Transit Detection of a Starshade at the Inner Lagrange Point of an Exoplanet

by sanxiyn • 9 years ago

106 66 9 years ago

yodon• 9 years ago

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.

weberc2• 9 years ago

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?

codezero• 9 years ago

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 :)

the8472• 9 years ago

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

drostie• 9 years ago

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.

btilly2• 9 years ago

You know, this sounds like a great idea.

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

tensafefrogs• 9 years ago

But then they could find us.

mmjaa2• 9 years ago

Doesn't matter, still had planet supporting life.

pavel_lishin4• 9 years ago

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

downrightmike3• 9 years ago

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

jerf3• 9 years ago

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.

btilly2• 9 years ago

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.

taneq4• 9 years ago

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.

abecedarius• 9 years ago

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.

mmjaa2• 9 years ago

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

a_gopher• 9 years ago

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...

Eerie• 9 years ago

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... :-)

https://www.youtube.com/watch?v=RjtFnWh53z0

logfromblammo2• 9 years ago

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.

downrightmike3• 9 years ago

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-...

ceejayoz• 9 years ago

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

louithethrid• 9 years ago

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

ChuckMcM2• 9 years ago

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.

maxerickson2• 9 years ago

The capability represented by interstellar travel is absolutely terrifying.

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

westbywest• 9 years ago

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.

TeMPOraL2• 9 years ago

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.

ChuckMcM2• 9 years ago

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?

jerf3• 9 years ago

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.

mabbo• 9 years ago

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...

InclinedPlane2• 9 years ago

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.

noonespecial• 9 years ago

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.

valuearb• 9 years ago

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.

caio1982• 9 years ago

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.

bcbrown• 9 years ago

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.

ekianjo2• 9 years ago

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.

yellowapple2• 9 years ago

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

fapjacks3• 9 years ago

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.

E6300• 9 years ago

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.

ansible• 9 years ago

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).

InclinedPlane2• 9 years ago

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.

marcus_holmes• 9 years ago

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

fapjacks3• 9 years ago

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.

graycat• 9 years ago

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.

downrightmike3• 9 years ago

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.

Transit Detection of a Starshade at the Inner Lagrange Point of an Exoplanet

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