# Physiological adaptation of life on a planet orbiting a red giant.

Let's say there exists an Earth-like exoplanet which orbits a normal star, with a similar process regarding the evolution of life on earth, yet the star became a red giant during said evolution. Assuming the exoplanet was in the goldilocks zone during and after the transition from normal star to red giant and the red giant still having a lifespan of a billion years subsequent to the transition...

How would life adapt to take advantage of this if there were no sapient organisms?

Would the planet become dominated by photosynthetic plants due to increased efficiency from solar energy conversion rates?

Would mutations become more common due to increased solar radiation?

If the planet had a slow rate of rotation might the plants develop a new storage medium for the solar energy when left without a constant source?

Would animals tend to be cold blooded and/or would they develop photosynthetic traits, possibly with semblance to the synthesization of vitamin D in humans?

• Glad to see you've done your research about red giant stars lasting up one billion years. It's possible the exoplanet might remain within its Goldilocks zone both before & after its red giant transition. If it was only in the habitable zone during & after the transition, it would be extremely cold. A snowball exoplanet. Not good for the evolution of life. Mich depends on how the Goldilocks zone moves as it goes from normal star to red giant. You should clarify that in your question. This is an interesting question, but you may need to focus it more. – a4android Mar 15 '18 at 3:56
• One thing you should note is that a red star produces less UV and energy than a yellow star. The plants will be dark trying to absorb as much energy as possible, and the weak solar radiation will not penetrate water very deeply. – Dan Clarke Mar 15 '18 at 4:23
• @2a4android Perhaps the planet's orbit began to move as the star grew. We've seen wandering planets in other systems, and physicists think it might have even happened to the outer planets in the solar system's early years. The increasing solar wind over billions of years could definitely raise a planet's orbit, with other planets giving an extra gravitational kick as they moved too. And if such a planet moved in tandem with its habitable zone, it could have life billion of years older than us. – Mattias Mar 15 '18 at 5:14
• Let's ignore the implausibilities regarding the formation of life, before the star became a red giant. If the star was similar to Betelguese, yet was at a distance relative to the exoplanet where it could maintain the planets surface temperature at around 30 degrees Celsius. According to this the star is about 3000 kelvin, so if it was closer to the planet could it sustain a warm surface temperature? – Lutro Mar 15 '18 at 17:38
• @Lutro, the plants won't care much about the UV, but the reason for plants to absorb as much of the different wavelengths as possible is the low energy output. A cool red star doesn't produce as much energy as a yellow or blue start, so plants can't be choosy. – Dan Clarke Mar 15 '18 at 21:12

## Temperature and luminosity

Let's start with some calculations. For the sake of argument, I'll assume that we're talking about a planet that's identical to Earth orbiting a star that's identical to the Sun. To be as generous as possible, I'll assume that this red giant expands to only about $$\sim$$200 solar radii - a mere 0.93 AU - and cools to around 2800 Kelvin. Therefore, its radius has grown by a factor of 200 and its temperature has dropped to half its value on the main sequence. Now, we can approximate the star as a black body, meaning that it has a luminosity described by $$L\propto R^2T^4$$ as per the Stefan-Boltzmann law. Therefore, the red giant will have a luminosity $$\sim$$2500 times that of the Sun - near the lower end of models for our Sun's future. The effective temperature of a planet scales as $$T_{eff}\propto\sqrt[4]{\frac{L}{r^2}}$$ where $$r$$ is the distance to the star, and so at its current distance, the equilibrium temperature of the Earth would be about 7 times its current value, about 2029 Kelvin. Now, fortunately for any life-forms on the planet, the radius of the planet's orbit may increase as the star loses mass, which happens significantly faster during the red giant phase. Generously, Earth could move to an orbital radius of $$\sim$$1.5 AU. If we were to assume an even greater mass-loss rate for this star, it's not too far out of the question that Earth could move as far away as 2 AU. Therefore, if we recalculate the temperature, we find that $$T_{eff}$$ drops down to 507 Kelvin, or a mere 453$$^{\circ}$$ F. Hey, it's still better than Venus!

