Habitability of our solar system when Sun becomes red giant

Question

I'm working at a story about the future of life in our Solar System.

Humans have gone millions of years ago, but they left behind them a group of highly intelligent but not superhuman AIs with the task of caring of natural life on Earth. These AIs are (sort of) sentient, but they are designed that they can't evolve and become superhuman.

The AIs have taken care of life for a billion years, or more. They have allowed species evolve and disappear, but they have not allowed mass extinctions nor cathastrophes. In these millions of years, these AIs, known as The Caretakers, have helped life in a lot of crises like cometary impacts, avoiding colissions.

They have also avoided these "great" crises (see https://en.wikipedia.org/wiki/Timeline_of_the_far_future#Future_of_the_Earth.2C_the_Solar_System_and_the_Universe) :

• 200 million years in our future a massive gamma ray burst from a close star could have destroyed Earth's ozone layer, but Caretakers are intelligent enough to study the sky and foresee this burst, so they could build a orbital screen that covered Earth from the gamma rays.

• 600 million years in our future the carbonate-silicate cycle begins to be disrupted by the increased power of the Sun. This could kill plants because of the lower levels of CO2 don't allow photosynthesis. The Caretakers have been burning fossil fuels for millions of years to avoid this. They also build space screens when global warming becomes an issue.

But now, a billion years into the future, Caretakers can't avoid death of life on Earth. Sun is becoming too powerful. AIs are sentient, and individual, not a hive mind. And they have diferent opinions.

-The most conservative faction, called "The nihilists" think that their duty is over and want to commit suicide.

-A faction, called the Telurians, want to stay on Earth, wrapping the planet in a coccoon that protects life and "air conditioning" all the excess heat to the space.

-Other faction, the Selenians, want to do the same, but into big domes in the Moon.

-The Caretaker faction called "The martians" want to terraform Mars thinking it will be more easy and stable than a warming Earth. They think that growing power of the Sun will help them to do this.

-The last faction, "The jovians", want to terraform some moons of Jupiter of Saturn, also with the growing power of the sun.

A very important issue: the Caretakers want to preserve life of Earth "as it is". They will allow some degree of natural evolution, but they will not use genetic engineering to change life for it can adapt to Mars or Europa.

Also, they are not much more clever than humans and they are not much more advanced than our current technology. No FTL, no hyperdrives, no supermaterials, no unobtanium. But they have thousands of years to work and they have all the resources on Earth and the solar system.

Mi questions are:

1.- Are there any other potential catastrophes in the future of Earth that Caretakers have to care about?

2.- Which faction have more chance to survive? Do you have any advice to give them?

3.- Can you suggest another new faction with a different plan?

Answer 1

Destined To Fail

Let us get two things clear first and foremost:

1- When the sun peaks out at its red giant phase, its outer layers will reach (and perhaps extend) the orbit of Earth. This means that Earth will be inside the sun at that time. No cocoon or whatever will enable life on Earth or Earth's moon.

2- As the sun expands, it will radiate out its inner heat reserve of billions of years, making it grow hotter as it expands. While Earth will simply be annihilated, Mars would be roasted to cinder. Forget colonizing Mars in hopes of long term habitation.

Telurians, Selenians and Martians ... their plans are destined for failure.

The Problems

However, Martians might just get the things right. While terraforming a planet is not as quick or easy as they portray in movies or fiction, if (within a million years) they are able to terraform Mars to support Earthly life, it will provide a stable base to export life to further outer reaches of the solar system as the sun continues to expand. That is, Mars cannot serve as the final destination for life in the solar system, but it can serve decently as a temporary (serving efficiently for half a billion years or so. that is my estimate, I haven't done detailed research on this subject) base, from where the caretakers can send it further outwards.

One problem which is often overlooked when considering colonization of other planets or moons is that of gravity. Complex Earthly life has evolved for at least half a billion years under Earth's gravity. Not only our skeletal and muscular systems, but also our circulatory and sensory systems are extremely fine-tuned to serve optimally under Earth's gravity. Even if we could enable the correct atmospheric mixture on Mars, a sudden shift from Earth to Mars would be extremely hard for majority of complex life forms (specially for larger creatures). It would be even worse in the case of moons.

Then there is the case of atmospheric pressure. Objects of lesser mass can sustain thinner atmospheres. And without a strong magnetic field to shield the object (planet/moon) against solar wind, the atmosphere will slowly be blown away into space. This means that even if we could somehow create an Earthly mix of atmosphere on Mars, the atmosphere would be very thin (since Mars has a much lower mass, it can only hold lesser atmospheric mass) and breathing there would feel like breathing on Mount Everest: laborious and gasping. Then there is the issue of marine creatures. Thinner atmosphere would mean that there is lesser amount of oxygen dissolved in the water. You can hardly support a large population like that.

What about sun's ultraviolet radiation? On Earth, the ozone layer saves us from that. But there is no ozone layer on Mars or any of Jovian or Saturnian moons. It will not be an easy task safeguarding against sun's increasing ultraviolet radiation.

Plausible Solution(s)

In the light of above considerations, the only viable solution appears to colonize another terrestrial planet around a star which would not go supernova or red giant in at least 10 or so billion years. Considering that even with our very crude, entry-level technologies, we have discovered a large number of planets around other stars, it looks probable that most stars in our galaxy have planetary systems. While it is indeed difficult to find Earth's twin, it is not entirely impossible. Migrating to such a planet would be the only possible solution.

With the sun slowly growing larger and hotter, it would have to be done as soon as possible. The way I would have done this (if I had all the resources of Earth and large number of super AIs under my command) is as follows:

Phase 1: Build a space station in Martian orbit. Considering that the caretakers are AIs and have no biological constraints, thousands of caretakers should be available to work on this project and once built, the AIs should be available on the Martian space station all times.

