Exoplanetary systems

Well, at least they didn't announce this "stunning new announcement" a month in advance without telling anyone what it was just to build up public anticipation again. Thank you, NASA, for not doing that again.

Based on simple geometric analysis, "Super Earths" are almost certainly uninhabitable without significant amphibious technology and pressure suits.

For round numbers, if the planet has 8 earth masses and the same average density, it would have 2 earth radii and therefore double earth surface gravity, with 4 times earth surface area. If the planet has proportionate atmosphere and water content, it would have 8 times the volume of water and atmosphere, making the surface pressure 4 times greater than earth: 8 times mass of atmosphere divided by 4 times surface equal double the atmospheric mass per unit surface area, then double the gravity gives 4 times the pressure.

Same logic applies to water. Even with a proportionate volume of water, the entire planet's surface would be covered by water, except maybe a few rare mountain peaks, since the highest possible mountain peak also ends up being cut in half, etc. Again, the entire surface would be permanently covered in oceans.

Now to prove the tallest mountains would, on average, be only about half as tall as the tallest mountains on earth consider the power of wind and water erosion at 2 earth gravities and 4 atmospheres pressure. At any elevation above sea level, the gravitational potential energy of water runoff would be double that of the earth, and the same would apply for rock formations simply collapsing under their own weight, etc.

So mountains would be eroded in half the time, and oceans would be, on average, twice as deep anyway, which means almost nothing would ever be above water.

This is true for a planet of exactly the same compositions and proportions as earth, just scaled to 8 times mass...

We can consider similar logic for smaller planets between 1 earth and 8 earth masses.

Based on the same logic, a larger-than-earth terrestrial planet needs a significantly lower proportion of air and water than the earth by mass, else there will be no dry land and the air will be too dense...

Perhaps a mega-earth spins much faster, so is significantly oblate ?? That would pull water away from the poles...

Perhaps a mega-earth is in the throes of an ice-age, which locks much of the water at the poles ??

Perhaps it is a 'snowball earth', like Antarctica, with nunataks and dry valleys...

Perhaps a mega-earth has seas rather than oceans ??

There are many possibilities that would provide plenty of dry land. Agreed, mountain elevations will be less, and erosion faster than Earth's...

Nik_2213:

I was just hitting the high notes assuming same average composition as earth. A more detailed analysis is too much for 1000 characters.

There are other more complicated issues at stake for a super earth. As you increase either gravity or atmosphere, or both, your surface pressure goes up. This effects the temperature at which substances such as water and CO2 will freeze/thaw/sublimate and vaporization/condensation.

At higher pressures and with higher gravity, water should freeze at slightly higher temperatures, and the temperature for vaporization will be higher too. As always, the exact behavior of these media will depend on the planet's relative distance to it's host star as compared to that star's brightness. Yet because the pressure and gravity is so great things should tend to get ground to a flat state, and this would include the glaciers themselves in a "cold" super earth.

For example, you might think the ice will build up in ice caps at the poles, but this probably isn't going to happen, because with twice the gravity you need much more energy to power convection to move the water from the tropics to the poles. So for example, if the planet was the same average temperature as earth, bu t has twice the surface gravity, it would be very difficult for water to evaporate to form clouds, because.

At 760Torr, water vaporizes at 100C.

at 1600Torr, water vaporizes at 120C.

I don't even know of a phase diagram that would show 3040Torr, but I suspect it would vaporize somewhere around 150C...

So on an 8 earth mass planet of similar composition and temperature, weather as we know it wouldn't exist. At least the weather would derive very little, if any, of it's energy from convection of water vapor and condensation as is the case on earth.

Weather might be driven by convection of somethng else, or purely by day/night cycle.

Which is to say, the atmosphere would probably behave similar to a gas giant, with banding features caused as materials become sorted by composition and temperature.

Most wind would be driven by the planet's rotation, day/night cycle, and tidal forces, not the water cycle.

Oh yeah, I forgot water's phase transition from solid to liquid and vice versa is opposite most things, which makes this situation even weirder.

It's actually HARDER for water to freeze, i.e. lower temperature required, at a higher surface pressure. Which is counter intuitive, but suggests that there probably wouldn't be any polar ice caps at all if the average temperature was the same as the earth...

