I need to create a race that should be utterly older than humanity (and as such, much more powerful). My idea is to have that race originate from a rock planemo - a rogue planet, orbiting the center of the galaxy without a star.

The idea is that this planemo originated so long ago that the cosmic background radiation was the source of energy for its life forms.

Is that possible? Would the background radiation last long enough (in the past, close to the start of the universe) that it would have been able to feed photosynthesis and support life? How old this planemo should be?

Later, this civilization gained enough technology as to not need neither background radiation nor a host star to survive. The ancient build portals across the galaxy allowing a thriving economy to develop. Their fate is unknown.


Can background radiation sustain life on a rocky planemo (rogue planet)?


3 Answers 3


Quick answer: No, even though the conditions you asked about were actually in place briefly.

There was a time, when the universe (as we know it) was about 13-16 million years old, when it was far, far denser than it is now, when the CMB was in the temperature range of liquid water. The Universe was also mostly hydrogen and helium, with a sprinkling of lithium. Perhaps some really precocious stars formed, and blew up, enriching the local environment. This is possible:

Lifetime table

Any and all planets, even rogues would have had liquid range water.

Unfortunately, this window lasted only a few million years. So the 4 billion year RNA -> NASA evolution we experienced here would have to happen about 1000 times faster, which is, um, rather unlikely.

Furthermore, the same enriching process that would have created the materials for the star (supernovas) would still be going on, probably sterilizing all planetoids on a regular basis.

Moreover, a planetoid even if it had formed superfast would not have had time to cool off to liquid water temperatures, especially since it would be getting blasted with warm CMB radiation from all directions.

  • $\begingroup$ I believe handwavium can work here and make the ancients evolve in a quick burst right into enough tech to space travel. The hard part of intelligent life is all the evolution before intelligent life starts, but, as soon as people gather around a civilization, going to NASA is quick. We spent 4k years from the first civs to the first space ship. While life spent some billions of years to form humans. I believe that a more active ancient universe generated more large scale extinction events that open space for new species. $\endgroup$
    – Jorge Aldo
    Mar 22, 2015 at 15:43
  • $\begingroup$ Note: Population III stars would have dominated, so stars would have slightly different lives. $\endgroup$
    – HDE 226868
    Mar 22, 2015 at 16:14
  • $\begingroup$ @HDE226868 Which would be precisely the metal factories I was talking about. $\endgroup$ Mar 22, 2015 at 16:44
  • $\begingroup$ @SerbanTanasa The impacts wouldn't be too immediate, though. $\endgroup$
    – HDE 226868
    Mar 22, 2015 at 16:45
  • $\begingroup$ @HDE226868, time enough for a couple of generations, methinks. $\endgroup$ Mar 22, 2015 at 16:45

Not unless life evolves extremely quickly.

There are two conditions for cosmic background radiation to be able to support life:

  1. It's partially composed of photons at wavelengths required by photosynthetic pigments.
  2. It's bright enough to transmit a useful amount of energy to any life out there.

If either of those conditions is not met, life cannot survive (and as the first stars won't be born for another 100 million years or so, there likely isn't another feasible energy source). It turns out that any photosynthetic life forms on your planemo will only have a few million years in which to evolve - not nearly enough. If they were magically transported there from the future, they might have a chance, but not if they formed on the planemo itself.


Let's first see what limits we can place on life based on the evolving temperature of the CMB. It makes sense to talk about the cosmic microwave background in terms of its redshift (which, in cosmology, tells us how far away something is, and therefore how far back in time we're looking). For instance, at some redshift $z$, the temperature of the CMB is $$T(z)=T_0(1+z)$$ where $T_0\approx2.725\text{ K}$, its approximate temperature today (at $z=0$). Similarly, the relationship between the age of the universe and redshift is approximately $$t(z)=\frac{2}{3H_0\sqrt{\Omega_M}}\frac{1}{(1+z)^{3/2}}$$ where $H_0$ is the Hubble constant today and $\Omega_M$ is the dimensionless matter density of the universe; today, $\Omega_M\approx0.04$.

