Frontiers in Artifact SETI: Waste Heat, Alien Megastructures & Tabby's Star

Jason Wright


Science colloqium

Presenting sponsor: Leonard Tramiel

Good afternoon, ladies and gentlemen welcome to the colloqium. My name is Frank. I am a researcher at SETI Institute. Today I am going to be the host of this session. Let me first introduce our speaker, Jason Wright. Professor of Astronomy and Astrophysics at Penn State University. He is currently on sabbatical, spending time at the Center for Expoplanets at Penn State, at the Break-through Listen Laboratory at Berkeley, and visiting us here at the SETI Institute. His primary research is using precise measurements of nearby stars to discover and characterize the planetary system closest to the sun. He also organizes the search for extraterrestrial civilizations with large energy supplies.

Three years ago, I heard about his work on the search for Kardashev civilizations with WISE satellites. I invited him to talk, ... with Freeman Dyson and... and that was a hangout at the time because nobody was looking for K-2 civilizations. There is a hangout video available of that. We age gracefully. Recently in the search for K-2 civilizations, the discussion is getting more interesting thanks to the discovery of Tabby's star. So now he is going to talk about waste heat and alien megastructures. Please welcome him.

Thank you.

Jason Wright

It's good to be here. There's a nice Google Hangout with Freeman Dyson and myself and Matt where we talked about a lot of this. I also had an earlier SETI institute club view and so I want to pick up where I left off with a lot of that. The SETI research I do is under an umbrella I do, called G hat, glimpsing heat from alien technologies. And since I did that hangout and that previous talk, I came out with a bunch of papers explaining what we're doing. I'll quickly go over some of that, and then we will get into some more recent stuff you might have seen in the news involving Tabby's star.

So our first paper motivates our search and explains what we're doing and how it's different from traditional SETI.

The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification

So what ... let me just start with something I've been saying in a lot of places and trying to emphasize. What is SETI? It's a program for the search for extraterrestrial intelligence. It's a lot of things. It's researchers, places, it's things, but most of all I think of it as a field of study related to astronomy, radio engineering, astrobiology, and it's a multi-interdisciplinary field of study with many components that complement each other. The most famous component is of course Jody Foster from the film Contact. This is what most of us have in mind when we think about SETI because it's perhaps the most famous component of the search. The philosophy behind this goes back to a classic paper in 1960 by Giuseppe Cocconi and Philip Morrison called "Searching for interstellar communications" where they suggest searching for interstellar communication by looking at radio waves. They realized or published a paper arguing that it should be possible to communicate across interstellar distances with modern radio technology. Radio astronomy was just coming of age, and in this paper they did the calculation and showed that perhaps surprisingly that sending a signal with the technology we had in the 60s to the nearest star, would be a signal detectable with current technology. We had a comparable receiver. Drake was also working along these lines at the same time.

So that line of communication SETI today has many forms, perhaps most famously the Allen telescope array which is operated by the SETI Institute to pursue that vision and search for radio signals from extraterrestrial civilizations across the galaxy and beyond. The search, though, is largely funded privately. This whole field, SETI, is very little if anything in the way of federal funding through the traditional ways that we fund science research. It relies on private philanthropy and people getting excited about it. Most recently, there was the $100 million commitment by Yuri Yilner to launch the breakthrough-listen initiative to really push communication SETI forward and make it a much bigger project than it currently was. So this is now ongoing and the center is doing a lot of work to make this happen. One of the places this is being done is the Green Bank Telescope, operated by the National Radio Astronomy Observatory in West Virginia. It's the largest steerable dish in the world. It's a great instrument for detecting extremely weak radio signals from space.

The breakthrough-listen project has purchased a large fraction of the time available on the telescope and has installed all sorts of cool hardware on the backend to improve its ability to detect signals from other stars. So that's what we think of when we think of SETI a lot. I am not a radio engineer. I am not a radio astronomer. But I was interested in helping th SETI project. And so, I don't build things like this. So I was wondering what else I could do to contribute to the effort.

