# Life around Cepheid Variable stars

Are there any unique challenges life would face evolving on a planet orbiting a Cepheid Variable star? I'm aware this is a broad question, so to narrow it down, consider this a question about Great Filters, in the sense of the Fermi Paradox. What sorts of challenges unique to a Cepheid Variable star could prevent life from evolving to human-level intellect?

Stars that become Cepheid variables stay in this phase of their lives for only a short period of time, and after they leave the main sequence. While their properties vary (in particular, Cepheids are divided into two groups, Type I (Classical) Cepheids and Type II Cepheids) there are a few characteristics they have in common:

• Masses of anywhere from 3 to 20 solar masses
• Ages of a few tens of millions to a few hundred million years
• Luminosities on the order of $$\sim10000L_{\odot}$$
• Pulsation periods of a few days
• Luminosity changes of 0.5 to 2 magnitudes

Most of these are problematic for various of reasons. The key one is that there's not enough time for life to evolve around a Cepheid, in all probability. Furthermore, given how hot these stars are, most of their light will be at ultraviolet wavelengths, rather than optical wavelengths. This means that photosynthesis would require pigments unlike the chlorophylls we see on Earth; I'm actually not sure whether pigments optimized for these wavelengths even exist.

I'd argue that the swings in luminosity wouldn't be a problem for life, at least, for the same reason that eccentric orbits aren't problematic for life: only the mean flux a planet receives matters. If you could solve the timescale and spectral problems, the pulsations likely wouldn't be too problematic.

I should note that these issues may arise for other types of (lone) variable stars, too. Take the cases of (for instance) Beta Cepheid variables and rapidly oscillating Ap stars, which have masses similar to Cepheids. They're hotter than solar-type stars and more massive, and so they evolve quicker and emit substantially more UV light than visible light - not great for most kinds of life. On the other hand, cool, low-mass stars like Mira variables are likely to be more conducive to habitability, even if - as is true for most variable stars - their time as variable stars is brief.

There is a very tenuous and subtly balanced foundation on which the evolution and sustainment of life on Earth rests; change.

Evolution requires change in the environment in order to force adaptation, but too much change too quickly and you override the incredibly slow speed of evolution and wipe everything out. We know for instance that even on Earth there have been several extinction events on the earth that have been devastating, but not total. that has allowed life to flourish once more when the event is over or the effects of it have abated and the Earth has once again become more conducive to life upon it.

Even the tilt of the Earth helps with that; the seasons drive key behaviours in animals and plants that also drive change and adaptation. The rotation of the Earth, and even the phases of the moon all support different activities on the part of life on Earth that drive change, albeit at a slow rate.

Your Cepheid Variable stars are larger than our Sun, but often much more bright or energy rich, so there's a good chance that the Goldilocks zone of such a star is much further out, meaning that the 25% variation between peak and trough of luminosity, while still severe, may be of a softer impact on your temperate planet if it is sufficiently distant from the star. As Ryan points out in comments also, given the period of your variability is likely not to extend past 50 days, a very wet planet may not even notice the variability too much because of the thermal mass of the water.

A desert planet on the other hand might feel it more so; there's even a possibility that their cycle of variation allows for a short form of 'season' on a planet that doesn't have an axial tilt for whatever reason, or a moon to generate tides and the like.

Of course, the exact opposite could also be true and your star's cycle of variation could push your planet periodically out of the Goldilocks zone, by either freezing it or cooking it once per cycle1. In such a case, your Great Filters apply because the star resets abiogenesis on a regular basis and forces life to form again, or at the very least disrupts its progress. In such a circumstance, life can't evolve to the complexity that affords sentience because there is too much disruption.

Even our existence is arguably highly improbable which is what the Fermi Paradox seems to imply. By this I mean that the chances of something going seriously wrong in the last 3.5 Billion years since there has been some form of life on earth would have been an almost certainty. We've had mass extinctions, and arguably those have slowed the development of sentience, but at the end of the day we are still here because of all the things that haven't happened to our fragile planet over the intervening time.

In the end, I'd argue that life around a Cepheid Variable star is less likely to grow beyond unicellular forms than life on Earth because the star is not proving the stability that ours does, but there are still a lot of variables in play to consider, and as has been discussed above in some very rare circumstances, the variability of the star could actually be a good thing for life. But the chances of that being the case are extremely remote and the variability of the star is in fact more likely to disrupt the evolution of life than support it in most conceivable scenarios.

Of course, it's possible that if you have a very short lived form of life that reproduces at an exceptional rate (with massive die-offs at times as well), you could end up with a particularly hardy strain of life that evolves so much faster than ours; in such a case, it's possible that your life could react faster (biologically speaking) to environmental changes caused by your star. In such a case, your life may well get up to complex animals (or whatever evolution creates that is an animal analogue), but then that life would be so short lived that the learning experiences needed to figure things out and pass on the knowledge to the next generation might not be possible. You end up having to evolve into longer lived creatures, that are then more susceptible to the variations of your star. But, they may have a small chance of becoming sentient.

I'm deeply suspicious that your planet won't support life, certainly not continuous life in a manner that would allow sentience to evolve. That said, it might be possible even if it's clearly not probable.

1. As discussed earlier in the answer, this may be less of an issue for all but planets on the very edge of habitability due to the presence of water, and therefore an 'evening' of temperatures over the period of variability thanks to a high thermal mass. Thanks Ryan L for pointing this out.

• I wonder about your Goldilocks zone point. Having read the wikipedia pages for Cepheids, I see that they vary over the course of 1-50 days. Planets and atmospheres are pretty big heat sinks, would merely 50 days of 25% more/less light be enough to cook/freeze everything? – Ryan_L Jul 26 '19 at 2:38
• @Ryan_L This is a good point and I agree that it's unlikely under ideal conditions, but there is still cycles of ice-ages & heat-ages where the variation could push an edge case over the boundary of habitability. I guess what I didn't quite make clear in my language was that the planet would have to exist within the margins of habitability at the same rate of variation; in other words, if the variability is 25%, then you'd need to be inside 25% of either edge of normal habitability to make sure your life can still thrive. I'll look at editing to make this clearer. – Tim B II Jul 26 '19 at 3:16
• But my point is that 50 days is so short in geological timescales, that a 50-day cycle seems more like weather than climate. Sorry if that wasn't clear. – Ryan_L Jul 26 '19 at 3:53
• @Ryan_L Ah. Yes, you have a valid point when I read more clearly. Of course, much lower water levels could significantly reduce thermal mass, but it's still a fair point. I'll think it through and update my answer. – Tim B II Jul 26 '19 at 4:07