Plausibility Check – Habitable Planets around Red Giants


While planets orbiting twin stars are a staple of science fiction, another is having humans live on planets orbiting red giant stars. The majority of the story of Planet of the Apes takes place on a planet around Betelgeuse. Planets around Arcturus in Isaac Asimov’s Foundation series make up the capital of his Sirius Sector. Superman’s home planet was said to orbit a the fictional red giant, Rao. Races on these planets are often depicted as being old and wise since their stars are aged, and nearing the end of their lives. But is it really plausible to have such planets?

Stars don’t last forever. Our own Sun has an expiration date in about 5 billion years. At that time, the amount of hydrogen fuel in the core of the Sun will have run out. Currently, the fusion of that hydrogen into helium is giving rise to a pressure which keeps the star from collapsing in on itself due to gravity. But, when it runs out, that support mechanism will be gone and the Sun will start to shrink. This shrinking causes the star to heat up again, increasing the temperature until a shell of hydrogen around the now exhausted core becomes hot enough to take up the job of the core and begins fusing hydrogen to helium. This new energy source pushes the outer layers of the star back out causing it to swell to thousands of times its previous size. Meanwhile, the hotter temperature to ignite this form of fusion will mean that the star will give off 1,000 to 10,000 times as much light overall, but since this energy is spread out over such a large surface area, the star will appear red, hence the name.

So this is a red giant: A dying star that is swollen up and very bright.

Now to take a look at the other half of the equation, namely, what determines the habitability of a planet? Since these sci-fi stories inevitably have humans walking around on the surface, there’s some pretty strict criteria this will have to follow.

First off, the temperature must be not to hot and not to cold. In other words, the planet must be in the Habitable zone also known as the “Goldilocks zone”. This is generally a pretty good sized swath of celestial real estate. In our own solar system, it extends from roughly the orbit of Venus to the orbit of Mars. But what makes Mars and Venus inhospitable and Earth relatively cozy is our atmosphere. Unlike Mars, it’s thick enough to keep much of the heat we receive from the sun, but not too much of it like Venus.

This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area, showing that Gliese 581 d is located inside the habitable zone around its low-mass red star. Based on a diagram by Franck Selsis, Univ. of Bordeaux. Credit: ESO

The atmosphere is crucial in other ways too. Obviously it’s what the intrepid explorers are going to be breathing. If there’s too much CO2, it’s not only going to trap too much heat, but make it hard to breathe. Also, CO2 doesn’t block UV light from the Sun and cancer rates would go up. So we need an oxygen rich atmosphere, but not too oxygen rich or there won’t be enough greenhouse gasses to keep the planet warm.

The problem here is that oxygen rich atmospheres just don’t exist without some assistance. Oxygen is actually very reactive. It likes to form bonds, making it unavailable to be free in the atmosphere like we want. It forms things like H2O, CO2, oxides, etc… This is why Mars and Venus have virtually no free oxygen in their atmospheres. What little they do comes from UV light striking the atmosphere and causing the bonded forms to disassociate, temporarily freeing the oxygen.

Earth only has as much free oxygen as it does because of photosynthesis. This gives us another criteria that we’ll need to determine habitability: the ability to produce photosynthesis.

So let’s start putting this all together.

Firstly, the evolution of the star as it leaves the main sequence, swelling up as it becomes a red giant and getting brighter and hotter will mean that the “Goldilocks zone” will be sweeping outwards. Planets that were formerly habitable like the Earth will be roasted if they aren’t entirely swallowed by the Sun as it grows. Instead, the habitable zone will be further out, more where Jupiter is now.

However, even if a planet were in this new habitable zone, this doesn’t mean its habitable under the condition that it also have an oxygen rich atmosphere. For that, we need to convert the atmosphere from an oxygen starved one, to an oxygen rich one via photosynthesis.

So the question is how quickly can this occur? Too slow and the habitable zone may have already swept by or the star may have run out of hydrogen in the shell and started contracting again only to ignite helium fusion in the core, once again freezing the planet.

The only example we have so far is on our own planet. For the first three billion years of life, there was little free oxygen until photosynthetic organisms arose and started converting it to levels near that of today. However, this process took several hundred million years. While this could probably be increased by an order of magnitude to tens of millions of years with genetically engineered bacteria seeded on the planet, we still need to make sure the timescales will work out.

