Black Holes & Life: Myth or Reality?

[Adb82]

I tryed to search more about CMB and blueshift radiations.
I got again something interesting i suppose, but i damn myself because its full of maths that i can't even try to understand at the moment: https://arxiv.org/pdf/2001.10991.pdf

This seem exactly what PeterDonis tryed to explain me as the most likely scenario for such a planet. Ill read it with more attention later, i just gave it a fast scroll at the moment, but would a planet in this conditions have a relativistic velocity? If i understand well they are talking about an orbit really very near to the horizon, this don't assume that the planet should move with relativistic velocity? That said, in the conclusions they also say "The resulting multiblackbody spectral profile of the incoming CMB with the peak in the ultraviolet band can be limiting for the evolution of biological life as we know it on Earth. The evolution of life on a black hole exoplanet orbiting deep in the extreme gravitational field would also be limited by the shortening of relevant time scales caused by the relativistic time dilation which factor reaches the value of thousands."

How those processes can be limiting for the evolution?

Still in the conclusions the also say: "...a supermassive black hole successfully hosting a habitable exoplanet should be old enough to accrete all the surrounding cosmic garbage", would this BH be so old to give the time to evolution to work even with so big time dilatation?

 

[PeterDonis]

would a planet in this conditions have a relativistic velocity

If "relativistic" means "close to the speed of light relative to the closest thing to stationary observers that are available in the local vicinity", then yes.

The qualifier about "the closest thing to stationary observers that are available" is that inside the ergosphere of a rotating hole (which is where these close orbits will be), there are no truly stationary observers, in the sense of stationary relative to an observer at infinity. The frame dragging due to the hole's rotation is strong enough that everything has to revolve around the hole in the same sense as the hole's rotation. But inside the ergosphere there are still zero angular momentum observers (ZAMOs), who are the closest thing to "stationary" observers available. A planet in the kind of close orbit the paper you reference is discussing will be moving at relativistic speed relative to such observers.

How those processes can be limiting for the evolution?

Because ultraviolet light breaks down the kind of complex molecules required for evolutionary processes to take place and produce pretty much any kind of life.

 

[PeterDonis]

i don't understand why it would be relavant for the life on the planet

If there were life on the planet, it would be likely to be killed off by a collision of the kind described, because of the huge energies involved--the relative velocity between the planet and the infalling object or matter would be not just relativistic, but ultrarelativistic.

 

[Adb82]

If "relativistic" means "close to the speed of light relative to the closest thing to stationary observers that are available in the local vicinity", then yes.

The qualifier about "the closest thing to stationary observers that are available" is that inside the ergosphere of a rotating hole (which is where these close orbits will be), there are no truly stationary observers, in the sense of stationary relative to an observer at infinity. The frame dragging due to the hole's rotation is strong enough that everything has to revolve around the hole in the same sense as the hole's rotation. But inside the ergosphere there are still zero angular momentum observers (ZAMOs), who are the closest thing to "stationary" observers available. A planet in the kind of close orbit the paper you reference is discussing will be moving at relativistic speed relative to such observers.Because ultraviolet light breaks down the kind of complex molecules required for evolutionary processes to take place and produce pretty much any kind of life

Yes that's what i was meaning with "relativistic", thanks for the clarification.

About ultraviolet light, yea i understood it, what i mean is: we are talking again only about maybe some micro life organism but nothing else? And there isn't any event that can theoretically solve this problem?

If there were life on the planet, it would be likely to be killed off by a collision of the kind described, because of the huge energies involved--the relative velocity between the planet and the infalling object or matter would be not just relativistic, but ultrarelativistic.

Yea I am sorry, i had already edited the post once you replyed, because i understood in the first time that they mean that's good for develope life, i didnt have clear the meaning of "threats", as they mean that's dangerous for life lol.

 

[PeterDonis]

About ultraviolet light, yea i understood it, what i mean is: we are talking again only about maybe some micro life organism but nothing else?

The paper you referenced doesn't seem to be limiting its analysis to any particular kind of Earthlike life. UV light would be dangerous to microbes too (UV light is used here on Earth as a method of disinfecting surfaces).

