The dying of the light

[Guest]

7 minutes ago, wumpus said:

Life exists near undersea hydrothermal vents (the only ecosystem that doesn't require sunlight), but only up to ~130C.  Venus is more like ~470C, something no Earth based life seems possible of being able to handle (although I wouldn't have expected >100C either).

It's not a question whether any current life forms from Earth could survive on Venus (they couldn't), but whether they would, when Earth turns into Venus over a billion years or so, gain that ability. That, IMO, is a fairly likely scenario.

Remember, it's the same life that not only adopted to living in an atmosphere full of highly reactive, highly lethal poison, but actually found a use for it, and now has serious trouble surviving without it. There's also the fact that this very life figured out, a while before that, a way to fill the entire atmosphere with a highly reactive poison, lethal to all competition except themselves, all as a side effect of their method of energy collection (sounds familiar?  ). And all that occurred over a much shorter timespan than a billion years, without the benefit of sexual reproduction, at that. Of course, that kind of backfired when the planet turned into an ice ball for a while (the Sun being dimmer back then), but that didn't do much to stop it, as we can all see.

[K^2]

11 hours ago, Dragon01 said:

Because Earth is a great big hunk of rock and unless we stripmine it all in the meantime, it'll still be a great big hunk of rock. Hunks of rock that big take some time to heat up and fall apart, even in the solar corona, which, despite impressive temperature readings, isn't very dense.

It's dense enough to generate tremendous drag. Yes, heating from corona, while significant, will cause ablation over very long time scale, except, Earth isn't sticking there. Even in high corona, the orbit can be maintained on the order of decades, maybe centuries, not hundreds of thousands of years that you need. And once the planet gets to low corona, it will decelerate very fast. Most likely, it will break up. Sun's surface is right on the threshold of Roche limit for Earth-sized object (0.8R for rigid and 1.55 for fluid), and with a help of hydrodynamic forces of low corona, I'm pretty confident calling it. [Edit: I'm being silly. Of course, this is in relation to Sun's current radius. Once Sun becomes a red giant, Roche limit will stay well inside, so it's not a factor at all. Hydrodynamic forces might still break up the planet, but I'm not nearly as certain of it. Rest of the post stands.]

Even if the Earth somehow didn't disintegrate due to combination of gravitational and hydrodynamic forces, it would decelerate very rapidly at that point, ending up literally sinking in much lighter hydrogen-rich atmosphere of the Sun. And as much drag as it's going to generate, it's not a very slow process. Add to that pressure and convective forces, and Earth has about as much of a chance as single ice cube in a hot cup of tea.

Edited August 17, 2020 by K^2

[mikegarrison]

I thought the models tended to show that while Venus will definitely end up inside the sun, Earth will quite possibly be far enough out to avoid that fate.

[Nuke]

i think the surface of the earth will become uninhabitable long before the sun consumes it, if it does at all. the habitable zone will creep out gradually giving humans enough time to relocate. mars might self-terraform as it gets a better position inside the habitable zone and we can certainly help it along by setting up an artificial magnetosphere (perhabs drilling down to the core at the poles and putting a giant superconducting solenoid at each end, powered by fusion reactors) and importing as much water as possible. then we can move on to ceres or the jovian moons as it expands further. then again we might be type 2 by the time that happens.  

Edited August 17, 2020 by Nuke

[K^2]

3 hours ago, mikegarrison said:

I thought the models tended to show that while Venus will definitely end up inside the sun, Earth will quite possibly be far enough out to avoid that fate.

I've seen some recent papers from studies of other stars that suggest that we might have been severely underestimating extent of corona at red giant stage. I don't recall details, and whether that'd be relevant to our own Sun when it reaches that stage, but it might be enough to offset whether or not Earth, and possibly even Mars, end up decaying from their orbits due to drag. If I'll find anything useful, I'll add info. Right now, take that with a considerable quantity of salt.