It turns out the timescales will be different for different masses of stars. More massive stars burn through their fuel faster and will thus be shorter. For stars like the Sun, the red giant phase can last about 1.5 billion years, so ~100x longer than is necessary to develop an oxygen rich atmosphere. For stars twice as massive as the Sun, that timescale drops to a mere 40 million years, approaching the lower limit of what we’ll need. More massive stars will evolve even more quickly. So for this to be plausible, we’ll need lower mass stars that evolve slower. A rough upper limit here would be a two solar mass star.

However, there’s one more effect we need to worry about: Can we have enough CO2 in the atmosphere to even have photosynthesis? While not nearly as reactive as oxygen, carbon dioxide is also subject to being removed from the atmosphere. This is due to effects like silicate weathering such as CO2 + CaSiO3 –> CaCO3 + SiO2. While these effects are slow they build up with geological timescales. This means we can’t have old planets since they would have had all their free CO2 locked away into the surface. This balance was explored in a paper published in 2009 and determined that, for an Earth mass planet, the free CO2 would be exhausted long before the parent star even reached the red giant phase!

So we’re required to have low mass stars that evolve slowly to have enough time to develop the right atmosphere, but if they evolve that slowly, then there’s not enough CO2 left to get the atmosphere anyway! We’re stuck with a real Catch 22. The only way to make this feasible again is to find a way to introduce sufficient amounts of new CO2 into the atmosphere just as the habitable zone starts sweeping by.

Fortunately, there are some pretty large repositories of CO2 just flying around! Comets are composed mostly of frozen carbon monoxide and carbon dioxide. Crashing a few of them into a planet would introduce sufficient CO2 to potentially get photosynthesis started (once the dust settled down). Do that a few hundred thousand years before the planet would enter the habitable zone, wait ten million years, and then the planet could potentially be habitable for as much as an additional billion years more.

Ultimately this scenario would be plausible, but not exactly a good personal investment since you’d be dead long before you’d be able to reap the benefits. A long term strategy for the survival of a space faring species perhaps, but not a quick fix to toss down colonies and outposts.

24 Replies to “Plausibility Check – Habitable Planets around Red Giants”

  1. “This means we can’t have old planets since they would have had all their free CO2 locked away into the surface.” I’m sorry to be ignorant, but why has that not happened on Venus or Mars?

    1. Good point.

      For both these planets there is a lack of tectonics, so carbonation is slower.

      For Venus the runaway greenhouse made it too hot for effective carbonation IIRC, and somehow it lost its water that means less tectonics, less weathering and less oxygenated silicates I think. (Water loss probably mainly by hydrogen loss.)

  2. In “Planet of the Apes”, the Superman franchise, and Asimov’s _Foundation_ series Betelgeuse, Rao, and Arcturus are inhabited by intelligent life. You are assuming that only natural processes are in effect.

    Let’s say that Type I civilizations developed on a planet around these stars while they were on the main sequence. A Type I civilization would have the capability to either move existing Goldilocks planets with the habitable zone as it moved outward (Luna would be a good gravity tractor for us, in our case – or maybe some sort of orbital resonance with Venus or Jupiter), Terra-form outer planets as the habitable zone expanded to their orbits, or both.

    A Type I civilization inhabiting nearby star-systems may even, in fact, seed those star systems with intelligent, self-replicating robots to prepare the planets for eventual colonization. They would do this by making sure the planets had enough free oxygen, carbon-dioxide, water et al.

    Certainly the existence ET life much less of intelligent ETs is unproven. But if you are going to use these fictional examples in your argument, you have to accept the possibility compatible entities in any counter argument.

    1. Huh? Before Kurzweil computing capacity crackpottery there was Kardashev energy capacity crackpottery!?

      Um, right. That is exactly why first order theories are natural. I’m not sure what you mean by “fictional examples” of life, I’m looking at a lot of (natural, natch) life on this planet right now!

      1. The “energy level” thing is obviously garbage, but personally when I think of type 1 civilizations, I think of them like this (I think this is a more common usage than the “energy capacity of civilizations” anyway):

        Type 1: A civ capable of utilizing all the resources of a planet
        Type 2: A civ capable of utilizing all the resources of a solar system
        Type 3: A civ capable of utilizing all the resources of a galaxy
        Type 4: A civ capable of utilizing all the resources of a (the) universe

        By that measure on Star Trek they are a type 1 civ. The Borg would be bordering on type 2. The Q *might* be nearing type 4, though it’s hard to say. The Ancients/Ori from Stargate would be bordering on Type 3.