Finally, in the case of situations such as trenches on such a large planet, water in deep trenches at super-earth gravities eventually crystalizes under it's own weight, forming "Ice 7". so if there are any "super deep trenches" of something like 100,000ft or more below the ocean surface, then the water at those depths will actually solidify and become a crystal under the weight of the water above it. This wouldn't necessarily require a "water world," just a very, very deep trench on an otherwise proportionate size and composition "super earth".

So depening on exact composition and geology, the behavior of ocean currents and weather on a planet like this would be drastically different from anything we've ever observed directly.

The only reason they haven't discovered thousands of earth sized planets yet is that our detection equipment simply isn't good enough at present.

This is now officially the Quantum_Conundrum crank thread.

Sure life might be different, but it’s still got oxygen and water, and humans can live comfortable in environments of up to 4 g's, and more than 3 atm's. Not all big planets have big atmospheres, and vice versa. Look at titan, it has more atmosphere than the earth but it’s definitely smaller. And look at Venus, arguably the same size but significantly greater pressure. There are likely a huge range of earth sized planets with varying atmospheric pressures. I'm certain there are plenty of smaller earth sized planets, in range for water to be liquid, especially when considering how many small ones naturally formed in our system.

And considering that water is the most common compound in the universe, found almost everywhere, and the polar nature of water naturally enables gaseous oxygen to form, especially if in the presence of UV radiation, produced by most stars, at least until ozone blocks it out, which coincidentally is naturally produced when water reacts with uv radiation. As far as I’m concerned chances are of countless worlds with a breathable atmosphere and drinkable water exists regardless of whether life already lives there or not, especially when considering the fact that we live in a galaxy of hundreds of billions of stars.

This is now officially the Quantum_Conundrum crank thread.

Explain anything that I've said that was wrong.

Ramael:

No we can't live on a planet with 4gs. You're smoking crack. Maybe, just maybe 1.5gs, and definitely not 2 or more.

If a perfectly fit 185lbs athelete were on a planet with 4gs he'd weigh 740lbs. That would be like lifting and carrying 555lbs all day long, and sleeping with that much weight on top of you, etc. The world's strongest man is much bigger than 185lbs, and can overhead press about 460lbs @ earth gravity...one time, or carry 800lbs about 100ft before total exhaustion.

You're in fantasy land buddy. This isn't a marvel comic book, and it's not Star Trek or Star Wars either.

Our pressure tolerances are much better than our ability to tolerate high gravity, as evidenced by people who can dive very deep, but even that has limitations.

Even at 2gs, such as on the surface of our hypothetical 8 mass terrestrial planet "super earth," our 185lbs athelete (think college back or wide receiver,) would weigh 370lbs. This perfectly conditioned, thin, muscular athelete would be in about the same condition there as a morbidly obese person is here on earth.

I haven't even got into considerations like building construction or plant life in 2gs, but many of our crops and shrubs would not grow in 2gs because they'd be smashed under their own weight either whle trying to grow, or else afterwards when you try to ship it you can only stack half as much in a stack before the bottom is smashed, etc.

Trees would have their maximum height cut in half, and their limbs would reach the breaking weight at about 2/3rds the diameter and break under their own weight long before full growth.

In general, construction is at least twice as hard because everything weighs twice as much and needs to hold up twice as much, etc.

For an 8 mass super earth, escape velocity is 22.388km/s.

Moreover, from an altitude of an addtional 2 earth radii above the surface, the escape velocity would still be 15.831km/s.

You'd need a rocket the size of a sky scraper, if using conventional chemical power and propellant, in order to land on or lift off from an 2g planet's surface. Or you'd need something about twice as big as the Saturn V in order to leave orbit if you were already orbitting at an altitude of 2 earth radii.

Landing would be easier than getting back off the surface, but it's hard to fathom what someone would have to concoct to land on something like this, and highly dependent on the atmosphere. If the atmosphere is proportionate you might be best served with some sort of HUGE parachute or balloon...but we're talking twice earth gravity. The Apollo capsule would be roasted like a marshmallow.

When people go to space, they experience g forces for only a few minutes during launch, and then it's over.

Experiencing continual high g forces would be very deadly.

If anyone has ever done the "Gravitron" ride at a carnival, you are experiencing about 1.5, maybe 2, total net g forces by the time you do the vector math between the earth and the horizontal angular motion, i.e. the centripital force, and factor for overcoming coefficient of friction. This is how you are able to go up the "ramp" of the wall.

