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Transit Detection of a Starshade at the Inner Lagrange Point of an Exoplanet (arxiv.org)
106 points by sanxiyn on July 11, 2017 | hide | past | web | favorite | 66 comments

Here I was hoping that Kepler had found a star shade.

Interesting point that at some point your observing apparatus gets good enough that you can 'see' the structures built by sufficiently advanced civilizations (sure they cloak their ships in orbit but you can see how they make their home world comfy!)

At one of the SETI seminars there was a discussion about when would be the "right" time to alert a newly discovered intelligent species that they aren't alone in the universe. There was a lot of back and forth about indigenous tribes in the Amazon, some of who learned of other tribes by the arrival of missionaries, some by loggers, and some who were out walkabout and came upon the strangers. How you meet outsiders has a different impact on how it affects you.

So if you were aliens and you didn't want to 'alarm' or 'damage' humans, what would you use as a signal that it was probably a good time to say "Hello" ? I've always felt that once you could detect they were having conversations on other planets you would now "know" we weren't alone and someone could appear in orbit and say Hi. Others felt it would only be safe if humans felt reasonably confident in their own ability to meet them at their level (so perhaps at least colonies on other solar system bodies). One person at our table was firmly in the only when it is unavoidable, which is to say they are about to send a probe to an inhabited planet or come across a construction like a station that is not easily concealed or moved.

Star Trek had its solution: no contact or interference with civilizations that didn't develop warp drive yet. The goal of the rule was to not contaminate the development of another civilization. In today's terms, you could generalize it to: no contact with a civilization until it's about to, or already did, figure out that there are others in the universe.

Which is kind of in between of the positions you described in the comment.

Beyond "it showed up on a popular television show", there is very little reason to believe in the Prime Directive being a real thing. It already appeals only to a particular subset of humanity, and there's little reason to believe it would appeal to all aliens. And there's a lot of humans who would consider it actively immoral, too.

It's one of those Fermi paradox answers that requires all of the supposedly millions or billions of intelligent species to all happen to have adopted one particular answer uniformly and for all time. Those answers aren't very compelling without a rock-hard explanation as for why it's absolutely mandatory that all star-faring civilizations will A: adopt that ideal immediately and B: keep that ideal intact for millions or billions of years despite all the changes they may go through.

The Prime Directive was a tool for creating drama, not a well-thought-out philosophical statement on the inevitably of certain ethics. Heck even in Star Trek itself, only the Federation has this policy, which it violates accidentally and otherwise fairly often anyhow. The other major civilizations show no sign of having this policy at all.

It is also an easy answer to try to explain the Prime Directive in a dramatic setting in such a way that our real world might still be in a space full of aliens, despite the fact that if space was really crawling with aliens there shouldn't be any such thing as virgin territory anymore and everywhere in the galaxy should be showing millions of years of proof of alien interactions, just as it's increasingly hard to find a square meter of Earth's surface that doesn't have some sort of unambiguous evidence of the presence of a technological civilization, if examined with powerful enough tools.

Which they violated whenever the script required it :-)

It's the 'figure out there are others' bit that made something like the Prime Directive problematic. Given our advances in sensing and observing technology we are much more likely to 'see' an advanced civilization before we can actually fly out to meet them. If you know that there is intelligent star faring people living on a 'nearby' star, and they happen to have been observing your progress, do they introduce themselves or wait for you to figure out how to reach them?

>Given our advances in sensing and observing technology we are much more likely to 'see' an advanced civilization before we can actually fly out to meet them.

I don't think this is really true (though I could be wrong, I am not an astronomer). Sure, we may be able to "see" evidence of an advanced civilization, but without warp drive, we won't have real proof. All we'll have is some observational data showing that, for instance, the light from their host star dims in a really weird way that's unlike anything we've seen before in our observations, or that there's some object sitting in their L1 spot, etc. We can hypothesize that this data indicates artificial structures, because what natural phenomenon could possibly cause such things? But that's not proof; there really could be some natural thing causing those readings.