It turns out there's another strand of SETI that goes back also to 1960 to a paper by Freeman Dyson. He pointed out that if you have a civilization which uses, an old civilization perhaps-- we think old civilizations are likely to be very much older than us, because we just got started. And the galaxies are billions of years old. And they might use a lot of power. They might use significantly more power than we do, many orders of magnitude more than we do. They might use a significant fraction of starlight to power whatever it is that alien civilizations do. And if they use that much energy.... we think about using energy, but of course you never use "up" energy. You conserve energy. Once you use the energy to do the useful work in your computer or whatever it is, you then have to get rid of it in a higher entropy state, and that's what's called waste heat. So your computer boots up, it does work, then it radiates that heat away. So energy is never used up. It's just converted into higher entropy, a lower temperature than when you got it. So you have this very hot star that is producing a lot of energy, you collect it, you use it, then you radiate it away at lower tempeatures, at longer wavelengths, which usually come out at infrared for most reasonable temperatures.

And so, Dyson said that if this is- it's a very general approach. It doesn't matter if they are potentially signalling us, it doesn't matter what they are doing, if they are using huge amounts of energy, then that energy still has to go somewhere when they are done with it. That's bedrock thermodynamics. And so he suggested that we look for stars that have an excess of infrared radiation coming out of them. This would be indicative of the tail pipe of the alien civilization.

So where do you look for this? How big could these civilizations be? Is that really practical? In 1964, Kardashev published a paper where he speculated how much energy could a civilization use. He wanted to know, you know, they have a lot of energy available to them presumably. This sets a limit on how powerful their radio transmissions might be and what we might look for. He defined a Kardashev type 1 civilization as one that could harness an entire planet's energy supply, that is, all of the starlight that falls on to that planet. A type 2 civilization could harvest the entire star's output.

.. A type 2 civilization could harvest the entire star's output with a swarm of solar panels. And a type 3 civilization not only does that, but also has interstellar travel, and it does so to all of the stars across the galaxy. So once you have harvested most of the stellar energy in an entire galaxy, that would be a type 3 civilization. The time scales here are such that the time it would take a civilization to do such a thing, by most measures, is much smaller than the amount of time that galaxies have been around. There's no physical reason that there couldn't be such civilizations across the universe, collecting all the energy from stars and galaxies.

In the 1980s, a satellite called IRAS All-Sky map (1983) was launched to map the entire sky. One of the science outputs of that would be stars that have red Xs and perhaps have these type 2 civilizations. This was complicated by the actual map which ended up looking like this ((10:22)) revealing that the galaxy is filled with glowing dust at infrared wavelengths. That creates a background against which you see stars giving off infrared light. This makes the satellite much less sensitive to waste heat. So it wasn't actually a very efficient way to search for Dyson spheres or stars giving off a lot of infrared light. We also learned that there are a lot of other different kinds of stars that actually give off infrared radiation, further complicating this technique.

Nonetheless, one astronomer, Richard Carrigan, published a paper in 2009 called "IRAS-based whole-sky upper limit on Dyson spheres". He finally went through the entire catalog and checked every bright star and studied it carefully to figure out if we had any good candidates in the dataset. He found some candidates, but on further inspection he found that they were carbon stars, asymptotic giant branch stars, H2 regions, which are all astrophysically interesting and has created great science coming out of this, but nothing that looked like a confirmed alien civilization. So this put an upper limit on how many alien civilizations of these types that exist in the galaxy.

We can go a step up to galaxy-spanning civilizations, Kardashev type 3 civilizations. The only search to date that I am aware of, before ours, was also at Fermiab from a guy named James Annis, "Placing a limit on star-fed Kardashev type III civilizations" published by the British Interplanetary Society. And he looked for galaxies that were too dim. The idea was that if you surround a star with solar panels, less light would get out than should. So he wondered, are there any galaxies out there that just are, the surface brightness is too low? You know the mass dynamically from how the stars move around the galaxy, are there any that just don't have enough light? He wasn't looking for mid infrared light, he was looking for a lack of optical light from stars coming out. Anyway, he did the search in galaxies in the cluster and didn't find any strong candidates for the extreme case of Kardashev type 3 civilizations in our local galaxy cluster.

