Premise: Can a Chthonian planet develop life?

The questions is much less if it could develop living organisms, but more if it can develop the building blocks necessary for organic life: An atmosphere, a magnetosphere, liquid water, maybe plate tectonics, etc.
For the purposes of the question, consider it possible that the planet moves into a hospitable zone around its star, or gets captured by another star in the hospitable zone, etc.

Under those circumstances, could a planet like TOI-849 b develop these things over time?


3 Answers 3


A magnetosphere might be possible, depending on the composition of the chyhonian planet. And it may get the chemicals needed for life by chance. By going rogue and then being captured by a star that is still forming, it can gather gas from its coalescing protoplanetary disc, water from comet bombardment etc.

Such a planet might be much more hospitable to life than Earth. If our measurements of TOI-849b are correct, its surface area would be 4.5 to 5.5x that of the Earth (back of napkin calculation here). It would thus be able to harbour many more ecossystems in ita biosphere, making it more resilient to global extinctions.

On top of that, many scientists believe that tectonism is a very important factor that led to life on Earth. A super earth made from a chthonian planet could have a very active crust, so some fine tuning would be needed for it not to have too much - but the size and mass are a great positive in this regard.

Also, the 4G surface gravity means it would be much better at keeping an atmosphere than even Earth is.

There is a great article on all this on space dot com: Super Earths May Be Superior at Fostering Life. TL;DR:

Earth is a marginal planet when it comes to conditions we would like to see for complex life to sustain itself, Sasselov notes. In the family of Earth-like planets, the sweet spot for complex chemistry and biochemistry to emerge and sustain itself lies in planets larger than the Earth.

  • $\begingroup$ Can you tell me how you got 4G surface gravity? my back of the napkin maths for TOI-839b got me 1.87G. $\endgroup$
    – MarsMagnus
    Commented Feb 2 at 13:50
  • $\begingroup$ @MarsMagnusI used this calculator in Wolfram Alpha, using 3 Earth radii and 40 Earth masses. Got 43m/s^2 as a result, which I then rounded to 4G. $\endgroup$ Commented Feb 2 at 19:43
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    $\begingroup$ @The Square-Cube Law This calculator omnicalculator.com/physics/acceleration-due-to-gravity gives 43.65 meters per second per second, or 4.451 g. Anyway, surface gravity doesn't matter for retention of an atmosphere. What matters is escape velocity. This calculator omnicalculator.com/physics/escape-velocity gives an escape velocity of 40.845 kilometers per second, or 3.651 that of Earth. $\endgroup$ Commented Feb 2 at 21:45

No Surface Water

While rocky planets have a great variety of surface features it is very unlikely that water is present. At least near the surface.

Like the Moon and Mars, hydrogen and helium are the first elements to go if the atmosphere is stripped from the planet. So as a chthonian planet it has none of those near the surface. And it would also be low on other lighter elements that don't make chemically strong bounds with other elements like sodium, potassium and magnesium. It will also be comparatively rich in heavy metals.

But during and after migration changes can happen. It is theorized that Earths hydro layer comes from bombardment of icy comets. Or hydrogen locked up in metal latices in the mantel of the planet might be pushed to the surface by volcanic activity.

Still the crust of the planet (if it has one) would have less silicon and magnesium than Earth. And would be rich in heavy metals that are toxic to earth life. If there is live it will have adapted to these circumstances.

Heavy metals are not suitable to form an alternative chemistry for life. They cannot form versatile complex molecules that are stable. Chemist even doubt if silicon is an alternative for carbon and suggest that carbon based life is the only option.

That said, take a look at the scaly-foot gastropod/snail for inspiration of what carbon based life might look like when presented with a different pallet of resources.



I assume your planet is not too hostile because otherwise the question would be pointless.

Since we still don't know how life begin, ie. abiogenesis is still a mystery, a definite "yes" and definite "no" are out of question. Given enough time, any event whose probability > 0 can happen, no matter how unlikely. Also, any event whose probability is < 1 might not happen... or might not have happened yet. It just cannot be predicted.

However, it's possible that your planet has life that has come elsewhere. For example, there is a lichen from Earth that has survived in space for 18 months. In actual space, in vacuum, without any protection from radiation. It could have survived longer, but it was a sample and it was removed for study.

A sturdy enough organism could survive interplanetary or interstellar space, maybe hibernating, and start growing on your planet if it was not too hostile for life. This is know as panspermia. A single organism that grows there can mutate and evolve and eventually even create an ecosystem.

In practice, that means: if your story needs life on the planet, then there can be life on that planet.

  • $\begingroup$ Ah, the question is much less about the life itself, and more about the outside circumstances like atmosphere, magnetosphere, etc. $\endgroup$
    – MarsMagnus
    Commented Feb 2 at 11:24
  • $\begingroup$ Yes; I am sorry, I forgot to add the link about the lichen surviving in space. That is about such conditions. Check the page where "survived in space for 18 months" takes you. $\endgroup$ Commented Feb 2 at 11:33

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