So the ETs don't really have to establish contact until we have warp drive and the actual ability to go see these artificial structures up-close. They can just leave us to sit around our planet wondering what these observations mean, and slowly come to grips with the idea that there may very well be some intelligent ETs out there with starshades, and argue over it for a few centuries as we'd surely do. They have no obligation to come visit us and clear up the matter. And considering how backwards, irrational, and violent a species we are, there's no good argument I see for the ETs to rush first contact with us, and every reason to delay it as long as possible.

We'll be able to see the spectra from distant planets soon. That will amount to "seeing" advanced civs.

Going to them with warp drive without landing and talking to them would be equivalent. What would you be able to do? Basically sense EM radiation from there with better resolution, but it's essentially the same thing, "seeing".

You'll have to explain how seeing spectra from distant planets conclusively proves intelligent life. I don't see how it does. Spectra will tell you that a planet has molecular oxygen, for instance, carbon dioxide, etc., which would indicate an atmosphere like ours, which would indicate life similar to ours (doing photosynthesis), but you'd get that on a world occupied solely by plants, or with only non-intelligent life. Anyone looking at Earth before about 2M years ago (or anyone looking at it now, but more than 2M ly away) will see exactly this: a life-bearing planet with no life intelligent enough to build a civilization.

Finally, even if we could conclusively detect a civilization on a distant planet, that still doesn't explain why they have any obligation to contact us. They can easily just ignore us, and I for one can see why they would. The only time they'd really need to strongly consider establishing contact is when we have sufficient capability to visit them, and that's never going to happen the way we're going. It'd probably by prudent on their part to spy on us though, since we're a potential threat.

>Which they violated whenever the script required it :-)

Pretty sure it only ever came up when it was being violated. A rule literally made to be broken, lol.

Kind of like Asimov's 'Three Laws', really. It's a talking point and a foil for the plot. It was never intended to be a realistic and workable solution.

On the other hand, (at least in the I, Robot stories) the Three Laws actually seem to be fine as a realistic/workable solution; all the conflict revolves around robots being programmed to not obey them, whether due to added rules before Rule 1, elided rules, or reordering of rules.

Prime-Directive-related conflicts, meanwhile, seem to revolve around cases where the Directive itself is flawed (or at the very least not ideal for a given situation).

The funny thing is that the popular culture somehow missed the memo, and often treat the Three Laws as if they were really wise. You can see this happening in every other article about AI these days.

I've even heard people talking about them as if they're some kind of laws of nature, like the laws of thermodynamics.

The trouble is that we really have no way to make faster than light travel, and probably won't for hundreds or thousands of years, if ever. Meanwhile, our ability to see and detect our neighbors gets better and better every year.

It could be that we go 100,000 years without ever being able to reach other stars, but spend the entire time passing messages, like prisoners in individual jail cell passing notes to friends in nearby cells.

... Damn, that would be a really great setup for a sci-fi story...

Vernor Vinge's "A Deepness in the Sky" posits something like this; FTL travel is impossible, but it is possible to help civilizations bootstrap via an internet-like broadcast system that is inherently built to operate at the speed of light.

(Those who have read the novel(s) in question will probably find my description of the Qeng Ho network incomplete at best, but I don't want to write too many spoilers.)

I think the whole point of the "Prime Directive" was to provide absurd, immoral unintended consequences when it was slavishly followed with religious zeal. High drama then ensued when the Captain then had to choose to break the rules and risk his career to do what was obviously (to the viewer) right. Like a galactic example of zero tolerance rules in an elementary school. Sometimes the aliens chew their toast into the shape of pistols and shout bang, bang.

it's also a good scapegoat to people wondering why they only fight the monster of the week on the many preindustrial planets they meet then adbandonig them to their quotidiane misery afterwards instead of, you know, dropping a basic medicine handbook and a civil engineering primer so they don't all die from infection and water shortages.