That's where we came in. I got thinking about this when we saw a talk at Penn State given by my friend Mike Cushing who showed that the WISE satellite, which had recently published a lot of its data, had discovered a brown dwarf with an effective temperature around 300 kelvins. This is a failed star, around the size of Jupiter, much more massive but not as massive as a star, so it's not using hydrogen. And so, it's only warm because it got put together when it was hot, and it's been cooling off ever since then. And it's basically a room temperature object that WISE had discovered in space. And also that's about the temperature taht humanity, our civilization, gives waste heat off at, which means that the WISE satellite was sensitive to exactly the wavelengths that you might expect to find us at, and it was also searching the night sky in a way much more sensitive than IRAS had. So this was a new opportunity to improve on the IRAS results.

So our strategy was to use that data to go and see what we could find. We were able to do this through a grant that ultimately came from the Templeton Foundation, so again this is private philanthropy funding. The one problem is that if you just look for points of light that have infrared radiation, you don't know if it's a star right away. WISE detected 100 million sources. Going through all of those is a challenge. It will be easier in the future once we have more data to figure out what they all are. For now, that was a bit daunting. So we restricted ourselves to looking for type 3 civilizations, the way that James Anis had. That is, looking for galaxies where you can actually see the whole galaxy in the image, and not just a distant galaxy that is just a point of light which could be anything.

Here's an image of a nice spiral like our own. This is Andromeda galaxy. And you see the dust lanes in the spiral arms. That dust is actually growing very brightly at infrared wavelengths. And the soft glow everywhere you see is just from 100 million stars giving off light. So if we put on infrared goggles and look at this; so we will switch over to a Spitzer image of the same galaxy... you see all that dust that used to be dark is now bright and glowing in the infrared. This tells us a couple things. Just seeing infrared light from a galaxy is not surprising. Dust does that. But it also tells us that dust tends to be clumpy. You can look between the red lanes and you can see regions where there's hardly any dust glowing at all. If instead we had seen a smooth distribution of infrared light, then the infrared light would be coming from a smooth distribution of infrared stars, not just a clumpy distribution of dust, and that would have been very interesting. So by being able to resolve these galaxies, we can distinguish these cases where they might be filled with alien civilizations versus ones that are sort of bright because of dust.

I am going to show one of these laters, so I just want to explain what it looks like. This is a spectral energy distribution of a galaxy. The black line and the red line goes on top of it towards the left, representing how much energy an old elliptical galaxy gives off. This galaxy has essentially no dust at all. On the left you have optical light. On the far right you have far-infrared radiation. Right in the middle, those purple bands that go up and down are the wavelengths that the WISE satellite senses. So the right-hand ones that are about 10 or 20 microns .. are very little emission that you would expect to see from a galaxy with no dust at all. If just 1% of the star light were being intercepted by that galaxy, being used and irradiated again at around 300 kelvin, it's not too sensitive there, there would be a significant excess of the current light and WISE would have seen that it was too bright. And if you have a galaxy that is filled with dust and star formation and it is inherently bright, you would get something like the green curve here. This is a famous starburst galaxy called Arp 220. It's very bright at those infrared wavelengths at around 10 and 20 microns. But even then, it's not as bright as you would expect if 90% of the starlight were being intercepted. So what this means is that there's no natural source that looks like a galaxy where 90% of the starlight is being intercepted. So that's our first upper limit. If we can't find anything with more infrared radiation coming out than that, it's some sort of extreme beyond natural that sort of thing, and below that, we're going to have to sort out-- these are the starburst galaxies, and these are the dusty galaxies, and these are the galaxies that are strange and we should investigate further.