[mikegarrison]

1 hour ago, K^2 said:

I've seen some recent papers from studies of other stars that suggest that we might have been severely underestimating extent of corona at red giant stage. I don't recall details, and whether that'd be relevant to our own Sun when it reaches that stage, but it might be enough to offset whether or not Earth, and possibly even Mars, end up decaying from their orbits due to drag. If I'll find anything useful, I'll add info. Right now, take that with a considerable quantity of salt.

Well, I guess we'll have to wait for a bit to find out who's right.

[Guest]

15 hours ago, K^2 said:

It's dense enough to generate tremendous drag. Yes, heating from corona, while significant, will cause ablation over very long time scale, except, Earth isn't sticking there. Even in high corona, the orbit can be maintained on the order of decades, maybe centuries, not hundreds of thousands of years that you need.

So, I went and ran the numbers. Mind you, that was very much a "spherical cows" calculation, but should be good enough to get the order of magnitude. Using the standard drag equation, authalic radius to calculate reference area, and 10^-16 g/cm^3 value for solar corona density (from Wikipedia and for Sun's current state, but I don't think it's a lot denser on a red giant), I'm getting "tremendous drag" on order of 5600MN*Cd (not sure what Cd for Earth inside a Solar corona would be, but hopefully not too large   ). Given this, assuming no significant changes in Earth's mass and a Cd of 1, coronal drag would initially slow it down by 10^-8 m/s per year. That's not a whole lot, compared to the orbital velocity of 30km/s.

The above calculation, of course, makes some very silly assumptions, but I suspect that in terms of orders of magnitude, this should be pretty close. Besides, doing this "properly" would involve a lot of guesswork, anyway, since neither aerodynamics of planets orbiting inside stars nor exact parameters of the Sun after it becomes a red giant are quite settled science (in fact, it sounds like a fun PHD thesis for someone into theoretical physics/astronomy). It appears to gives it more time than I thought it'd have, in fact. It's draggy, and its days are certainly numbered, but it should last long enough in the corona for some bizarre lifeforms deep in the rock to live a fulfilling life.  

Edited August 17, 2020 by Guest

[sevenperforce]

16 hours ago, K^2 said:

It's dense enough to generate tremendous drag. Yes, heating from corona, while significant, will cause ablation over very long time scale, except, Earth isn't sticking there. Even in high corona, the orbit can be maintained on the order of decades, maybe centuries, not hundreds of thousands of years that you need.

Right, I was going to say this and then I saw you already had. By the time Earth is anywhere close to the solar corona, the solar wind will start decelerating it pretty rapidly.

59 minutes ago, Dragon01 said:

So, I went and ran the numbers. Mind you, that was very much a "spherical cows" calculation, but should be good enough to get the order of magnitude. Using the standard drag equation, authalic radius to calculate reference area, and 10^-16 g/cm^3 value for solar corona density (from Wikipedia and for Sun's current state, but I don't think it's a lot denser on a red giant), I'm getting "tremendous drag" on order of 5600MN*Cd (not sure what Cd for Earth inside a Solar corona would be, but hopefully not too large   ). Given this, assuming no significant changes in Earth's mass and a Cd of 1, coronal drag would initially slow it down by 10^-8 m/s per year. That's not a whole lot, compared to the orbital velocity of 30km/s.

Huh. 

I wonder what happens if I use the derived drag equation from Newton's momentum exchange approximation. Assuming Earth remains at its current orbital distance, it sweeps out a volume of 1.199e26 cubic meters each year (cross-sectional area of Earth * orbital semimajor axis * 2π). I'm also getting 1e-16 g/cc from a couple of sources. This means the area swept out by the Earth annually would contain 1.199e13 kg of matter. Earth, of course, is 500 billion times more massive than that, so Earth would only lose 1/500,000,000,000th of its orbital velocity each year.

Looks like our intuition was just wrong.