    2. Not only is this a possible explanation, but it is THE explanation for the Foundation Series. In one of his later books (Foundation’s Edge, I think) Asimov establishes that few to no planets are truly habitable when human’s first arrive. All of them undergo at least some terraforming. So there is no particular reason why the Galactic Empire couldn’t have set up a capital around Arcturus, so long as the star does have any violent fluctuations in brightness or solar (Arcturian?) flairs.

  3. I always had the feeling that in old scifi “red” star:= old regardless of size, either because of a powerful metaphor or weak science.

    If you go to M stars, there is also atmosphere loss. IIRC Earths and superEarths may keep a substantial (habitable) atmosphere ~ 10-15 Gy tops, I believe I have a reference somewhere.

    Presumably star type vs oxygenating photosynthesis will not be a problem after the discovery of near IR productive chlorophyll f. It should be enough for the puniest M stars.

  4. I thought the red giant _was_ the helium-burning phase, looks like I was wrong.
    How big / luminous is the helium-burning star then?
    Will the Sun live through that stage, or not, or maybe?

  5. Manu, the term, “red giant” can be applied to many phases. The initial hydrogen shell is just the first of (potentially) many periods in which a star can be considered a giant. After the hydrogen shell burns out (or sometimes overlapping depending on stellar mass), helium fusion kicks in and the star can become a giant again. It can go through many periods of swelling and contracting, with different shells and fusion processes, but regardless of which one you’re on, it still looks red and giant. I just went with what is likely to be the longest lived one since we need long timescales to create a good atmosphere.

    1. Thanks! Now I remember reading something like it somewhere.

      Another question comes to mind: is the picture fuzziness caused by telescope resolution limits, actual giant star fuzziness, or both?

  6. Stop destroying my childhood!!!! First Star Wars, now Superman. What’s next, Indiana Jones? It’s like every movie Johm Williams scored is just fantasy….

    If a star enters the red giant phase and begins to shed material, would that effect the gravity, causing the planets to slowly migrate outward? That might affect the time frame for habitability.

    Also Kryptonians figured out how to grow entire cities from crytals, so there. :-p

    1. Yes, that would have an effect on the orbit of the planets. IIRC, Earth’s current orbit is well inside the outer atmosphere of the future red giant sun, but it’s possible that due to Sol’s mass loss Earth will move *just* far enough away to not be completely incinerated (but still deep fried). Maybe. It’s still up in the air.

    2. When I was originally starting to research the topic I considered this as well, but the timescale is so quick (on astronomical timescales), that the planet doesn’t have much time to change its orbit before the habitable zone has already swept by.

      1. Since most of the material is ejected from the photosphere, it’s just hydrogen and helium. Chemically, it would do very little. The energy may heat the planet some but probably not too significantly compared to the increased stellar flux.

    3. I think life on Earth might last a bit longer than another billion years. The reason is Jupiter perturbs the orbital radius of Earth some. There is from my calculations a slight drift in the radius of the Earth orbit. Four billion years ago the radius would have been .83AU, which given the reduced energy output of the sun would make temperatures comparable to today’s. In a billion years the radius will be 1.03AU and 1.15 in 5 billion years when the sun enters the red giant stage. This does push the future for life here another billion years, for the increased solar irradiance is about compensated for by this outward drift. This means complex life might have another 700-1000 million years. The increased solar irradiance will begin to accelerate and over take this drift. In 2-3 billion years Earth will come to resemble Venus.

      The added radius of the orbit may not be enough to prevent the Earth from spiraling into the red giant sun. The Earth will be in the outer atmosphere of the swollen sun and the friction may drag the Earth in.


      1. I’m sure I’ve read both here [UT] and elsewhere that life will become virtually impossible here in earth within the next 500 million years as it doesn’t actually take the sun to become a red giant i.e. expand and change colour to make the environment harmful to complex life. Even before it becomes red the changing sun heats the oceans and atmosphere wiping out almost everything other than bacteria.A few million years after that the oceans boil off. This, from what I remember takes place by around 750 MY.