Okay. So, um, what we were basically trying to show that there aren't any leads in th WISE data set, or that there are. This is an image from WISE. The blue things are stars. At these wavelengths, stars look very blue. And the red thing in the middle is an extended little object. It looks sort of galaxy shape. And it's pure red. It's much brighter at red wavelengths than blue. That's the sort of extreme thing that we can easily find with a search through the WISE database. And in fact, this was one of the best candidates that popped out of our search. Now it turns out that this was an artifact of the spacecraft. There was a bright star at the previous pointing, and sort of like hitting a flash bulb in your face, you see the flashbulb for a few seconds afterwards, and that's what this is. This is an echo of a previous point towards a bright star. But it shows that we would be sensitive to this. What we found is that we didn't see any of these. Perhaps that's not too surprising. But it's the first time that anyone has shown this. If we don't have galaxies that are all infrared emission, with virtually no optical light escaping..

So, our big paper with our results of that first search is that the sorts of galaxies that are infrared bright (MIR-bright) are very rare. We looked at 100,000 galaxies. We only have 50 that look significantly infrared-brighter than we would expect. One of them was Arp 220, so that was a nice reality check. They are all probably starbrust galaxies, like Arp 220. And indeed there was a follow-up paper by Michael Garrett ("The application of the Mid-IR radio correlation to the G hat sample and the search for advanced extraterrestrial civilizations" ) which looked at the mid-infrared radio correlation and he showed that our sample of 50 follows just basically like we would expect, comparing the green and red, essentially what you would expect from the starburst galaxies, so there are almost .. they are almost all certainly starburst which is sort of what we knew. But this is the first big upper limit. It's the first time that someone has looked for waste heat. And we didn't expect to find anything right away, because we're just learning how to do this.

There was a.. when I was explaining this, a friend pointed me to a referre comment on a famous paper by Davis from 1955, who, um, uh, would eventually go on to discover neutrinos. They published a paper looking for solar neutrinos. Their first attempts did not have the sensitivty required to detect it. The referee was not impressed by their paper and wrote, "Any experiment such as this, which does not have the requisite sensitivity, really has no bearing on the question of the existence of neutrinos. To illustrate my point, one would not write a scientific paper describing an experiment in which an experimenter stood on a mountain and reached for the moon, and concluded that the moon was more than eight feet from the top of the mountain".

To which I reply, if you have no idea how high the moon is, that's a great first try. That only seems silly because you already know the answer. So, you got to start somewhere. Indeed, I am sure that people in antiquity, probably tried exactly that to see if they could get any closer to the moon on a mountain top. And so this is just a first try. We can actually do much better, now that we know what we're doing. We can better model dust and look at galaxies in more detail and say yes that's dust (or not). There are all sorts of contaminating sources that we had to sift through. We have better ways to go and exclude those from the search. We can focus on sources where our sensitivty is high. We can look at galaxies where we know that they don't have any dust, such as elliptical galaxies, measure their infrared output and there will be sensitive and we know there will be no dust, so it will be hard to see any infrared radiation at all. So anything we see will be interesting.

So that's where we were, with our waste heat search when the private philanthropic money ran out. That's moving very slowly now. Another opportunity came up, which you might have heard about, and which I am going to talk about next on a completely different angle. We're not going to... we were talking about kardashev type 3 civilizations before, we go back to those stars with too much heat, that's a type 2 civilization. On your way to a type 2 civilization, though, before you block 90% of the starlight, you block .. 10%.. and before that 1%. So how would you know if a star had 1% coverage of the energy coming out of it, from something on its way up? And the answer is that sometimes the solar panels pass in front of the star.

This is a technique we used to find planets orbiting other stars. Every time a planet goes in front of a star, if the alignment is just right, the star will appear to get slightly dimmer. So for scale, if it's a planet the size of Jupiter, around a star the size of the sun, the star gets about 1% dimmer which you can actually detect with high-quality amateur equipment from the ground. Many people have done that now that we know where to look, and which stars have these planets.