I think the classic picture of life on a planet orbiting relatively close to a red giant involves a completely inhospitable surface. I agree; 453$$^{\circ}$$ F is way too hot for life as we know it to live without shade. Even thermophiles couldn't survive; the famous Strain 121 and Strain 116 - which can live at 250$$^{\circ}$$ F - would be boiled. I would argue that things won't get much better at night. Heat transfer in a planet's atmosphere is incredibly complicated, so I can't give you hard figures, but the thermal inertia of air would have to be much, much lower than it currently is for any significant cooling to take place at night. Any organisms that tried to live nocturnally would be out of luck.

We therefore have to go underground for shelter, which is a problem, because it means that photosynthesis might not be possible. Chemosynthesis is a possibility, as is thermosynthesis - which, as I've talked about before, was a possible metabolic mechanism for early life on Earth. If I can invoke thermosynthesis here, then I'll point out that, as you'd need a large heat gradient for it to be effective, this planet might be just ripe for it to develop and thrive. As to how exactly it could be implemented - well, I'll leave that to you, for now.

It does seem that cyanobacteria can live deep underground, processing hydrogen gas using a photosynthesis-like chain that doesn't use light. It's pretty incredible, and means that subsurface life is indeed possible. Perhaps these cyanobacteria could survive the extreme temperatures and conditions on the surface.

## Periods of rotation and revolution

Now, Kepler's third law states that the length of a planet's orbital period ($$P$$) is related to its semi-major axis ($$a$$) and the mass of its parent star ($$M_*$$): $$P^2=\frac{a^3}{M_*}$$ Now, we assumed above that $$a$$ doubles and $$M_*$$ is approximately cut in half. Therefore, we can see that the new period of revolution is four years.

Now, there shouldn't be a significant change in the period of the planet's day, because that would involve a change in its rotational angular momentum. There's no source for it to offload its angular momentum to, and as its mass and radius will remain the same, so will the length of one day. The one way this could change would be if it has a moon - like Earth does. Over time, tidal forces transfer angular momentum from the planet to the moon, slowing its rotation. You haven't stated whether or not a moon exists; for simplicity, I'll assume one doesn't.

I don't see any significant changes happening here. If the length of a day remains the same, then the only difference is that the year has doubled, which doesn't seem terribly absurd. The seasons - such as they are - will also be twice as long, although with arguably no surface life, the change could be tough to see. The only effect this would have on life here would be that the available heat reservoir for thermosynthesis would oscillate over a longer period. That change, though, should be slim, and certainly wouldn't endanger life.

## Type of incoming radiation

Dan Clarke mentioned something important, and I'm going to expand upon it. The spectrum of a black body isn't uniform; it's peaked at some characteristic wavelength $$\lambda_{\text{max}}$$. We can calculated this wavelength from Wien's law: $$\lambda_{\text{max}}=\frac{b}{T}$$ where $$b$$ is a constant, 2.897$$\times$$10$$^{-3}$$ m$$\cdot$$K. Using this, we find that the Sun has a peak wavelength of roughly 502 nm. This red giant, with a temperature half that of the Sun, has a peak wavelength of 1004 nm, in the near-infrared section of the electromagnetic spectrum. Moreover, there would be less emission of ultraviolet light than from a Sun-like star.

As I wrote about here, different photosynthetic pigments are more effective at different peak wavelengths. For peak emission around 1000 nm, certain bacteriochlorophylls will be efficient. If photosynthesis was possible, the dominant life forms would be purple and green bacteria. Now, it doesn't seem like photosynthesis could happen - the surface temperature is too high. Still, I'm wondering if there's a way around this - perhaps, somehow, the atmosphere is extremely dense and drastically reduces the amount of light reaching the surface to a reasonable amount. If so, purple and green bacteria could be the major denizens.