  7. Another way to get CO2 into the atmosphere is from volcanism. Volcanically active planets could still generate enough CO2 for plants to use once the temperature warmed up sufficiently. If there was a technologically advanced civilization that didn’t manage to destroy itself over the lifetime of a star, they would naturally terraform their outer planets to make them habitable once the opportunity arose. Most likely they would also have tried moving their inner planets out of harm’s way diverting asteroids or large comets for gravitational assists.

    1. The trick is keeping volcanism active. Volcanic activity is driven by the molten interior of the planet which is supported by heat from initial formation of the planet as well as the radioactive decay of unstable isotopes. By the time a star reaches the red giant phase, Earth mass planets will have cooled to the point that they cannot support active volcanism. This effect was considered in the von Bloh paper I referenced.

  8. I wonder if an icy/rocky planet or moon might be a good choice. If the habitability zone reaches its orbit, then you could have oceans. I imaging the new oceans would then have an effect on the tectonics, which might cause quakes and volcanic activity. The thawing of ice might also be a good source of O2.

    As for the inner planets that get fried….I read somewhere that the outer regions of red giants are extremely thin, possibly thinner than our atmosphere, and closer to a hot vacuum. I imagine the engulfed planet would stay together in a decaying orbit inside the star. Could the inhabitants live underground? Ultimately, they would meet their end as I’m sure the planet’s orbit would decay to the point where it would get too hot or succumb to tidal forces, but it might buy them some time.

    I did forget that well before the Sun enters the red giant phase, it’s tempurature and brightness will increase, sending Earth into a hot zone. I think this is supposed to happen in about a billion years.

    Not that it matters, as I’m confident we will destroy ourselves much, much sooner, anyway.

  9. This is pretty speculative. The conversion of a G-class star to red giant shifts the habitable zone out. With the sun it will mean Jupiter and Saturn will be “balmy,” or in the habitable zones. The moons of these planets might then melt, so they become ocean planetoids, or objects with some sort of complicated chemical soup for their surface environment. Earth will become absorbed. In the 1-2 billion years before the sun becomes a red giant it will increase its temperature, so Mars might be habitable for a time. Of course to be habitable for actual life requires the chemistry to be copasetic. Will Mars and then later Ganymede or Titan evolve so they can give rise to life? It is impossible to say. If Mars already has life of some variety in its subsurface it might enjoy a period where that life is able to evolve more rapidly with greater solar energy stocks available. Maybe the same holds or Europa or Ganymede.

    Kurzweil’s singularity concept has become a bit of a buzz of late. Time magazine featured a bit on this. I will say I think there is a germ of something to this. It might not mean that cybernetic systems take over in a standard science fiction sense. We might end up becoming neurally interfaced with them. For various reasons I don’t think some super-algorithm will be developed at an AI lab at Caltech or MIT that will become the super-cyber-colossus machine that takes over. In spite of the machine Watson’s winning the game Jeopardy, it is impossible to say this machine had any experiential knowledge of the answer’s it gave. The emergence of cyber intelligence might be more a matter of connectivity between processors, and maybe our brains as well.

    It could also be that the singularity could manifest itself as the collapse of a complex society. Already this stuff is becoming hugely complex and demanding, where it might be in a few decades it all becomes anthropologically unsustainable.

    As for the Kardashev energy scale, that is more speculative. I doubt IGUS (information gathering and utilizing systems) evolve to gain ever greater control over everything with no bounds. For biological beings like us it seems unlikely we can control things on these scales. The ideas of ring worlds and Dyson spheres are bogus, for the gravitational potential with respect to the central star is constant. A Dyson sphere would not be stable. I doubt there are civilizations which gain control over a whole galaxy, and certainly not an entire universe. Maybe if we hit this singularity its extension into space with satellites and spacecraft will result in self-replicating and evolving IGUS which migrates out into the solar system and maybe beyond.


    1. “It could also be that the singularity could manifest itself as the collapse of a complex society. Already this stuff is becoming hugely complex and demanding, where it might be in a few decades it all becomes anthropologically unsustainable.”

      Quite possible I’m afraid. In case of, let’s say a major economic disruption preventing manufacturing and/or transportation of goods, what proportion of mankind (or small local groups thereof) has the skills to produce the bare essentials for survival, compared to 100 years ago?

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