The Prime Directive and Asimov's 3 Laws of Robotics aren't well-thought out, thoroughly researched concepts that are designed to be followed. They are reasonably sensible sounding ideas that have inherent weaknesses that give rise to interesting story elements. So much of Star Trek and so much of Asimov's stories revolve around the failings of the laws they purport to follow. They are stories, they are not meant to be guidelines for either AI or interstellar contact.

> So if you were aliens and you didn't want to 'alarm' or 'damage' humans, what would you use as a signal that it was probably a good time to say "Hello" ?

You don't say Hello if you are at 10 000 lights years away and have no means of traveling faster than light, because 10 000 years ago humans had no means of Radio communication anyway.

And for all we know, even if intelligent life arises everywhere in the cosmos, there is absolutely no model that says that intelligent life is supposed to evolve in the direction of mankind. After all, the Earth is full of intelligent life, well spread among numerous species (dolphins, crows, mammals, apes, etc...) yet only one species among them has been able to move to the next level. There could be many things happening in the meantime to prevent such a single species domination, and indeed for most of the time mankind was just another ape fighting for survival.

So, for me that's the big missing factor in Fermi's equation: species may not end up evolving in the same way that we did, and I can well imagine planets swarming with life yet having no mankind-like species that would be reaching out in the first place.

Well, and not only that, but some other fairly simple things that make our Earth truly rare indeed. For example, our solar system is "inside out" with the big fat gas giants on the outside. This is proving to be pretty unusual. We've modeled that there used to be another Saturn or Neptune that ended up causing our gas giants to move outward in their orbits. This might make it extremely easier for Earth-like conditions to form, where life could evolve beyond some kind of muck living off chemicals in environments with immense pressure and heat. Can't build a factory there. And we've got a moon. A fairly large moon, at that. Perhaps Jupiter and our own moon have been absorbing the space debris for some billion years, allowing conditions on Earth to become better still. I think the Rare Earth theory doesn't make our Earth rare enough.

FYI: "species" is both singular and plural. "Specie" is a word that describes money.

Thanks, I was not sure about the spelling anymore. Edited my previous comment.

The sci-fi book The Dark Forest (sequel to the Three Body Problem) discusses this problem. I don't want to give away any spoilers, but I found a lot to agree with in the book.

The capability represented by interstellar travel is absolutely terrifying.

Especially if it involves anything more than a beeping solar sail.

The sci-fi series Three Body Problem sustains the argument that intelligent civilizations evolve into being compelled to mask their existence from interstellar detection, as a matter of survival. General idea is there are always bigger in the ocean, and the time required for interstellar travel is so long that non-zero probability of any newly detected life being hostile becomes intolerable. I.e. civilizations survive over Gyr timescale by concealing themselves, and destroying any neighbors who make themselves observable.

Eric Nylund's "Signal to Noise" and "A Signal Shattered" posits something similar - they're out there, and they're out to get you - but FTL communications exist and the others are more rabidly mercantile than invasive.

(To be fair, I've only read The Three Body Problem, and I'm not sure how it evolves from there.)

The idea that any rational/game-theoretic civilization ends up rabidly genocidal, or close, has become an increasingly well known premise in SF. Modern use maybe dates to "The Killing Star" by Pellegrino & Zebrowski? It probably says something about the way we have come to view ourselves, compared to the era when it was argued that any rational/game-theoretic civilization would necessarily be enlightened and friendly and seek out opportunities for mutual benefit.

(Of course the more general idea, that it might be smart not to be noticed until we know what's out there, goes back much farther.)

I've been thinking about this, and I think it's easy to say all humans are destructive, violent, control freaks when global communications can always drum up an example of this. People are being arrested for allowing their children to walk to school, or shop in a store at the mall by themselves. It'd be hard to convince me that this frenzy isn't caused by the vast accessibility of information which always seems to bring us horror stories from around the globe.

Any single member of a collective may not be a good representation of the collective as a whole, or its actions.

i'd think we would want to know as soon as possible, so we could at least attempt to prepare militarily. Remember the fates of all those tribes that met peoples with superior technology.

Their fates were not determined by the other's superior technology but rather by the other's inferior social values that now, at last, we know are horrendous.