You asked about mutations. Ultraviolet light is a cause of mutations, and given that the temperature of the star has changed, so has the amount of ultraviolet light - it's gone down. The same goes for any x-ray and gamma-ray radiation (which would be negligible to start with, for a Sun-like star). Therefore, we should see a slight decrease in mutation rates. But then again, there won't be much light out in the open anyway. At any rate, having an ozone layer would be less important.

## The AGB phase

The asymptotic giant branch (AGB) phase of a star's life - occuring immediately after the star leaves the red giant branch - presents slightly different hazards for life on an orbiting planet. AGB stars lose mass rapidly through strong stellar winds, sometimes losing up to $$10^{-4}M_{\odot}$$ per year! This is a major problem. The central stars of planetary nebulae - having just left the asymptotic giant branch - can ablate away the atmospheres of orbiting planets, and I suspect that conditions wouldn't be more hospitable during the AGB phase. In other words, I would conjecture that it's possible that Earth's atmosphere could be stripped.

In addition to the mass-loss problem, there's still the issue of a vastly increased luminosity (perhaps $$\sim10^4L_{\odot}$$) at the very end of the asymptotic giant branch. Even if the planet retains its atmosphere, and even if the planet moves far enough away to remain in the habitable zone while its parent star is on the red giant branch, it will likely be scorched during the AGB phase.

None. Unless that thermosynthesis is working out already.

I believe that unless the organism had some kind of plant-based cell it would become cold-blooded, but they'd have to adapt underground to avoid extreme exaustion.

• Hi, Twimes. Can you explain your answer in a bit more detail? Thanks. – HDE 226868 May 31 '18 at 19:57

This all kind of depends on the density of the atmosphere of said planet. Does it have a good ozone layer? If so then, radiation should not be a problem. The planet would probably mainly be populated by an invasive species of plants, as it could be hard to survive for long periods of time in the dark. Animals might also be invasive, or they could be migrational (and move towards the light were plants are still healthy (and other animals are still alive)) as of predators. Maybe there are some animals that follow the migrating plant eaters, or maybe there are some plants that will use animals as "batteries" for the long night. As of mutations, I can not say.

I'd say as a safe bet due to the solar radiation from the fluxing red giant, there would probably not be an ozone protecting from harmful radiation, so most life would be confined to water where the radiation is absorbed. this makes life even on the surface of the water unlikely as radiation isn't completely absorbed by the water. but eventually, the creatures in deep water could build up a resistance to radiation (if genetic stabalization was possible). and could eventually inhabit land. although a vast number of creatures would still live in the water.

I would say that the land would be relatively barren causing most creatures to be carnivorous and predatory. the prey would have to adapt to be able to run and/or hide from the predators, meaning both predator and prey would have to be fast and be able to run for a long time. the creatures able to survive would most likely survive on algae, moss, and other leeching plants that can live on no/ extremely little soil.

another consideration is the fluxes of energy from the star, turning it hot enough so send ponds to a boil one day, and cold enough to freeze rocks open the next. so everything would have to adapt to these constant temperature fluxes by developing thick skin and a way to store water to use for hot days and draw away for cold ones, expanding and shriveling the creatures slightly respectively. the creatures would also have to adapt to the light level changes; either having large eyes to intake available light, or using another sense entirely. finally, the plants living on the planet would likely mutate from the radiation giving way to everything from spiky algae to rock hard bushes. that's really all I know for now.

• Your first paragraph isn't correct. The star would be cooler, and so ultraviolet emission would go down significantly. Ultraviolet radiation is the prominent player in the ozone cycle, and with less UV light, there wouldn't be any threat to an ozone layer. You're definitely right about temperatures getting extremely hot, but I don't think the temperature changes would be more drastic than the ones on Earth; there's no mechanism for the air to cool rapidly at night. The temperature wouldn't change much from the day side to the night side. – HDE 226868 May 31 '18 at 19:56