Now imagine that non-stop all the time. That's what you'd have on the surface of a 4 earth mass planet: 1.58g.

This notion that humans would be walking around on a >2g planet is a pipe dream.

So returning to the world's strongest man example, Zydrunas Savickas, the reigning champion weighs 385lbs.

Now "pound for pound" he may not actually be the most powerful person alive, but in overall brute strength, he is the current reigning champion.

So now on earth he did the "Whisky Barrel Carry" at ~800lbs. So 800 plus 385 = 1185 for the maximum of what he lifted and walked with for about 100ft, being the only person to actually complete this event this year with dropping them at least one time.

On a 3g planet, his own weight would be 1155lbs, which means that on a 3g planet just sitting or standing or walking would be non-stop continual effort equal to maximum exertion at a strong man competition.

On a 4g planet the world's strongest man would not be able to lift his own body weight to stand upright even one time.

Now if you take some body builders who are much smaller, but stronger "pound for pound" they would fare only SLIGHTLY better than this...

There's this funny phrase right below the comment box, it says "Brevity is the soul of wit"... :)

There's this funny phrase right below the comment box, it says "Brevity is the soul of wit"... :)

Yes, well, not everything can be explained in two sentences or less. It's why we have things called paragraphs.

The 1000 character limit is woefully inadequate for real discussion of topics like this particular thread.

There's this funny phrase right below the comment box, it says "Brevity is the soul of wit"... :)

Yes, well, not everything can be explained in two sentences or less. It's why we have things called paragraphs.

The 1000 character limit is woefully inadequate for real discussion of topics like this particular thread.

Which is why the physforums exist. I'm guessing you'd be laughed out of there though.

wow. Are our 'scientists' on this website offended because I understand this stuff better that you morons, and was able to prove it using real world examples of human limitations?

Anyone here want to try living in a 4 g environment, now that we have some idea what it would be like? You idjits gave him 4 and 5 stars for the claim humans could live in 4gs environment.

Want to talk about ME not knowing science, and you morons don't even know basic stuff about gravity and limitations on human biology.

Which is why the physforums exist. I'm guessing you'd be laughed out of there though.

At least I know a human being wouldn't live very long under his own weight at 4g, unlike some people on this thread, to whom other ignorant readers gave 4 and 5 stars.

It's mostly not even the same people on physforums as those who come here.

Anyway, anyone who gives a damn can derive everything I did using Newton's law, which is close enough for non-relativistic situations.

wow. Are our 'scientists' on this website offended because I understand this stuff better that you morons

I can assure you that a scientific argument has never been won by claiming to be a self-professed genius and the opposition 'morons'.

wow. Are our 'scientists' on this website offended because I understand this stuff better that you morons

I can assure you that a scientific argument has never been won by claiming to be a self-professed genius and the opposition 'morons'.

In reality no. On this board it happens daily...

As to the existence of terrestrial planets I think the single most important factor is the number of hot Jupiters we've found. A large percentage of the systems we observe have these planets. Recent observations tend to rule out the migration theory of their formation...alternate theories tend to rule out formation of terrestrial planets in the goldilocks zone.

It's no surprise we haven't found any TRULY terrestrial planets as yet because we simply haven't the "eyes" for them yet. I'm going to stick my neck out and say that when we do have the "eyes" for them we still won't find them...certainly not in the numbers some purport.

Planetary systems in general seem to be defying isotropy. More accurately they seem to be isotropic with respect to each other, but not with respect to our system.

As to the existence of terrestrial planets I think the single most important factor is the number of hot Jupiters we've found. A large percentage of the systems we observe have these planets

That's because we've only been able to observe such systems so far. Things are slowly improving though.

Recent observations tend to rule out the migration theory of their formation

They do? It certainly isn't the mainstream view.

alternate theories tend to rule out formation of terrestrial planets in the goldilocks zone

Huh?

(continued...)

(...continued)

I'm going to stick my neck out and say that when we do have the "eyes" for them we still won't find them...certainly not in the numbers some purport

Why? Do you have an agenda?

Planetary systems in general seem to be defying isotropy.

???

More accurately they seem to be isotropic with respect to each other, but not with respect to our system.

Didn't you just say we don't yet have the 'eyes' to see different types of systems? And yet you're injecting some kind 'uniqueness' argument for our little corner of the universe? I do smell an agenda!

wow. Are our 'scientists' on this website offended because I understand this stuff better that you morons, and was able to prove it using real world examples of human limitations?