[Lisias]

On 8/16/2020 at 4:29 AM, eatU4myT said:

Let's suppose for a moment that our descendants in 1 billion years have forgotten that the sun was expected to become a red giant. Perhaps they lost all the astronomy text books in a freak yachting accident, who knows. At any rate, they have access to all of the scientific instruments that we have sent into/pointed at space, and also to the results that we have obtained by using those instruments in our time. Which instruments and observations would be the first to suggest to them that something was happening to the sun?

You **NEED** to watch this video:

 

[eatU4myT]

Or at least, the first 3 minutes of it! Interesting stuff.

2 hours ago, Lisias said:

You **NEED** to watch this video:

 

 

[DDE]

3 hours ago, Lisias said:

You **NEED** to watch this video:

TW: footage from 2012.

At 7:50: I always knew it. The end of the world is Mongols.

Spoiler

 

 

Edited August 17, 2020 by DDE

[K^2]

3 hours ago, Dragon01 said:

order of 5600MN*Cd (not sure what Cd for Earth inside a Solar corona would be, but hopefully not too large   ).

Checks out. Cd is going to be of order 1 for basically anything. You'd need a better model to get precision, but for order-of magnitude, just take it to be 1 and you'll always be in ballpark.

3 hours ago, Dragon01 said:

coronal drag would initially slow it down by 10^-8 m/s per year. That's not a whole lot, compared to the orbital velocity of 30km/s.

Getting slightly larger number, but close enough. So one last step this needs to be formal is figuring out whether that actually is a lot or not. Best way to do this is to take this as energy loss and look at how that impacts orbit.

On the following plot, I've taken specific force, multiplied it by orbital speed, and assumed constant energy loss. Then I plotted how the orbit decays with that energy reduction. I did actually bump up density to 10^-14 g/cm³, because that's the highest number I could find for coronal density of the current Sun, and I think you'll see why in a moment. The x axis is time labeled in years and y axis is semi-major axis in AU.

So that's like 5% decay in a billion years.

 

Thing is, if we don't get dramatically higher density at 1AU, Earth will be fine. I mean, yeah, atmosphere will be gone and surface scorched, but Earth will still be right where it's at now. No plunge into the Sun.

So let me revisit the point of contention.

On 8/16/2020 at 3:45 AM, Dragon01 said:

Even when the Earth ends up inside the Sun, for all we know, we might just end up with creatures adapted to survive and thrive inside a star.

I'm reading this as Earth actually becoming part of Sol proper, not just passing through its extended atmosphere. If all you meant was, "Earth gets bathed in Solar corona, and life persists somewhere within," yeah, maybe. But that's not actually being inside a star. Mercury is going inside the star. Venus is going inside the star. Jury's out on Earth, but if it does go inside the star, the descent is not going to be like the above. It can't, you've just demonstrated that. And what the above graph really tells you is that deceleration will be smooth and gradual, right up until the point that you start hitting denser pockets of stellar matter, and then it won't be fine and gentle anymore. That's a completely different profile.

To get an idea of what the descent will be like in reality, you should model density as an exponent. Now, it's not a perfect model, but it puts you on the right track. And you can run the numbers, but actually, we already have KSP doing that for us. Sort of. The model for aerobraking is similar. Again, not for precision, but kind of order of magnitude sort of thing. Put a ship in circular orbit just skimming the atmosphere. It can actually hang there for a while, because drag starts out really low, but you also aren't getting any heating or anything else of note, and a light puff of engines will pull you straight out. That's what we're seeing above. But once you actually start descending, the process is very quick. You can hang on the outskirts for a long time, but not through your actual descent. And this will be the case for any planet spiraling into the Sun.

Same goes for heating. While Earth is bathed in corona, the surface will be boiling, cooling everything down. Temperatures will be in high hundreds, possibly low thousands of K, but well within what we're used to dealing with. I'd have to take a look at heat conduction through rock to see how long the deep-living bacteria have, but I suspect it will be a while. Possibly long enough to outlive the Star if the Earth doesn't fall in.