The CMB was released during recombination, when $T\approx3000\text{ K}$. This corresponds to a redshift of $z\sim1100$, and a time of 375,000 years after the big bang. Now, 3,000 Kelvin isn't much - it's about half the surface temperature of the Sun. That means the radiation peaks at a wavelength of $\lambda\approx966\text{ nm}$, which is already outside the visible spectrum. However, the CMB will still consist partly of visible light until it reaches roughly the Draper point, which corresponds to an age of 2.7 million years. Given that photosynthetic pigments on Earth are largely sensitive to visible light - which will only compose a small part of the CMB, even after it's emitted - it seems that after a few million years, they'll be out of luck.

Energy density

Since the cosmic microwave background is (approximately) a black body, we can analyze it in terms of Planck's law. Andersen et al. 2018 actually did this for the CMB at various times in the early universe, and determined what it would look like to a human observer. They found that it was actually too bright for human eyes up to 1.2 million years, and too dim after 5.3 million years - roughly twice as late as our simple estimate using the Draper point.

We can use this as a proxy for how photosynthetic organisms would receive radiation, given that green, chlorophyll-rich plants thrive in the same environments humans do. It seems unlikely that they could survive far beyond 5 million years - again, assuming that they're similar to the planets we have on Earth. If we consider exotic photosynthetic pigments that operator well at far longer wavelengths, perhaps things could be a little different.

In the Dark Ages

As I said before, the first stars - massive, luminous objects called Population III stars - won't form until 100 million to 200 million years after the Big Bang. This means that there should be a very long period of time called the Dark Ages between when the CMB is formed and then becomes invisible to human eyes and when these stars form. It's called the Dark Ages because, well, it's dark. There's so significant source of visible light in the universe.

Life seems unlikely to evolve within 5 million years, as Serban Tanasa indicated, and I agree. If it did, though, an interesting question would be whether it could survive these hundreds of millions of years until the first stars are born, perhaps by going dormant in some way. (I don't really have an answer for this, to be honest; I doubt it but can't really prove it). One of the problems, of course, with thinking about the evolution of life under strange conditions is that we only have on planet of data - and the lifeforms on it are carbon-based. I'm not sure how well we can extrapolate life on Earth to the not-carbon-based life on this planemo.

The impossible planemo

Something that I've ignored up until this point is that your planemo can't exist at all at this point in the universe. It would be composed of heavy elements and silicate compounds. Unfortunately, those compounds can only form after the first stars in the universe are born and die. For the same reason, any life on this body could not be carbon-based, as there wouldn't be any carbon to go around.

Indeed, galaxies as we know them won't yet have formed even by the time the background radiation becomes too dim for significant photosynthesis to take place. Structure formation takes quite a long time, and when the universe is a few million years old, galaxies are still far in the future. Overdensities exist, and those overdensities will eventually collapse into protogalaxies and then galaxies, but that's about it at the moment.

I include this only for the sake of completeness; I think we've ruled out the possibility of life even if we handwave away the setting!


The laws of thermodynamics say no

Unfortunately, for heat to be useful, you need a heat gradient. If the entire sky is at 300K, then you can do no more work out of it than if it is at 3K - that is, none at all.

You can only use heat when it flows from a hot to a cold place, basically. Which is incidentally why all the schemes about eliminating waste heat by turning it into electricity are impossible - waste heat is what you need to evacuate so the cold end of your system is cold again, so heat can flow to it again. That would be akin to refilling a hydroelectric dam by turning low water to high water and energy, things simply don't work that way.

So for your planemo to be inhabitable, you need either a hot sky and a cold sink, or a cold sky and a heat source. Earth is the latter, with the Sun and (very distant second) Earth's own primordial/radioactive decay heat.

You could theoretically have a hot sky/cold sink with a planet orbiting a black hole, but it would need to orbit exceedingly close to the black hole, which means absurdly high orbital velocity, which means any pebble will turn into a Dino-killer, if the tidal forces don't rip the planet apart before that. Also it would be a planet, not a planemo. And as already pointed out, at the time the sky was hot enough for liquid water, there wouldn't be any existing planet, star or black hole yet.

What could theoretically work is a planemo born with big reserves of primordial heat (from the accretion) and enough radioactive material to at least partially renew it. Depending on how those life forms work, they could exist using the planet itself as a heat source. Europa-type ice-covered ocean worlds, and/or super-Earth with an enormous atmosphere would be your best bets. However, such life would probably be very different from what we know. As temperatures are probably much lower, I would expect very slow life forms.

  • $\begingroup$ This answer is the most fundamental reason why CMB cannot be a useful energy source for life. $\endgroup$
    – Qami
    May 28, 2020 at 15:19

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