There is, of course, the fact that the Native Americans maintained a closed-borders against the rest of the world (With thriving cities), until herpes andor smallpox was inevitably transferred among the population. There isn't much that a military presence can do for that though.

The article calculates that solar-radiation reducing shields for earth-sized exoplanets (like some propose for mitigating climate change) will be detectable by the next generation of astronomical telescopes.

I'm confused... Does this mean we are looking for life sufficiently intelligent to build these radiation-reducing shields? Or are we looking for natural shields (i.e., a moon?) that might make a planet naturally resistant to climate change?

The L1 point is a saddle point in the local effective potential: it is therefore not stable enough for an object to stay there without station-keeping.

So yes, the goal here would indeed be to detect life capable of creating large structures and keeping them actively in orbit. There is some tongue-in-cheekness to this sort of research, but as the abstract notes, also some seriousness. The tongue-in-cheekness is that we're nowhere near being able to build something this large between Earth and the Sun; as the abstract notes, it has to be around as large as the Earth in order to usefully eclipse the Sun; that's a very very large structure, and we're certainly able to get into space right now: and yet we have no prospects even in the next many thousand years for engineering anything at that scale, unless folks are right about an impending technological singularity.

The seriousness, on the other hand, comes from one consideration: blotting out the Sun is probably easier than fleeing to nearby star-systems. It's very hard to say exactly how much energy it takes to flee to a nearby star-system, but a lower limit for the technology that I can presently envision is maybe 10^20 J of kinetic energy; my imagination might not be very good though, but I think that's reasonably conservative. Now the solar power irradiating a disk-that-blots-out-the-Sun is known to be 10^17 W or so. So the claim is, to even harvest the level of energy that is required to try and scout out neighboring star systems, one needs Sun-blotting scales of energy-harvesting technology; the length scale could maybe be smaller by a couple orders of magnitude to scout, but one can imagine that the energy needs of billions of entities fleeing their home planet would be several orders of magnitude larger, too.

> blotting out the Sun is probably easier than fleeing to nearby star-systems

If we want to later use the sun-blotter as a solar panel to power our interstellar exploits, then yeah.

If we just want to make the Earth a little cooler, we'd be far better off seeding the stratosphere with some dust, creating a mini 'nuclear winter' / 'Krakatoa effect' in a controllable way.

L1 orbits are not stable, so there would be no natural shields. Not for significant amounts of time anyway.

Presumably this would detect both, but I think they are operating on the assumption that the shield would be just enough to shield the planet and that finding a moon sitting in L1 orbit that also has that shape would be unlikely. I didn't read the whole thing yet, but maybe they cover this.

As with a lot of stuff in space, we assume a lot to be unlikely until we find out that it's not :)

This is some interesting speculation, and I applaud the researchers who think of things like this to look for.

I still estimate that by the time a civilization has planetary-scale engineering capability, that they won't need to make things like starshades.

If you have molecular nanotechnology, you can either adapt yourselves to whatever location you find, or just skip the biological body business, and directly upload your consciousness to a computer network.

The 2nd option is far more mass and energy efficient to support large numbers of sophonts, and I expect that any civilization to endure long enough will have the majority of its population living online instead of offline. If that even ends up being a thing, and they all don't just merge into a single entity (going in the direction of Star Trek's borg).

I always think of the "they're made of meat" story when considering this [1].

How do we even know what life looks like out there? Or what it has turned itself into in order to survive, travel and spread? Or even what it started out as?

We can't even talk with dolphins, even though we have a very recent evolutionary common ancestor. They're sentient life, tool-users, do we think they'd build a starshade to solve this problem? Shame we can't ask them, or understand their answer.

We know trees are alive, we know they communicate with each other. We don't think they're sentient. But how would we know? What would a sentient tree do differently from a non-sentient tree? What would a sentient tree do about the sun getting hotter?