But if it does fall in, that final descent into the star is going to be brief and it's going to be violent. And the time between deep bacteria not even experiencing any changes and the planet being ripped apart into atoms is not going to give you enough time for any kind of evolution to work.

So either Earth stays pretty much where it is in orbit, scorched, but not fundamentally changed, in which case, sure, there might very well be life surviving within it pretty much until the core freezes solid. Or Earth falls in, in which case, the ride will be short.

[magnemoe]

1 hour ago, DDE said:

TW: footage from 2012.

At 7:50: I always knew it. The end of the world is Mongols.

  Hide contents

 

 

Next problem. 

Stuff like the heat dead of the universe will be a bit harder to solve. 

[K^2]

6 minutes ago, magnemoe said:

Next problem. 

I think I know where you learned to solve problems.

Spoiler

 

 

[Guest]

1 hour ago, K^2 said:

So either Earth stays pretty much where it is in orbit, scorched, but not fundamentally changed, in which case, sure, there might very well be life surviving within it pretty much until the core freezes solid. Or Earth falls in, in which case, the ride will be short.

I think the dispute is partly hinging on just what "being inside the star" means. The corona is, indisputably, part of the star, after all. It is a whispy "atmosphere" of it, but since there's no true solid surface (it's all different kinds of plasma), any other definition of "inside" that's more restrictive is going to be more or less arbitrary. There is a definite boundary between the chromosphere and corona, but no real surface as with planets. I suppose you could try to define a Karman line as that, which should end up somewhere inside that boundary region. That said, at the scales we're talking about, the transition is abrupt enough, so that might not matter (see below).

Exponent is not a good model for the solar corona, because its density is almost constant. The graph looks like this:

Assuming it doesn't hit the spicules too much, it doesn't even matter where exactly inside the corona the Earth will end up. There will, most likely, not be much spiraling going on, because the distance between the point where the density of the corona starts to increase and the start of the photosphere is a mere 4000km, less than the Earth's radius. Now, the photosphere has similar density to Earth's upper stratosphere (10^-4 kg/m^3). Earth will not plunge into it as much as crash into it (or rather, given that the Sun would have to expand towards Earth's orbit for it to happen, the Sun's surface will smack into the Earth).

That is, of course, assuming it has similar density in a red giant as it does in the Sun, but those transition regions shouldn't get much taller. Of course, I don't think we have that kind of data on red giants, anyway.

Edited August 17, 2020 by Guest

[K^2]

3 hours ago, Dragon01 said:

I think the dispute is partly hinging on just what "being inside the star" means. <snip>

Exponent is not a good model for the solar corona, because its density is almost constant. The graph looks like this: <snip>

There are exoplanets out there currently within the corona of their star that are cool enough to have liquid water on the surface. They probably don't, because they probably have their atmosphere stripped, but bellow surface, they can be habitable even by our, terrestrial standards. If you are happy calling that thriving inside a star, then you are technically correct, and we don't have a disagreement. But it's kind of like if you did Mars fly-by, skipped off the atmosphere, and then said you've been to Mars. Technically, sure...

Nonetheless, if that's what you meant, I don't have any objective disagreement, just a subjective "meh" feeling about it.

 

And exponent is an excellent model for the graph. You're just thrown off by a constant bias, which, as we've seen above, is pretty much irrelevant to planet's motion. If your model is exp(...) + constant, and constant is tiny, then you can just model it as an exponent. That place where density curve goes almost horizontal? That's the fun part. Planets touch that, and NOW they're heading into the star-proper. This is the only part that interests me from perspective of "What happens to the planet once it's inside the star." Until then, it might as well be in slightly harsher solar wind.

And yeah, technically, since this is a log scale, exponent would look like a straight line, and you can see that within chromosphere there's a good chunk that is actually almost perfectly exponential. If you extend that line through the top of chromosphere and into the corona, you'll get a model that's good enough for rough estimates. Again, getting that density to within an order of magnitude will get you correct qualitative predictions, and that line looks like it'd do. So for modeling behavior of a planet falling into the star, I'm happy enough calling that an exponent until you hit photosphere.