The Kepler spacecraft was designed to detect things much smaller than planets the size of Jupiter, like planets the size of Earth. So, it searched over 150,000 stars in its prime mission of four years, to see planets passing (transiting) in front of stars. It was very sensitive to this. Before it launched, though, an astronomer named Luke Arnold published a intriguing paper in which he pointed out that you know it's not just planets you will be sensitive to. If there are huge ones with solar panels and it happens there's an alien civilization doing this, if the panel passes in front of the star then you will see that too. He wanted to know could you distinguish a planet from a panel if the panel didn't have a circular aspect to it? Why would they build it in the shape of a circle? Not because they would do this, but just to illustrate his point, he said could you for instance tell the difference between a circle and a triangle? Megastructure aspect vs planet aspect. So if an alien civilization had a for whatever reason a triangular one, would Kepler notice the difference? These are both the size of Jupiter. And so if you ask what this shape is, this characteristic shape where it gets dimmer and then brighter again, for a circle versus a triangle, are they any different? He subtracted the two cases after calculating them. And it's mostly at the corners where there's a difference, but yeah. There would be a detectable difference. For Jupiter-size objects, Kepler would be able to tell that this thing is not round. Planets happen to be round, more or less. So that would be interesting.

He also got very creative and said, hey if they build these things, why not build them in funny shapes to send us information? So he imagined more complex structures with leuvers that could signal and flash us morse code and things and get us very complex signatures as well. It's a neat idea, that you might communicate information that way. You might have the solar panels fly in formation to share an orbit, which planets don't do. And then you would see perhaps patterns of them go by, and if they aligned them in certain ways that were obviously unnatural, he said that would be a good way to communicate "here we are" like arranging solar panels in prime number sequences in the same orbit so that there's no way you would mistake this for a planet.

If there are solar panels out there, Kepler should be able to find them. He said well that's a neat idea, once Kepler finished its prime mission, did it see any? We should be able to see these things are out there or not out there. I went to write a paper about this, "Part IV: Transiting megastructures" and ask all the ways that these sorts of megastructures would be distinguished from planets and then ask what limit could we put on these different things we might see.

Our team identified 10 possible anomalies that you might see around a star if it had these giant solar panels.

Ten anomalies of transiting megastructures that could distinguish them from planets or stars:

AnomalyArtificial mechanismNatural confounder
ingress and egress shapesnon-disk aspect of the transiting object or starexomoons, rings, planetary rotation, gravity and limb darkening, evaporation, limb starspots
phase curvesphase-dependent aspect from non-spherical shapeclouds, global circulation, weather, variable insolation
transit bottom shapetime-variabe aspct turing transit, e.g. changes in shape or orientationgravity and limb darkening, stellar pro/oblateness, starspots, exomoons, disks
variable depthstime-variable aspect turing transit, e.g. changes in shape or orientationevaporation, orbital precession, exomoons
timings/durationsnon-gravitational accelerations, co-orbital objectsplanet-planet interactions, orbital precession, exomoons
inferred stellar densitynon-gravitational acceleration, co-orbital objectsorbital eccentricity, rings, blends, starspots, planet-planet interactions, very massive planets
aperiodicityswarmsvery large ring systems, large debris fields, clumpy, warped, or precessing disks
disappearancecomplete obscurationclumpy, warped, precessing, or circumbinary disks
achromatic transitsartifacts could be geometric absorbersclouds, small scale heights, blends, limb darkening
very low massartifacts could be very thinlarge debris fields, blends

Luke Arnold had the first one, that the shape of the transit would be different. It turns out that there are a lot of different things that you might expect. He said there might be variable depths, that is the variable sized objects passing in front of the star. There might be many different panels, and therefore it might not be periodic in the same way that a planet is, because of many solar panels in orbit. Um, they also don't have a lot of mass probably, so it's possible that they experience significant non-gravitational acceleration, it's just the pressure of the light striking them might be enough to alter their orbits. And also it turns out that you can tell that from light curves.

And then there also might just be a swarm of them; it might not just be one giant panel, there might be a whole swarm of panels. Some might be bigger than others. When the big ones go by, you might see that, and it might be a very confusing complex light curve that you would see.

And then we also suggested some ways you could diagnose them and figure out what was going on. All of the things listed there is a natural way that you could create that signal. Indeed some of these are seen by Kepler. If a planet has rings, then it does not appear as a circle. It's a non-circular. So you should get a strange shaped light curve. If it has a massive moon, like the mass of Earth, it will make it wobble and throw off the timing and the moon itself will block the starlight which will get you a strange transit through. We tried to think of all the different ways you could create these anomalies naturally. This just goes to show that there are natural scientific reasons to look for them as well. This is not just a SETI search. You go looking for these anomalies in your data set, and you will either discover aliens, cool, or you will discover one of these other amazing things like exomoons and rings and things like that. It's natural science as well as SETI.