I, for one, read and appreciated you comments more than all the others. Any scientist has appreciation and respect for Newton's laws. Your calcutations were correct. The "morons", as you say, are not scientists but real honest-to-goodness morons.
I hope this helps to clear things up.

I, for one, read and appreciated you comments more than all the others. Any scientist has appreciation and respect for Newton's laws. Your calcutations were correct. The "morons", as you say, are not scientists but real honest-to-goodness morons.
I hope this helps to clear things up.

Sockpuppet, we're laughing at you/QC because he has completely ignored every other fundamental construct of classical physics, ie: Boyle's laws of gasses.

That's because we've only been able to observe such systems so far. Things are slowly improving though.

Patently false. If we were seeing large percentages of systems with multiple gas giants with distances similar to those in our system you'd have a point. We don't see this, and you don't. We most certainly have the eyes to detect such systems.

They do? It certainly isn't the mainstream view.

Of course it is. Recent observations have been showing hot Jupiters with orbits that tilted 20 degrees or more with respect to the plane of the stellar disk where they were born. This rules out migration.

alternate theories tend to rule out formation of terrestrial planets in the goldilocks zone

Huh?

Do some research before you spout off. Look up the Kozai mechanism.

Why? Do you have an agenda?

No I have a brain. Do you have an agenda when you say there are probably billions of stars in the galaxy because that agrees with current observations?

Patently false. If we were seeing large percentages of systems with multiple gas giants with distances similar to those in our system you'd have a point. We don't see this, and you don't.

You're even contradicting one of your own statements. We don't see it because we CAN'T. The tech isn't there yet.

We most certainly have the eyes to detect such systems.

Again, no we don't! The most prolific detection method has been the wobble method, which is an indirect method that only picks up massive planets close to their stars. A less prolific detection method is the transit method, which also is biased in detecting closely orbiting planets due to the time it takes to complete multiple orbits needed for comparison. This is being extended by observatories that can scan thousands of stars over increasingly longer periods of time.

Of course it is. Recent observations have been showing hot Jupiters with orbits that tilted 20 degrees or more with respect to the plane of the stellar disk where they were born. This rules out migration.

Nope, it doesn't. A few anomalies do not invalidate what is still very much a mainstream view.

Do some research before you spout off.

Indeed!

No I have a brain.

Yes you do, but you're prone to losing it every so often.

Do you have an agenda when you say there are probably billions of stars in the galaxy because that agrees with current observations?

You've lost me there...

Posting with an alternate account becaue mine will no longer post to this thread..

You're even contradicting one of your own statements. We don't see it because we CAN'T. The tech isn't there yet.

Show me where I said we can't detect Gas giants. I said we can't detect terrestrial planets. Please read more carefully.

Again, no we don't! The most prolific detection method has been the wobble method, which is an indirect method that only picks up massive planets close to their stars. A less prolific detection method is the transit method, which also is biased in detecting closely orbiting planets due to the time it takes to complete multiple orbits needed for comparison. This is being extended by observatories that can scan thousands of stars over increasingly longer periods of time.

We've found planets out to 330 AU. Go fish.

cant post more than a few words again fing physorg.

I guess I can keep making accounts...

Posting with an alternate account becaue mine will no longer post to this thread

Why not?

Anyway, you said that a large percentage of the systems we're detecting have hot jupiters. I said that's due to current tech limitations. You then said "that's patently false" and confused the issue by talking about jupiters at distanced similar to our system.

I've only ever been talking about our ability to detect hot jupiters and conversely, our inability to detect truly terrestrial exo-planets.

Sockpuppet, we're laughing at you/QC because he has completely ignored every other fundamental construct of classical physics, ie: Boyle's laws of gasses.

I have never made a sock puppet account. I only have this account.

I agree we can't detect terrestrial planets.

What you fail to see is that we are seeing not systems out there that even LOOK like ours. Forget about not being able to detect terrestrial planets. If they're there we'll find them. Basically I'm saying since we're not seeing systems that look like ours it's unlikely we'll find the terrestrial planets when we DO have the tech.

We DO have the technology to find systems that look like ours, the only thing in those systems we wouldn't be able to see are the terrestrial planets. If we were seeing that I'd be very optimistic about finding many of them. Since we aren't I'm not. That simple.

I agree we can't detect terrestrial planets.