[sevenperforce]

The velocity of the solar wind is what, 400 km/s? I wonder if that pushes the Earth into a higher orbit. 
 

The actual gravity of the Sun won’t change as it swells. Thank you Shell Theorem. 

[K^2]

1 hour ago, sevenperforce said:

The velocity of the solar wind is what, 400 km/s? I wonder if that pushes the Earth into a higher orbit.

No. There is no net angular momentum transfer and at equilibrium, no net work, as there is zero average movement in radial direction. All it's going to do is reduce orbital velocity for given distance by the tiniest of amounts and probably circularizes orbit over very, very long time.

[Hannu2]

On 8/16/2020 at 2:18 PM, Dragon01 said:

That's what the second line about "some thermodynamic barriers" addressed. However, do not dismiss that idea so quickly. In particular, you'd be surprised what can survive inside a rock. What I've been thinking of is life evolving on the surface, in the increasing heat, and colonizing deep regions of Earth. There are already bacteria found very deep underground, and as the surface temperatures increase, I would expect the biosphere, now adapted to higher temperatures, would penetrate even deeper. Now, heat transfer through a rock is pretty slow, and while the transition might not be slow on the surface, it certainly will be so a few kilometers underground. Life may not be able to survive on Earth by the time it ends up inside the Sun, it might well be able to last in it.

I've seen enough extremophiles to treat any "it can't be life that evolves from anything on Earth" type statements with suspicion. Some of these organisms are really puzzling, and quite a few of them turned out not to be relics of the very first organisms, but modern organisms that took up the niche (and probably outcompeted any then-living fossils that lived there). Seriously, what chances would you give that Chernobyl mold evolving from anything we know? It took life one billion years to go from plankton to humans and everything else. It'll be several times more than that before Earth even gets near the Sun. 

You have not seen exthermophiles without very complex bonds between carbon atoms or other atoms. In DNA, in proteins etc. They are possible up to about 100 C and can occur in very high external pressure, but such bonds are not stable at star's atmosphere. Or you can not find any alive things from magma in Earth, because chemistry in which all life is based do not work at around 1000 C. There should be new natural laws unknown to us to make life in stars possible. New interactions and new particles which could produce complex structures.

Changes in Sun are so slow that whole Earth is sterilized millions of years before visible surface hits to Earth. It will probably melt the whole crust to global magma ocean.

[DDE]

3 minutes ago, Hannu2 said:

chemistry in which all life is based

How presumptuous. We have an unsurprising bias towards the study of organic chemistry, nor does life has to be based on molecular bonds, or even on baryonic matter. Neutronium life is wild speculation, but not impossible.

[Hannu2]

32 minutes ago, DDE said:

How presumptuous. We have an unsurprising bias towards the study of organic chemistry, nor does life has to be based on molecular bonds, or even on baryonic matter. Neutronium life is wild speculation, but not impossible.

What do you mean with impossible? Do you have a consistent theory of quantum chromodynamics which predict chemistry like behavior in neutronium matter under conditions in typical neutron star? Or what do you based on statement that such life may be possible? As far as I know neutronium is some kind of liquid, maybe superfluid, and does not form any complex structures. There is absolutely no known reasons to expect any reactions like lifeforms. It is purely science fiction.

However, it would not help Earth's life. Current life can not be evolutive step towards neutronium life, which works with completely different physical phenomena. It should begin from scratch. And it can never happen in our solar system, because the Sun is far too light star to generate a neutron star remnant.

[DDE]

18 minutes ago, Hannu2 said:

Or what do you based on statement that such life may be possible?

On the extremely broad definition of life. So long as there's an energy sink and a capacity to store information, we shouldn't exclude anything.

[Hannu2]

1 hour ago, DDE said:

On the extremely broad definition of life. So long as there's an energy sink and a capacity to store information, we shouldn't exclude anything.

Is there enough capacity to store enough information known lifeforms need and communicate it between parts of the system in neutron liquid? Is there usable energy source and sink and ways to control energy flow?