We wanted to put upper limits on all of these, but the point was that the really obvious stuff, like the gigantic solar panels and so on, the stuff that would definitely not be a planet, well Kepler didn't see any of those or it would have been all over the news, right? Right? Well... it turns out that, there's a lot of Kepler data. The way that they go looking for planets often involves algorithms. There's 150,000 stars, 4 years of light curves, it's just too much for people to check every one with their eyeballs. So most of the results you have heard about have come out of these automated pipelines that know what they are looking for. And if they see something strange, they don't say "hey what's this" they say instead "it's not a planet, throw it out".

Some regions of parameter space, turned out to have unexpected signals popping up... this is KIC 12557548 (Rappaport et al. 2012). And check out this light curve. What you're seeing is the brightness of the star with time, over the course of 15 days, and every, uh every day or so, it gets about 1% dimmer, or 0.5% dimmer. This is consistent with something like Jupiter going around it for an orbit of once per day. The depths are not constant. That's not resolution, that's real. It's variable depth. And that's exactly what Luke Arnold said we should be looking for, variable depth, with a common orbit. So that's kind of neat.

Let's look a little closer at one of those dips, by the way that we would expect from a Jupiter sized planet. So whatever is going in front every day or so, doesn't have a circular aspect. So, there we go again, there it is. What are we looking at? The best model is that we're looking at an evaporating planet. The planet itself is too small to block any light from the star. But for some reason it's giving off huge amounts of material. It's evaporating so close to the star. We see more of these they all have very short periods. There's a common story here. A blocking planet getting too close to the star, it will evaporate, and the material coming off will create a dust trail. Sometimes a lot of stuff is coming off, and sometimes not very much is coming off. The density of the tail varies very rapidly. When it's dense, you get a deep transit, when it's not dense you get a shallow transit. You always see first the head come in, and it quickly gets darker, and even once the planet has gone by, the tail is still there, so you get a soft fall-off. There are a few others like this now.

So that's neat. Those are there. We have a natural cofounder but there's cool science. One of the things we did is we said well if this had been an alien signal, it's possible that there's information embedded in there. Arnold's idea was that this would be a way of communicating information bit-by-bit. And so we asked, is there information in there? A grad student of mine, Kimblery Cartier, worked up a metric to try to determine how much information is in a signal like that. This is just to see where it lands. So on the left is zero no information, it's a steady drumbeat, it's exactly the same every single time, the only thing you need to describe depth is that number. All the way on the right would be random white noise. That would be that you are dealing with a very stochastic random process, or you are receiving highly compressed information. And, in between, you can see where things land. P1 and P2 were Arnold's proposal for what beacons might look like. They contain very small amounts of information. KIC-1255b scores very high, it's very random. This is a neat ida for how to quantify this in both time and frequency. So if we ever do see one that we think is aliens, we can try to quantify how random it is.

The lesson here is that the computer algorithm to find these things are often throwing these off, it requires a lot of special analysis to find this. And that's where the planet hunters come in. You might be familiar with the planet hunters website it's a citizen science project where citizen anyone scientists can login and get an account and examine those 150,000 light curves that Kepler produced for all of those, and this .. instead of an automated approach. So this is a real light curve of a real star, the star got brighter over the course of 30 days, that's probably an instrumental effect by the way, not really getting brighter. It was looking if there were dips in there, to identify by eye, for strange looking dips.

There was one in particular, though, that really got the planet hunters confused. They always found strange things and it almost always turned out to be instrumental issues, or something not all that interesting. There was one that got them puzzled. This was the talk page of one of the stars, KID 8462862. And this is one of the users, his handle is Night Hawk Black, trying to figure out how he feels when looking at. In the upper right, the light curve got dimmer and then brighter, and that doesn't look anything like a planet transit. And then here's another plot that shows an even stranger curve, that doesn't look like a planet, so they contacted the scientist astronomer in charge of figuring out these anomalies. And it took her a while to convince herself that it wasn't an instrumental error, but real. Once it was determined to be real, people got very interested.