Even if they started out like us, the evolutionary responses to living on other planets will change them. We're adapted to the conditions on Earth. If we live on other planets with different conditions then we will adapt, because that's what we do. We don't know how far those changes will go, because all the life we've ever seen has been adapted to Earth. Maybe high-gravity humans are less like sci-fi dwarves and more like Pratchett trolls.

We seem to be expecting sentient life out there to resemble us and have similar solutions for their problems. This seems dumb to me.

1: http://www.terrybisson.com/page6/page6.html

Maybe, but this conversation always inevitably ends up in this territory ("But what if aliens are crystals?"). And sure, we can't know. But we're going to have to make some assumptions, and we all do live in one universe, with a common constituent set of physical laws. Aliens that build spaceships probably also have airplanes and an internet. I don't think it's dumb to speculate with some reasonable assumptions, and while I appreciate that this conclusion always comes up and is not wrong, it's uninteresting.

We could make starshades today if we made it a priority. It would take a shade roughly only about 0.09% the size of the cross-section of the Earth to change the climate as much as all of our CO2 emissions. That's only a roughly 200km in radius shade. If we were spending, say, a trillion dollars a year (1.3% of global GDP, a sum less than what is spent on many other things) we could almost certainly build and launch such a structure in at most a few decades.

Paper doesn't address what measures the aliens might use to disguise any obvious signature of the starshade.

You'd think that the aliens might not be too keen to create huge beacon advertising their presence to all and sundry...

One cannot assume that an alien can think in a fashion similar to Earth animals.

Humans have the ability to imagine a possibility for which no supporting evidence currently exists--that species alien to our solar system might exist. That ability might not be shared by the species we can imagine. That is, humans can imagine a species that cannot imagine humans. Such aliens would not even realize that building a planet-sized object that cannot be explained by natural processes might present an existential risk.

We, of course, are afraid of hypervelocity impact weaponry, even while having no evidence whatsoever that any such weapon exists anywhere in the universe.

Of course, that just means that any species that did fear annihilation by near-c rocks smashing into their planet would simply disguise their star shade so well that we could not detect it. Meaning that any solar shade we can detect was not likely built by any species able to think like us.

We could clean up by selling them insurance.

I'm not sure how you propose a species that can't imagine things that don't exist would advance technologically to space travel.

Investigation of serendipitous results.

They may not need imagination if they can build dyson spheres, new telescopes could help verify where we should put our efforts at contact: http://www.sciencealert.com/researchers-just-found-a-second-...

There is no stealth in space.

Therefore, instead of hiding, the aliens are going to pro-actively fry every competitor civilization by using their solar shades as a Nicoll Dyson Beam... :-)


Best disguise would be reduced solare output or another inner sync planet giving partial shade..

You know, this sounds like a great idea.

Maybe we should be building ourselves one of these...

This isn't for piddly-ass "climate change" of a handful of degrees, this is "the sun is two times brighter than it was a million years ago and we're no longer in the habitable zone".

For our problems shades in Earth orbit would be just fine, and are probably the best solution overall, really, for dealing with climate change. Not just "man-made" climate change, but all climate change, full-stop.

Shades in Earth orbit raise the wonderful prospect of triggering Kessler syndrome. I'd rather not make near Earth orbit uninhabitable any time soon, thankyouverymuch.

By the time we're building one of these, we probably have a solution for Kessler. There's really no shortage of possibilities. Kessler syndrome isn't a fundamental of space travel, it's a momentary accident of our being able to launch things, but not launch things cheaply enough for us to justify launching things just to clean up the orbital space. By the time we're seriously discussing this sort of solution, clearly we can launch things cheaply enough to clean up inner space, or, more likely, simply build the cleaning solutions in space to begin with.

Though I can't help but enjoy the dark humor of refusing to fix one somewhat hypothetical problem with a hypothetical solution because it might create a different hypothetical problem. ("Hypothetical" here simply in the sense that none of these things have actually happened yet.)

