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Scenario run-down. Several billion years from now, Sol has finally exhausted its hydrogen supplies and has entered the red giant phase. It has already been increasing in lumosity for some time and Earth has become an uninhabitable desert. Humanity is a spacefaring species now and everyone has been evacuated in time to nearby star systems, along with a "Genetic Ark" of every life form ever recorded on Earth. Sol expands and engulfs the Earth (but not quite reaching Mars). Fast forward a few more hundreds of millions of years, it releases the halo and reduces to a white dwarf. At this point, humanity is settled in dozens of nearby systems, has become advanced in planetary engineering and starts to dabble in stellar engineering. An ambitious long-term project is put in motion: Restore the Earth to its pre-human state (and as much of the rest of the Solar System as possible) as a form of museum/memorial/preserve.

Before I start making up the necessary restoration tech, I need a realistic damage assessment. In particular:

  • How will Earth emerge (if at all) out of this? As a space rock scorched beyond recognition? Will it simply melt down and disperse in the Sun's heat? Or from the friction of atmospheric drag? Or will the drag slow it down so it falls into the Sun's core? Surely its orbit will be affected.
  • What about Venus and Mercury? I assume they will evaporate.
  • What about Mars? It's been orbiting quite close to the red giant's surface.
  • What about the gas planets?
  • How will the halo that the red giant releases at the end of its cycle affect the whole Solar System in general?

EDIT: Human technology level at time of the event is not sufficient to counteract/stop/delay the event in any way. I'm interested only in the natural results of Sun going through the red giant phase and the impact this has on the Solar System.

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    $\begingroup$ I think you want to read about the Sun's life after core hydrogen exhaustion. Particularly check out the illustration pictures and diagrams surrounding that section. When considering those sizes, remember that Mars' aphelion (point of orbit farthest from the focal point of the orbit) is a measly 1.6660 AU. If the Sun grows to 1 AU radius, as indicated by the illustration, then Mars will be just a smidge farther from its boundary than is Mercury today (Mercury's aphelion is a little under 0.47 AU). $\endgroup$ – a CVn Jan 13 '16 at 9:03
  • $\begingroup$ Don't worry, IMO it is a good question. Looking forward to some answers $\endgroup$ – Ciacciu Jan 13 '16 at 9:16
  • $\begingroup$ Its pretty optimistic assumption that humanity is still in a condition that enable them to care about the fate of their old homeworld... some billion years in future. If they do really care, they would zip their homeworld and move it to a more relaxed place in terms of danger-of-beeing-scorched-by-the-star-we-are-orbiting long before it even comes close to this state, wouldn't they? $\endgroup$ – Confused Merlin Jan 13 '16 at 9:57
  • $\begingroup$ @ConfusedMerlin This is where my world diverges from a realistic assumption. I'm fully aware that in the billions of years it will take Sun to end its current lifecycle stage we should already become incorporeals/uploads/whatevers with perhaps several singularities along the way. In my world, the technology didn't advance that fast - interstellar travel is in its pioneering phase by the time the evacuation takes place. So no planetary engineering (a.k.a. "let's move the Earth out of the way"), no singularity (only augments) and only several nearby systems settled. $\endgroup$ – Misza Jan 13 '16 at 10:13
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    $\begingroup$ @Misza That type of information should ideally go into the question itself, because it places some specific constraints on answers. Think of comments as ephemeral post-it notes, which can be deleted for almost any reason; anything that is important to someone answering your question should be edited into the question itself. Since this hasn't yet received any answers, you don't need to worry much about invalidating existing answers. $\endgroup$ – a CVn Jan 13 '16 at 10:38
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There have been some interesting studies into this. Wikipedia appears to think that the earth would be destroyed within 200 years of the start of the red giant expansion (Note that even if it isn't, the moon would apparently be destroyed so you'd have that much to do!).

However, there are other studies listed in articles here, here, and here, that say that it's unclear whether the earth would be swallowed up at all, and even if it was, whether it would affect the mass loss of the Sun so significantly that it would survive the process.

The first article states that scientists have predicted the presence of two planets orbiting a white dwarf at a radius that would be well within the red giant expansion. They theorise that the presence of the planets within the expansion would have accelerated the mass stripping process, potentially to the level that there would be enough core planet left at the end to have survived. It's worth noting that it doesn't say how big the planets were to start with! Given projections that the Earth would be destroyed within 200 years and the expansion phase would last around a billion years, it seems unlikely that the Earth would speed up the process enough to survive.

It's also worth noting that most articles make reference to the fact that the Sun's radius will fluctuate dramatically in that time - it's possible that the Earth will be close enough to the surface that these fluctuations will give it periods of respite, which could aid there being something left - of course, friction with Sol's atmosphere will probably pull it down into a tighter orbit if it gets swallowed, so not much chance there.

The second and third articles theorise that the earth's orbiting radius would be pushed out by the expansion phase and that it might be pushed out enough to avoid being swallowed up at all. This is alluded to in the Wikipedia article:

The orbital distance of the Earth will increase to at most 150% of its current value.[70]

The most rapid part of the Sun's expansion into a red giant will occur during the final stages, when the Sun will be about 12 billion years old. It is likely to expand to swallow both Mercury and Venus, reaching a maximum radius of 1.2 AU (180,000,000 km).

A 150% increase in orbit would push it out to 1.5AU, or an equivalent orbit of 0.3AU Theoretically, that's enough to take it out of the "vaporisation zone".

All this being said, there is a final article Here which says that even increasing to 1.5AU orbit, the earth will still be swallowed up. It does however raise two other interesting points:

  1. The earth's "original" orbit would only have to be increased by .15 AU ("only" I know) in order for the expansion effect to "push" its orbit outside of the danger area and survive in some shape or form.
  2. When the expansion starts, while the earth will be swallowed up, an awful lot of rocks on the outer edge of the solar system will suddenly be potentially habitable. While the white dwarf phase will change that again, returning the solar system to its "pre-human form" will be affected by this.

In Conclusion, the general consensus is that if the earth gets swallowed up, it will be vaporised, along with Mercury and Venus. The rest of the solar system will survive, though it will be dramatically changed in the process - virtually nothing will remain the same, and the ice planets may melt entirely. Some people thing the earth may be pushed out enough to survive, but it will end up like mercury at best, and will be mangled beyond recognition in the process. There is an outside chance that it would survive being swallowed up, but it looks pretty fanciful at this stage.

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