Here are four different events, with four different scales. The upper left is the first stuff that got people confused. There was 0.2% to 1% dimmings. On day 1206, there was a complex shape there. The one that gets me is in the lower left. Over the course of a week, it steadily got 15% dimmer. And remember, a jupiter-sized mass would be 1% dimmer. So whatever's blocking the starlight is many times larger than jupiter. But it's the steady dimming, and then suddenly it just snaps back to normal. I don't know what that is. And then, in the final quarter of what turned out the final quarter of Kepler's mission, you can see it's ... it's almost random. Just all kinds. The star gets dimmer by fractions of a percent to over 22%. The astronomer in charge of figuring this out, Tabetha Boyajian, was working on this trying to solve this for years. She ruled everything out. At one point she showed up in my office and asked, what do you think this could be? I said, I don't know, but the paper I'm writing where I said Kepler mission didn't see any of these, is wrong. How many more of these are there? How would we know? It took human eyeballs to identify this.

I told her that we really need to get this out and get more eyeballs on it to figure out what's going on. Even though she hadn't solved the mystery, she begrudgingly published what she had so far, and it was actually a lot.

Plant Hunters X. KIC 8462852 - Where's the flux?

.. the second author there is Night Hawk Black, the citizen scientist. Here's the abstract, trying to understand what's going on. And they needed to say something, they needed to have some proposal for what it could be. They concluded that the scenario most consistent with the data is that the passage of a family of exocomet fragments, from a single previous break-up event. So the idea is that each of those things you are seeing is a massive comet, much bigger than anything in the solar system. With an enormous tail, sort of like evaporating planets, except bigger and more dense. And not just one of these, but a whole family of them. And each time one of them went by, you saw the star get dimmer by some enormous amount. It's very, very clever. And that's been the working hypothesis as to what it could be. It's very contrived, though, because we don't know if such comets exist, we don't know why they would exist, and it turns out that it's going to have some other difficulties as well. Anyway, they published this paper and made a minor media splash, New Scientist picked it up about catching a cloud of comets orbiting a distant star. And that's where things were.

In the back of my mind, I was thinking "well, maybe, it could be". This was, after all, this is what my previous work said we should be looking for. I should probably put my time where my mouth is, and we should go to the radio SETI people. So we teamed up with Andrew Siemion at the Berkeley SETI Research Center. And he and Boyajian and I submitted a proposal to the Green Bank Telescope, the national radio-astronomical observatories, and we asked for time to use the break-through initiatives backend to look for radio communication because they are harvesting a lot of energy and they could be using it to make radio waves. We submitted the proposal, it was all very fun, and then Andrew was testifying before Congress on astrobiology while he was in Washington he met a reporter from the Atlantic, Ross Andersen. And I knew something was brewing when I saw this tweet from Ross, "Last night I had dinner with the director of Berkeley's SETI Research Center. We had a fun talk: ....". He wrote an article on SETI, but one of the questions he asked Andrew was, are you looking at anything interesting, do you have any odd leads? And he said, you know, there's this star....

So, Ross wrote another article, "The most mysterious star in our galaxy" .. it's a great article by the way. I really liked it. Right at the end, he interviewed me about it, and I really did give him this quote, "It looked like the kind of thing you might expect an alien civilization to build". I was not saying we found anything, only that it's worth looking at. It got a lot of attention. Here's one of the saner headlines that we got from buzzfeed... Some places did well, others did not do so well. It was on The Late Show. Neil degrasse tyson and Steven Colbert and Seth Macfarlane all discussed the alien megastructure star, for the record, Steven Colbert is on Team Megastructure and .. team ordinary explanation we haven't thought of yet. Seth just chuckled and said he read it on buzzfeed. He said, "they think it's the Borg". It was an interesting study to see how the media deals with this topic. And I think it's one of the reasons it's hard to get goverment money. It's hard to have a sane conversation about it, without buzzfeed lighting its hair on fire and everyone talking about aliens. There were some interesting meta-analyses, like "Why it's so hard for astronomers to discuss the possibility of alien life". I think it's one of the reasons why NASA is so careful to be sure they are talking about the discovery of microbial lifeforms, simple lifeforms, start small and not jump all the way to science fiction because people start thinking about bumpy forehead aliens and bad acting and bad scripting and it just gets all silly. And it's hard to do this without stuff spinning out of control. It's important to manage that.