We DO have the technology to find systems that look like ours, the only thing in those systems we wouldn't be able to see are the terrestrial planets. If we were seeing that I'd be very optimistic about finding many of them. Since we aren't I'm not. That simple.

I disagree.

Why do you think we are only finding "hot jupiters"? That's because they are so close to their stars that they have short orbits and occult the star very often or are close enough to cause an easily detectable wobble.

It is very difficult to detect even Jupiter size planets in Jupiter-sized orbits because they will only occult the star every several hundred years, and the wobble from that far out is relatively minor.

I'll stick my neck out and say that when the tech gets better, we will find that nearly every star has planets, and that there are many many billion earths in this galaxy alone.

...there are many many billion earths in this galaxy alone.

To be clear, I am not saying that all these will be in the habitable zones nor conducive to life of any kind, let alone our own. But I wager we'll be finding a number of earth size and larger planets in our own Oort zone when we get better technology.

Geo:

Well you have a reasonable position. We'll know who's right in a few years.

Honestly I hope I'm wrong. I hope the galaxy is positively littered with worlds like ours. I'm just very pessimistic at the moment.

What will be even better is when we have the "eyes" to analyze the atmosphere's of the planets we're "observing". Then we get to answer some real interesting questions.

As an aside I think the observations we've made thus far are a possible answer to the Fermi paradox. Honestly we should see some evidence of civilization on a galactic scale...if not in our own galaxy then in another. The "pot" has certainly had enough time to brew...

Geo:

Well you have a reasonable position.

LOL, when I said EXACTLY what geokstr said, you went off the handle, but when geo said it, you agreed!

As an aside I think the observations we've made thus far are a possible answer to the Fermi paradox.

I disagree. Our sample size is minuscule (and biased) and will continue to be far into the future. The vastness of the cosmos, the unlikelyhood of complex, intelligent life, the unlikelyhood of their existence coinciding with ours and that they can be detected over vast distances makes the Fermi paradox not in the least paradoxical.

Honestly we should see some evidence of civilization on a galactic scale

I doubt it. What detection would be an unambiguous signature?

if not in our own galaxy then in another

That's even more fanciful, IMO.

What will be even better is when we have the "eyes" to analyze the atmosphere's of the planets we're "observing". Then we get to answer some real interesting questions.

They're already learning how to analyze the atmosphere of exoplanets by subtracting the spectrum of the star from the combined spectrum when the planet occults the star. It will be a long time before we'll be able actually view it directly.

They're already learning how to analyze the atmosphere of exoplanets by subtracting the spectrum of the star from the combined spectrum when the planet occults the star. It will be a long time before we'll be able actually view it directly.

That's true, though I do remember reading about some new star-nulling system which would make nearby planets directly imageable. But I don't recall if such a system is actually on the drawing board or if it was just a theoretical proposal.

As an aside I think the observations we've made thus far are a possible answer to the Fermi paradox. Honestly we should see some evidence of civilization on a galactic scale...if not in our own galaxy then in another. The "pot" has certainly had enough time to brew...

Again I have to disagree. Who knows how long the average civilization lasts, given all the things that can happen. Even if it's millions of years, the galaxy is billions of years old. Hundreds of alien cultures could have come and gone already. Maybe they don't use radio waves, or care about exploration, or are water creatures or so many other possibilities that would make detection impossible.

That's true, though I do remember reading about some new star-nulling system which would make nearby planets directly imageable. But I don't recall if such a system is actually on the drawing board or if it was just a theoretical proposal.

You're right, but all they really get is a dot to analyze. The planet itself won't radiate sufficiently to allow direct spectrographic analysis, unless it's a brown dwarf.

"...I do remember reading about some new star-nulling system which would make nearby planets directly imageable. But I don't recall if such a system is actually on the drawing board or if it was just a theoretical proposal."

You may be thinking of the Apodizing Phase Plate coronagraph:

h
ttp://www.physorg.com/news/2010-10-planet-hunters-longer.html

The article describes an APP device recently developed and used on one of the 8m VLT scopes in Chile and mentions another coronagraph used on the 6.5 Meter Telescope atop Mt. Hopkins. Aside from exoplanet research, these devices should help astronomers studying quasars, blazars and other AGN galaxies, which share similar contrast ratios. They are also relatively cheap and easy to adapt to a variety of telescopes.

