I recently discovered that there is another kind of water : heavy water. Heavy water is made of deuterium, a heavier isotope of hydrogen. Heavy water itself is 11% percent heavier than "normal" water.

This got me wondering :

Is a world where all the water is heavy water possible ?

  • 1
    $\begingroup$ Worldbuilding is only bound by the rules which a worldbuilder chooses to impose upon themselves. If you want to say "this world only has heavy water on it" what's to stop you from doing so? $\endgroup$
    – sphennings
    May 23, 2022 at 14:38
  • 2
    $\begingroup$ Could you narrow it down much more? I can interpret this in multiple ways, like 'an Earth with heavy water instead of water' 'a world where magically everything is heavy water and if life is still possible' or 'would heavy water stay heavy water for a long time' as a few choice examples. The goal isn't clear, which woupd help with answering. VTC for now. $\endgroup$
    – Trioxidane
    May 23, 2022 at 14:48
  • $\begingroup$ Any life would need to evolve with the heavy water, since heavy water at high enough concentrations is toxic to terrestrial life. en.wikipedia.org/wiki/Heavy_water $\endgroup$
    – DWKraus
    May 23, 2022 at 15:01
  • $\begingroup$ For my part, Zeiss Ikon's answer justifies the question. Voting to leave open. That being said, the question could be clearer and include more info about what's needed. $\endgroup$ May 23, 2022 at 15:10
  • $\begingroup$ @JiminyCricket. Just because a question is answerable doesn't make it a good fit for this site. $\endgroup$
    – sphennings
    May 23, 2022 at 15:22

3 Answers 3


Is a world with no protium, only deuterium (and the unavaoidable contamination of a tiny trace of tritium), hence all the water is "heavy water" possible? Sure it is.

All that's needed to be plausible is a plausible mechanism to concentrate the heavier isotope, probably in multiple stages (like multi-step distillation to reduce impurities further and further).

A potential mechanism might be a naturally occuring mass spectrometer, perhaps in orbit of a neutron star. Gas ionized by x-rays and UV, orbiting through the star's hyper-strong magnetic field, will tend to separate into streams of heavier and less heavy atoms. A few million years of this might plausibly separate the gas enough that some later event that strips it away from the neutron star and makes it available for star and planet formation preferentially grabs the denser deuterium.

Now, whether a star capable of sustaining life on a planet can form from deuterium only is a separate question -- at the very least, it wouldn't be where its mass says it should be on a Hertzsprung-Russel chart.

As noted in comments, this may not be a mechanism that can produce enough mass to form an entire star, but might well do to form a rogue planet that can later be captured. After capture, there's still the possibility of surface contamination by comet bombardment, but that's generally an event from early in stellar system formation, where rogue capture might well occur after primary accretion is well over with.

  • $\begingroup$ I don't think you could get a whole star's mass from a process like that. The mass you filter would far outweigh the neutron star. If you're doing natural isotope separation you'll need to produce comets to deliver the heavy water directly to the planet. $\endgroup$ May 24, 2022 at 2:46
  • $\begingroup$ I had understood that most of (for instance) Earth's water came in the minerals that formed the rocky body, with comets only an incidental source. Forming a rogue planet this way, to later be captured, does seem more possible, however. Editing. $\endgroup$
    – Zeiss Ikon
    May 24, 2022 at 11:08


One of the less-known strange things about Venus is that it has 120 times more deuterium than Earth. Now, Earth is 1 D to 6420 H, so that is still only 1 in 50 hydrogens, but it is the start of something.

We're going to take Venus, and put it around a nice stable red dwarf that can survive for trillions of years. Venus may have been wet just 1 billion years ago, so I imagine we can let it simmer for maybe 10 billion years (part of me is still sore from where I pulled that number) to roast off most of the light H. We can also start by tinkering with the mass to make sure none of our precious deuterium escapes, so hopefully we retain a bit more than Venus has of hydrogen now.

Now what? We have to get rid of the acid, so let's drop, oh, some sulfur-metabolizing bacterium on our Venus that grabs up all the S, causing its atmospheric H2SO4 to become plain H2O + 1 1/2 oxygen molecules. Per ancient terraforming schemes for Venus, we'll say the bacteria grow in the atmosphere but fall to the ground as cinders, taking not just the S but also those 90 atmospheres of CO2, or at least the carbon from them.

When all is said and done, you have a very old, dry planet with some nice lakes of heavy water under a heavy oxygen atmosphere. Almost everything will burn on contact; perhaps you can drop some metallic meteorites that take up a lot of oxygen but don't react that much with the water.


Heavy water is D2O. You need a large quantity of deuterium to get large quantities of heavy water.

Deuterium is a precious gift from the Big Bang

Most of the deuterium present today was produced during the Big Bang nucleosynthesis event. Deuterium is made by separating naturally-occurring heavy water from a large volume of natural water. Deuterium could be produced in a nuclear reactor, but the method is not cost-effective.

So a large quantity of heavy water is possible only in an imaginary world. In real universe, it occurs only in very small quantities. On earth, the natural abundance of deuterium in the ocean is approximately 156.25 ppm.

Biochemical reactions

Deuterium behaves differently from normal hydrogen in biochemical reactions. While it's not deadly to drink a small amount of heavy water, ingesting a large quantity can be lethal.

In your imaginary world, full of heavy water, biochemical reactions will be different.


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