My (obviously unmodelled, hypothetical) low-earth-orbiting solution would be to use 'intelligent' rings of what I will call "dancing shadow leaves" (akin to the 'shadow square' system that provided day/night cycles for Ringworld). Vast numbers of relatively small 'leaves' made of graphene-like material would be connected into a large number of rings held together by conductive nanotube-like 'wire'. The leaves would use electrostatic charge to change their angle and to jump to slightly higher or lower orbit (to avoid orbiting debris). Being able to change their angle appropriately for their changing position above the earth they would shade or not shade different areas on the ground. A supercomputer capable of modeling the affects of such shading, would use the system to have some control of over the weather in some areas. Just the ability to cool areas of the oceans that spawn tropical storms might allow the system to pay for itself. And ideally the leaves would also be photovoltaic and beam power to the surface. As long as I am dreaming, the leaves would be transparent to the wavelengths most supportive of photosynthesis so that they provide shade without lowering CO2 uptake.

And before anyone chants "Ringworld is unstable" of course my rings's orbits would be constantly adjusted by using solar radiation pressure, electrodynamic tether thrusting and/or ion propulsion.

To be able to shade most of the planet would require ~100X the surface area of the L1 proposal. But using recent nanomaterials and the much smaller transportation distances for LEO might make up the difference in cost. In addition a much smaller coverage system might still be enough to keep critical areas of glacier and tundra below freezing for more of the year and keep the most critical tropical storm spawning areas of ocean cooler.

This paper is talking about placing one at the L1 Lagrange point. It's not on the Earth's orbit, it inside of it. https://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/La...

jerf brought up Earth orbit shades.

Ah, you're right. Sorry I missed that.

I'd hope that a million years from now we'd be able to move the Earth further away from the Sun, and/or simply turn the Sun down a bit and refuel it.

Let's see. It seems to me the obvious approach would be a very large number of solar sails manufactured in space. Eric Drexler wrote a thesis about that in the 70s, for transport instead of shading: https://dspace.mit.edu/handle/1721.1/16234

The general idea was to make lightweight thin films by metal vapour deposition onto wax (or something, iirc), followed by heating the wax to remove it. These films can be far thinner than what could survive launch from Earth and unfolding.

I'm not gonna reread it now, but say 50nm thick (which is above the knee of the opacity curve) times the cross-sectional area of the Earth gets you about 6 million cubic meters -- you wouldn't make one giant shade, but a lot of small ones; there's overhead for each one's control/power/cooling. Aluminum masses 2700kg/m^3, so we need to manufacture 17 million tons of solar sails (+ the overhead) somewhere up in space, I guess at a convenient asteroid that supplies the materials? And then navigate them to the Lagrange point, but solar sails are great for that.

Easy for me to say, but this sounds doable in this century if we wanted to enough. If you just want to reduce insolation by a few percent, then scale down the requirements to a few percent.

Added: I found the R. Angel paper the OP referenced, with the sunshade design: http://www.pnas.org/content/103/46/17184.full -- also a thin-film swarm, though not as thin, launched from Earth, and 20 million tons for 1.8% reduction.

Also, my sketch above left out stationkeeping overhead, which I was vaguely thinking could be small: rocking back and forth to alternately spiral out and in from your orbit. But that's wrong, it's not even in orbit. I'm not sure how well you can do.

But then they could find us.

Doesn't matter, still had planet supporting life.

Nah, we'll just make ours out of a cloud of vanta black, it'll look like a dust cloud.

If the alternative is them finding a tombstone planet, maybe it's not so bad.

So why has ET not used something like a Dyson sphere or a starshade to send us light signals as a form of "Hello"?

Maybe because by the time a planet is able to send such signals, they have discovered better means, faster than light speed, communications.

So, the "Hello" communications have been coming to us for maybe millions of years, but like our planet before we understood radio, we can't detect their communications.

We may even misinterpret the "Hello" signals because we don't understand their mathematics. They could very easily be sending a "Hungry" signal, because they are a race of locust.

Looking for umbrellas in space. What will we think of next?

I know I'm gonna get downvoted for saying this, but this is kinda dumb. Might as well look for Dyson spheres by looking for gravitational lensing around dark spots in the sky.

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