material. Eventually the gravitational potential energy is released as heat, but this is mixed up with the rest of the solar luminosity over maybe a million years or so.

If Earth survives the giant phase it will orbit the white dwarf core (at a distance set by mass-loss expansion of the orbit minus gas drag inspiral; about twice the current orbit if drag is small). At this point it will become an airless (due to luminosity-induced atmosphere loss earlier), frozen rock over a few hundred million years or so as it cools from earlier heating.


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