I understand the idea of philosophical thinking that everything is possible which is not proven to be impossible, but it does not lead to anything interesting. It is also possible that Walt Disney's cartoon world is real, city of Duckburg is somewhere in North American continent and Gyro Gearloose have made some strange mind disturbing device which prevents humans to detect any creatures or other objects of that story. It must be considered possibly true statement according to your philosophy, because nothing we can observe can ever prove it false. In science every statement must be able if be falsified based on observations to avoid such ridiculous mess of all possible fictive stories. Neutronium life is exactly like my example. We do not have theoretical base to predict such phenomena or observations which would tell something about such life. Anyone can say anything and think he is right because no-one can falsify his statement.

[sevenperforce]

12 hours ago, K^2 said:

No. There is no net angular momentum transfer and at equilibrium, no net work, as there is zero average movement in radial direction. All it's going to do is reduce orbital velocity for given distance by the tiniest of amounts and probably circularizes orbit over very, very long time.

The solar wind (and radiation pressure, to a lesser degree) is responsible for pushing the dust tail of a comet away from the sun, so it's definitely doing SOME work.

3 hours ago, Hannu2 said:

4 hours ago, DDE said:

How presumptuous. We have an unsurprising bias towards the study of organic chemistry, nor does life has to be based on molecular bonds, or even on baryonic matter. Neutronium life is wild speculation, but not impossible.

What do you mean with impossible? Do you have a consistent theory of quantum chromodynamics which predict chemistry like behavior in neutronium matter under conditions in typical neutron star? Or what do you based on statement that such life may be possible? As far as I know neutronium is some kind of liquid, maybe superfluid, and does not form any complex structures. There is absolutely no known reasons to expect any reactions like lifeforms. It is purely science fiction.

I'm torn on this point. On the one hand, it's true that we can't speculate endlessly without entering the realm of pure science fiction. I do not expect neutronium life to evolve, for example. But I also don't want to limit imagination. For example, there could be sentient clouds on Jupiter, converting solar energy into rotational eddies that maintain their structure. It's unlikely, but it's entirely possible within the realm of real physics.

Life is possible in any place where there is a source of energy and a sink for entropy. Both are required. One reason I don't believe it's realistic to imagine neutronium life is that there is no energy source (other than residual heat) and no entropy sink. Life is a machine that uses energy to artificially decrease its own entropy while increasing the entropy of its surroundings. So you need both to work.

Terran life crawls around on the surface of a giant rock, either breathing or creating corrosive gas. Our energy source is the sun and our entropy sink is the soil beneath our feet. Our ability to selectively convert energy into entropy is totally chemical, completely bound up in a very narrow range of temperatures and pressures. Life does not require these conditions to exist or to thrive, but it does require the existence of persistent structures capable of sustaining low-entropy regions. Intelligent life requires those low-entropy regions to be so thoroughly protected that they are able to hold information. And that is the question.

[Guest]

4 hours ago, sevenperforce said:

The solar wind (and radiation pressure, to a lesser degree) is responsible for pushing the dust tail of a comet away from the sun, so it's definitely doing SOME work.

Yes, but in case of a massive, symmetric body such as the Earth, it cancels itself out. If you've got an outward force that is constantly acting on a body in a circular orbit, work done at one point of the orbit will cancel out with that done at the opposite point, and the net work will be zero. This is not the case with an eccentric orbit, but in that case, the only parts of the state vectors not to cancel out will be the eccentric components (hope I put that right), and the orbit will be circularized. Hard to do in KSP right now, but in KSP2 you should be able to burn a ion engine radially for the entire duration of an orbit and see what happens.

This also happens with the cometary dust. It is separated from the main body of the comet and its orbit circularizes. It's just that, being dust, it also disperses so much that it ceases to be observable. To get net work out of the solar wind, you need to reflect solar wind, not absorb it, and the system has to be both asymmetric and able to control its attitude.