 

[Adb82]

The paper you referenced doesn't seem to be limiting its analysis to any particular kind of Earthlike life. UV light would be dangerous to microbes too (UV light is used here on Earth as a method of disinfecting surfaces).

Yea, its just that i don't get what kind of life can be expected to be found there. Not Earthlike, but can't anyway be any kind of "complex" life form, as the evolution can't take place with ulraviolet light, so it have to be something so small that i suppose it can't neither be percepted by human eyes (for exemple)? Or we can suppose (or anyway we can't exclude) a different kind of evolution with ultraviolet light? Because if i understood that paper, there is no way this planet can escape the ultraviolet light and have enough heat to survive at same time, or maybe in some part of its orbit the light wouldn't be ultraviolet and so we can suppose a very slow but still possible evolution?

 

[PeroK]

About ultraviolet light, yea i understood it, what i mean is: we are talking again only about maybe some micro life organism but nothing else? And there isn't any event that can theoretically solve this problem?

For any incipient life on this planet that really would be the ultraviolet catastrophe!

 

[Adb82]

There is nothing that can shield eventually the planet changing the spectrum to a less catastrophic one?

Im not sure what I am saying have totally sense, but if i understand well the ultraviolet problem, it happen because the blushift caused from the BH to the CMB it shift too much...so I am wondering if there is something that can eventually limiting the blushift to shift few less for be in the right spectrum...but if this would bring also everything to freeze on the planet it 's probably not worth the change.

 

[phinds]

those effects start normally in proximity of the event horizon

Just as a side issue for this thread, that is not true. For a supermassive BH, there is nothing much happening even AT the EH except that once you cross it you can't get back out but you would not experience anything unusual in terms of your freefall. The tidal forces are negligible.

 

[PeterDonis]

i don't get what kind of life can be expected to be found there

The paper you referenced does not really give an answer either way on that. It defines "habitable zone" basically as "temperature compatible with liquid water existing on the surface", but then it says the peak of the radiation frequency observed from the blueshifted CMB is in the UV range, which doesn't make things very habitable. So the paper is not actually claiming that life would exist on the kind of planet it is describing. It's just analyzing how various factors that might be relevant come out.

 

[PeterDonis]

there is no way this planet can escape the ultraviolet light and have enough heat to survive at same time

That's what the paper seems to be saying, yes; the blueshifted CMB peak is in the UV range everywhere in what it calls the "habitable zone" (where liquid water can exist on the planet's surface).

 

[PeterDonis]

im wondering if there is something that can eventually limiting the blushift

The blueshift is entirely determined by the planet's orbital parameters.

 

[Adb82]

The blueshift is entirely determined by the planet's orbital parameters.

yep, but if we change the orbit for get a "better" spectrum, than the planet going to freeze, so no chance for what i was trying to build.

 

[DaveC426913]

Have I glossed over a post that describes why the OP is looking for special relativistic effects, rather than general relativity effects? Is it possible there is some confusion on this matter?

Normally, when looking at black hole scenarios, one would expect relativistic effects due to gravity, but now we seem to be talking about relativistic effects due to velocity.

 

[PeterDonis]

Normally, when looking at black hole scenarios, one would expect relativistic effects due to gravity, but now we seem to be talking about relativistic effects due to velocity.

The time dilation factor for the orbits under discussion can be viewed as having elements of both. Heuristically, the "gravitational" part is due to altitude above the horizon and the "velocity" part is due to velocity relative to zero angular momentum observers (ZAMOs).

One can ignore this split, but if one does, one also has to not call the time dilation "gravitational", or indeed anything specific. The definition of "gravitational time dilation" depends on having a stationary spacetime (which Kerr spacetime is, but which, for example, FRW spacetime is not) just as much as the definition of "time dilation due to velocity" does. So we can either have both, or neither.

 

[Adb82]

Anyway, there is still one possible scenario if i understand it well, and its rapresented by a companion sun in a binary system with the BH and the planet orbiting around both of them. In this case the black hole shouldn't be a supermassive BH isn't it? But not being a supermassive BH the planet's orbit should be far from the EH for don't be destroyed from tidal forces (am i right?) and so i think also here the inhabitants can't experiment any of the relativistic effects we was discussing above. But can a similar scenario be enough good for the devolepement of life and the evolution process?

