When the Sun gets to the age at which it will rapidly expand what effect would it have to the outer planets and it's moons? Would it make new habitable areas of the solar system?
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Possibly but "Habitable" doesn't automatically mean "Suitable for Earth-creatures"
Mars will be heated significantly, enough to boil off any water. The same will happen to the moons of Jupiter, though there will be a period during which the ice on Europa melts and evaporates.
Titan is a more interesting case. It would be in the habitable zone of a red-giant sun. It could form Ammonia-water oceans and a complex atmosphere that may make it suitable for bio-genesis. However while it might be warm enough for humans, it won't be a home-away-from-home. There is no oxygen, for a start. Any future Earthlings would have to terraform it before moving in.
A deeper discussion of habitable zones around red giants is had by Bruno Lopez et. al. in 'Can Life develop in the expanded habitable zones around Red Giant Stars?'
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$\begingroup$ This article claims that "When the Sun becomes a fully-fledged, full grown red giant (in some 7.59 billion years), it will reach its largest radius at 256 times its current size. Interestingly, when this happens, the new habitable zone will stretch from 49.4 AU to 71.4 AU, well into the Kuiper Belt. So although Earth will be gone, there may be a host of new habitable moons in our solar system (time will tell)." Not sure how robust that estimate is though...? futurism.com/… $\endgroup$ Oct 31, 2021 at 17:56
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$\begingroup$ but could the sun's expansion cause chemical reactions in the outer planets due to the sun shedding it's mass? Kinda a natural terraform in a way. $\endgroup$– MichaelNov 3, 2021 at 5:03