You may be thinking of the Apodizing Phase Plate coronagraph

Yes, thanks, I do remember reading about that one, but I was actually thinking of a different system which was to be space based. It had a weird flower petal type design which would block and diffract the star light in a particularly advantageous manner. I wish I could member what it was called - I may have read about it New Scientist a couple of years ago...

LOL, when I said EXACTLY what geokstr said, you went off the handle, but when geo said it, you agreed!

You did nothing of the kind. Your initial response to me was fucking prickish to say the least. You basically said we don't have the technology to see terrestrial planets (which is what I said in my post too) and proceeded to accuse me of having some kind of agenda.

Basically you were being a dickhead, and then had the nerve to act surprised when I responded in the tone I did...

Why is it more fanciful to assume we might observe cosmic engineering in any other galaxy as opposed to our own...is our galaxy somehow special? Geee...do you have an agenda?

If terrestrial planets are as common as you seem to think they are then at least one civilization should have lasted long enough to hit type III on the Kardashev scale. Their mark on their galaxy would be as unambiguous as the lights from Las Vegas from orbit...

You did nothing of the kind. Your initial response to me was fucking prickish to say the least. You basically said we don't have the technology to see terrestrial planets (which is what I said in my post too) and proceeded to accuse me of having some kind of agenda.

I'm not going over this with you again - the record speaks for itself, so let's just move on.

Why is it more fanciful to assume we might observe cosmic engineering in any other galaxy as opposed to our own...is our galaxy somehow special? Geee...do you have an agenda?

You're not doing yourself any favors. I asked you how we could unambiguously detect cosmic engineering. Isn't it obvious that if such a detection is difficult in our own galaxy, that it would be much more difficult in anothor galaxy, which is even more distant?

If terrestrial planets are as common as you seem to think they are

I don't think I ever said that they were 'that common', just that we can't detect them yet.

at least one civilization should have lasted long enough to hit type III on the Kardashev scale.

That's an unwarranted assumption. Just because a terrestrial planet exists, doesn't mean it will be conductive to complex life. And eve if it is, there's no guarantee that creatures as smart as we, or better, will evolve. And even if they do, they may not have an interest in technology, like say whales or even some nomadic human populations. And then there's the matter of long-term survivability, ability to routinely access space, to be coincidentally around when we were, the list goes on.

Their mark on their galaxy would be as unambiguous as the lights from Las Vegas from orbit...

Doubt it. Again, how could you tell a point of light thousands or millions of lightyears distant is artificial in nature?

That's an unwarranted assumption. Just because a terrestrial planet exists, doesn't mean it will be conductive to complex life. And eve if it is, there's no guarantee that creatures as smart as we,

blah blah blah...

Given the size of the Universe you're wrong. No better way of saying it.

Doubt it. Again, how could you tell a point of light thousands or millions of lightyears distant is artificial in nature?

Uh well apparently you don't quite know where to stop with an analogy. It would most likely be the LACK of "points of light" we'd notice. They'd be building Dyson spheres around most of the stars in their galaxy, or even around the galaxy itself. You're imagination is pretty limited if you think we'd not be able to see the effects of an intelligence who's had millions if not a billion years to "leave their mark".

Hell who knows, maybe that's what dark matter "is"...

Well, I tried having a reasonable conversation with you, but it's evidently pointless.

Well, I tried having a reasonable conversation with you, but it's evidently pointless.

You did? I missed it.

And yeah it is pointless to continue a conversation you started out with the tone you did and expect it to change.

...the Kardashev scale...

...is nothing more than wild speculation, and might as well be called "science fiction".

The universe is so big as to be unimaginable. We think there's nothing past the edge of the universe, but we really haven't the vaguest clue. There are hundreds of billions of galaxies with hundreds of billions of stars in each in just the observable part of the universe. Every star is likely to have planets. The odds of other advanced civilizations are overwhelming.

But if Einstein is right and the speed of light is immutable, it is highly likely that Kardashev is whistling in the dark, and that civilizations becoming so technologically sophisticated they can use 100% of the energy output of a star is only fantasy. Or that every alien culture that lasts long enough will try to send out robot probes to colonize their galaxy, or build Dyson spheres, or any other science fiction writer's dreams.

But who knows? Maybe pulsars are traffic signs or warning signals.

I think an arbitrarily advanced civilization is going to leave signs of its existence on it's environment. I think it's a given. We'll just have to agree to disagree.