# Long lasting life on interstellar planets?

A short time ago I read that life might be possible on interstellar planets (i.e. planet-like objects which aren't bound to a star by gravity.) They may be insanely cold on the surface but inside they are warmer due to the radioactive decay. A planet consisting of the same materials as our earth but has $3.5$ times its mass could have liquid oceans beneath a thick sheet of ice. This ocean could stay there for about $5$ billion years. That's enough for life to evolve. It would live from chemosynthesis.

While this is already interesting, I was wondering whether longer lasting life is possible. If you imagine a planet of one Earth mass consisting of vanadium-50 (which has a half-life of $1.5*10^{17}$ years), it produces about one-fifth of the power of our Earth. At first that looks like nothing. But then, a fifty kilometre thick sheet of ice covering it can isolate it, just enough to keep a liquid ocean if the water is salty enough and the planet has some kind of atmosphere. It would stay that way something in the order of the half-life of vanadium.

This might be a little farfetched but at least in my opinion it is pretty cool to have one planet support life for $10^{17}$ years. Especially compared to the lousy $10^{10}$ years, our Earth has. However, in reality, it is unlikely to work quite as well, so I assume we need a more active and therefore shorter living isotope. (Here are a few candidates.)

So here's my question:

Is it possible that the universe generates (without help of whatever kind of sapience) such a planet?

With possible I mean to say: reasonably likely that it happens at least once during the life of our universe.

It doesn't have to be a vanadium planet, but at least one which consists for a large part of some long (meaning more than say $10^{12}$ years) living radioactive isotope and a huge ice layer on it. It should have enough of this radioactive isotope to have a liquid ocean for a long time.

I'm not asking for the specifics about life on this interstellar planet. It's almost sure bacteria-like. Also, I'm not asking whether it could really thrive under these conditions. While that might be an interesting question, I think we don't know yet enough to really answer it.

Edit: The main obstacle is whether so much radioactive material could be amassed, since these isotopes are rare in our universe. The cosmic abundance of vanadium is for example 0.0001% and most of it is stable isotopes. This is since most of the long living atoms are only generated by supernovae.

• Possible? Yes. Probable? don't think so. will need to find papers about that. – Mołot Nov 7 '16 at 13:18
• a Steppenwolf planet with at least 3.5 Earth mass could hide liquid ocean beneath thick layer of ice crust, however relying on fission alone means there won't be any serious volcanic activities therefore it would be a miracle to host macroscopic lifeforms since energy is scarce. – user6760 Nov 7 '16 at 13:43
• I read that. And I didn't ask for a macroscopic lifeform – lurch Nov 7 '16 at 13:57
• By "interstellar planet", you mean rogue planet, right? – HDE 226868 Nov 7 '16 at 16:36
• Actually, to be precise no. An interstellar planet may be an rogue planet (an exoplanet which got ejected from it's solar system) or an sub brown dwarf (something that is created like a star, but only became as big as an planet). However, generally people who are speaking about rogue planets actually want to say interstellar planet, so you're not completely wrong. – lurch Nov 7 '16 at 16:40

So let's break this down, there are a lot of steps here and if any of them fails your planet fails:

1. Interstellar planets - plausible

2. Oceans under ice - plausible

3. Heating from the core - plausible

4. Life around thermal vents in the ocean - plausible (we don't know if it evolved there but it can certainly live there).

5. Chemosythesis life - plausible

6. Insulated by a thick ice sheet - plausible

There is no known natural system that could concentrate a material to the point that even the majority of the mass of a planet is one unstable element.

1. Alternatively a planet containing a lot of radioactive elements - plausible

This is a lot more plausible, we have evidence of natural nuclear reactors happening on earth. We also know that at least some of the heat in our own core comes from radioactive processes, so in fact this is at least plausible. Whether it would last as long as you want is impossible to say but it would be viable for long enough for life to evolve.

A bonus idea:

There is another way though, have not a rogue planet but a rogue moon. An interstallar gas giant and the life is on an icy moon in orbit around the gas giant. Tidal heating could supplement or even replace your radioactive core and still satisfy your desire for life on an interstallar planet.

• I know about tidal heating as an alternative. However, I like the radioactivity more, since it's more stable. A moon could collide with some other moon. Also I've got no idea how long the tidal heating would warm the moon up, as the heating comes indirectly from changing the orbit. – lurch Nov 9 '16 at 14:23
• A moon's orbit is reasonably stable, especially if you make it only a two body system with no other moons. Yes the energy comes from orbital energy, but only a tiny amount, the orbit will still last for a very long time. – Tim B Nov 9 '16 at 14:58

Before anything else, I would like to remind you that you are on World Building forum, where people get together and strive to build new worlds according to your/their requirements. What is scientifically plausible and probable in astronomy falls under astronomy stackexchange forum.

Secondly, yes you are right when you say "I think we don't know yet enough to really answer it." We, as a species, only know about life on Earth, and know about no other type of life.

Can there be life in a liquid water beneath 50 kilometers thick ice sheet on a rogue planet? We don't know for real, but nothing that we know of, inhibits the origin of life under such conditions. So while it is theoretically possible, we can't tell how likely it would be.

Having a 50 km thick ice sheet means no light at all. Absolute darkness. That means no photosynthesis. No green plants. No eyes. No complex life. Bacteria at most. Those too, would be a fluke of nature. In the absence of lightning, you wouldn't be getting the amino acids which are the base of Earthly life.

Is it possible that the universe generates (without help of whatever kind of sapience) such a planet?

What is such a planet? A rogue planet, thrice the size of Earth with a huge reserve of radioactive elements in its core?

Yes, the universe can generate such a planet. There is no hindering factor. Most probably such a planet would start as a normal, stellar-orbitting planet which is later ejected out of its solar system due to gravitational tugging with one of the outer gas giants in the system and begin its endless vigil in interstellar space.

• First of all: I believe, the astronomy stackexchange would not like this question eider, since it is too far fetched for them. And yes, I'm trying to create a world according to my requirements. As you say I should do here. Also you are answering about every question I not asked. On you overlooked the major obstacle: Such radioactive Elements are rare. Otherwise the earth would be much hotter from radioactive decay. In the end your answer is more a comment than an answer – lurch Nov 9 '16 at 11:51