 

[PeterDonis]

a companion sun in a binary system with the BH and the planet orbiting around both of them. In this case the black hole shouldn't be a supermassive BH isn't it?

That's correct; the BH in such a system would be a stellar mass BH.

not being a supermassive BH the planet's orbit should be far from the EH for don't be destroyed from tidal forces

Yes.

i think also here the inhabitants can't experiment any of the relativistic effects we was discussing above

That's correct.

can a similar scenario be enough good for the devolepement of life and the evolution process?

Your guess is as good as anyone else's.

 

[DaveC426913]

...a companion sun in a binary system with the BH and the planet orbiting around both of them.

This is a bit ambiguous to me.

I interpret what you said as 'the planet is in a long-period orbit around the barycentre of the black hole - star system'.

A planetary orbit that is distant enough to be stable around a binary may have trouble getting enough light to sustain life. It may require additional tweaking.

 

[Adb82]

This is a bit ambiguous to me.

I interpret what you said as 'the planet is in a long-period orbit around the barycentre of the black hole - star system'.

A planetary orbit that is distant enough to be stable around a binary may have trouble getting enough light to sustain life. It may require additional tweaking.

Your interpretation is absolutly correct, but between not using my mother lengauge and needing a specific vocabulary, sometimes i feel like I am not able to comunicate exactly what i would like to say lol. I am sorry this bring everyone to make a double work having first to understand exactly what i mean.

About additional tweaking: a triple system with a BH and 2 stars would be the needed tweak or it just would make things even more complicated (problems with the obit of the planet?)? What if one of the stars would be a neutron star (i don't actually know even if its possible to have a triple system with a BH a NS and a star, but i was considering that neutron star will emit light but not so much heat, and if i understand the problem in that binary system is only the light and not the heat of the planet)?

About life in Earthlike forms around supermassive BH i found also this: https://arxiv.org/pdf/1910.00940.pdf
but it just seem confirm what you all already said till now, if I am not missing something on the paper seem it would be not possible.

 

[Adb82]

I searched some more info about triple systems with 2 stars and a BH.

The issues here should be the following:

1. Planet must be far enough from the black hole ( > Roche Limit ?)
2. Harmful radiations from black hole's accretion disk (but we can assume no accretion disk, or a tidally locked planet so that at least one emisphere will be habitable, so this isn't an issue at all probably)
3. Planet-Star relative distance must not vary too much.
4. getting enought light
5. Stability

about 1 i thought about a BH with M < 10 Solar masses. I once read in a thread that when considering a central body = 10 solar masses, and a satellite with a density of 5000 kg/m^3 (Earth's average density), then the Roche Limit would be as little as 0.8% of an AU. Seem enough small, but I am not sure a planet with Earth density can keep its atmosphere in this way...would be easier if the planet was bigger isn't it?

about 2, as i said this is probably the less problematic issue, but would the planet be tidally locked in this scenario?

about 3 and 4, seem it mean that the planet should be more near to the BH than to the 2 suns,
if we keep the 2 suns at 1.4 AU should have same flux of 1 sun at 1 AU (well, if i underdstand it), in this way the 2 suns would be more far away from the planet than one sun, giving the same flux, but as we said that in a kinda similar system (binary), the planet could need more light than just 1 sun, its reasonable to think the distance > 1 AU and < 1.4 AU?

about 5, assuming the two stars are twins very close to each other, orbiting around their own barycentre, while this barycentre orbits the black hole (or the centre of mass of the whole system), would this kind of system give enough stability to the planet orbit?

Ps i was reading also about that superEarths orbiting a neutron star ( https://arxiv.org/pdf/1705.07688.pdf ), if they have a very dense atmosphere, can in theory host life because the atmosphere will shield them and also heat the planet when the xrays hit. This same assumption can't be considered for planets orbiting supermassive black holes and getting heat in the same way from the accretion disk? Because in this way we wouldn't be forced anymore to choose an orbit in the UV spectrum, as heat will come from the the xrays hitting the superdense atmosphear and so the orbit can be in the blue spectrum, because even much colder it will not be the one source of heat for the planet. Also, in this way, if the planet is tidally locked, at least one emisphere would be safe from xray regardless the so dense atmosphear (howhever i suppose it stll need it for heating process). Is this possible?