Phase 2: Start building several large spaceships in space, in the vicinity of Martian space station. Each of these should be at least a mile long and 300 meters in diameter. 3 such spaceships need to be worked on, simultaneously.

Phase 3: Once a batch of 3 space ships is ready (which should take no more than 80 years), board one set of complex organisms (by one set I mean fish, amphibians, reptiles, mammals, birds etc) on one space ship and send the space ship to one of the potentially habitable planets which the caretakers discover through in our galactic neighborhood. Once the batch is dispatched (with caretakers crew on board, along with the organisms), start building the next batch.

Continue doing this until the sky comes falling (aka the sun roasts the Earth) and obliterates everything ...

Other Caretaker Faction(s)

Of course we would need the faction called Eden Angels, working with the plan I have suggested above.

Then there could be a faction called Weepers, who believe that the final act of mercy to all life on Earth would be to kill it painlessly, in order to save it from the horrific death of being slowly roasted by the sun.

Important Note

Considering that Earth has limited resources, caretaker factions with different survival plans (Telurians, Selenians, Martians and Jovians) would try to take over as many resources as possible, in order to successfully execute their own plan for saving Earthly life. This, along with possible Weepers, will create a sort of civil war between the caretakers until most factions are extinct and only one or two factions (with secret Weepers in the society for added mystery and tension) survive, both agreeing that it would be a better idea to avoid further confrontation and both controlling half of Earth's resources, utilizing them for their own escape plan.

Without this sort of civil war, the whole caretaker society would be a big mess, and neither faction would get enough resources to successfully initiate their escape/survival plan.

Answer 2

The AI's can simply start moving planets around the Solar System, and they will be able to succeed if they start right now.

The issue is needing to transfer momentum from one object to another. A spaceship swinging by Jupiter for a gravity assist already does this, the increase in the spacecraft's momentum is matched by a corresponding equal loss of momentum by Jupiter. We cannot measure this with the accuracy of current instruments, but it is quite real.

The Caretakers will need to send spacecraft to the Oort cloud and Kuiper belt to start harvesting the trillions of objects in deep space. Each one gets a solar sail and is kicked into an orbit reading into the inner solar system. As the comet passes the Earth, the orbit is calculated so the moving comet gets a "gravity assist" that would drop it into the Sun, transferring the momentum to the Earth/Moon system. The solar sail is deployed to boost the comet back into an orbit around the Sun, and it can be timed to swing past the Earth and carry out another manoeuvre to add more momentum to the Earth, boosting it into a larger orbit away from the swelling sun. The comet cores might have to make hundreds or thousands of passes to move the Earth away from the Sun, and ultimately they might have to park the Earth as far out as the current orbit of Neptune. Space Stack Exchange has some answers to this problem.

Other planets and moons can be moved in the same way.

Once the Sun starts to cool and contract towards becoming a white dwarf star, the process has to be reversed so the Earth and other planets get moved into a much closer orbit around the new, tiny star at the centre of the solar system.

Plan B (which can be used in conjunction with plan A) would be to use an industrial process called Star Lifting to reduce the mass of the Sun and convert it into a red dwarf before it balloons out into a red giant.

A mechanism for "harvesting" solar wind (RC = ring current, MN = magnetic nozzles, J = plasma jet).

The Earth and other planets would then have to be moved inwards to receive the same insolation (and perhaps platoons of mirrors would also be in orbit to reflect more light onto the planets). All the extra hydrogen being lifted from the Sun will be stored in the form of artificial gas giant planets, providing for a fairly spectacular night sky as dozens of Uranus sized gas giants fill the view.

The AIs can have a very busy and productive several billion years tinkering with the luminosity of the Sun if they get it down to Red Dwarf size.

Answer 3

Judging by this simulation

-A faction, called the Telurians, want to stay on Earth, wrapping the planet in a coccoon that protects life and "air conditioning" all the excess heat to the space.

No way, Earth will be swallowed

-Other faction, the Selenians, want to do the same, but into big domes in the Moon.

Same.

-The Caretaker faction called "The martians" want to terraform Mars thinking it will be more easy and stable than a warming Earth. They think that growing power of the Sun will help them to do this.

Still too hot, Mars will be the new Mercury

-The last faction, "The jovians", want to terraform some moons of Jupiter of Saturn, also with the growing power of the sun.

Jupiter moons will be still too hot. Saturn moons are BIG maybe.

Unless you allow technology much better then we have now I'll go with the

-The most conservative faction, called "The nihilists" think that their duty is over and want to commit suicide.

But first make a big party.

Answer 4

If the Caretakers have fusion power, they don't need a specific intensity of sunlight for power production for photosynthesis since they can use electrical powered artificial lighting for plants. But no doubt they will also use a lot of natural sunlight for power production. Waste not, want not. So build space habitats out of materials found in the solar system. Use fusion power for artificial sunlight for the plants. Stock habitats with Earth lifeforms.

Each habitat must be large enough for a flourishing ecological zone. It may become a problem building habitats large enough for ocean ecological zones.

Enough space habitats can be built that the total volume and surface areas of all the space habitats is many times greater than Earth's biosphere. Stock them with Earth life forms and move Earth life forms from habitat to habit until they have multiplied and filled them all.

Move the habitats outside the zone of destruction. Also move a lot of the raw materials from planets, moons, asteroids, etc. from the zone of destruction out to the safe zone. Waste not, want not. Wait though the entire solar red giant phase. Then terraform at least one remaining solar system body if possible and colonize it with some of the lifeforms from the habitats. Put artificial sun satellites powered by fusion in orbit around the terraformed body. Keep maintaining the space habitats with Earth lifeforms as a back up, which should have been started many millions of years before the red giant phase, anyway.