Anyway, that's not the end of the story. We didn't, by the way, we didn't get the telescope time at first but then buzzfeed hit and immediately attention came and everyone looked at the star and agreed, yeah it's weird, and the second time around we did get the time at Green Bank.

And then the star just got really weird. Bradley Schaefer ("KIC 8462852 faded at an average rate of 0.164 +- 0.013 magnitudes per century from 1890 to 1989") .... and he specializes in looking at these old photographic plates, from the last 100 years of the whole sky. There's this sort of lost art, that some astronomers still have, like Bradley, of actually looking at this, to go to these plates and by eye measuring the magnitude of the stars on all of these plates. So he went to Cambridge, went to the stacks and pulled out the plates for this star and charted the brightness of the star for the last 200 yars. And the grade points along the top of this chart are what typical stars do, it's a check star, the same type on the same plate, and the blue points are the averages of plate values across the century from 1890 to 1990, and how bright the star was. And those are two lines, suggesting more or less, what we're doing. The scales and magnitudes for the bottom line is basically 20%. The star is almost 20% brighter or was in 1990 when they stopped doing the plates, than it was in 1890. The star seems to have dimmed by 20% over 200 years. Stars don't do that. That is not a thing that stars do. And so he said, this is the first thing other than the Kepler light curve that seems weird about what turns out the media grabbed on to and named Tabby's star.

Photographic plate photometry like this is difficult, and it takes a real expert like Bradley Schaefer to do it right, other people look at the plates and came to different conclusions. The plates have all been digitized, you can actually access the data online at not quite full fidelity. It's a little harder that way. There were a pair of papers.

"The stability of F-star brightness on century timescales"

"A statistical analysis of the accuracy of the digitized magnitudes of photometric plates on the timescale of decades with an application to the century-long light curve of KIC 8462852"

There were two papers by these people, and they came to the opposite conclusion. The Lund paper said that KIC 8462852, that is, Tabby's star, which we believe is consistent with constant flux over the duration of the observations. Essentially what they are saying is that the quality of the photography is not good enough to make that kind of assertion. And the other paper comes to the same conclusion, in the case of this star which has been claimed to dim by this amount per century, cannot be shown to be the case. So this turned into a spat on the blogs and such to see whether you could really say that.

And then, it gets weirder. Some astronomers actually that I knew, Simon etc., said, well if this thing is getting dimmer, then shouldn't Kepler have noticed it getting dimmer during the 4 years staring at it? And the answer is, that's really hard. Kepler was not designed to do long-term photometry. It was designed to do short-term observations of planets passing in front of stars. It only looked at a little part of the sky around every interesting star. And as is the point moved around, starlight would fall out of the little region it was measuring, and fall in and you get these transit photometry that are artificial and instrumental and you can't really tell if the star was brighter before. However, it turns out every month that Kepler took a calibration frame, where it downloaded the entire sky as it saw, once every month. Those data should be good enough for those kinds of measurements. They very cleverly took the so-called full images over the whole Kepler mission, then learned how to do photometry on them, and measure how much brighter stars get brighter or dimmer. What they found is ... stars don't get brighter or dimmer over a 4 year timescale. They just stay the same. That's the way stars are. So they plotted all the stars that look like Tabby's star, all the stars near Tabby's star, and the stars don't do that. There is one outlier, though among all the stars they checked, which was again Tabby's star, which got 3% dimmer over the whole mission. Here's their photometry, over 4 years of the mission. The points are their photometry taken once a month.

"KIC 8462852 faded throughout the Kepler mission"

And, it goes from 1.01 at the begining down to 0.97 at the end. And that's a 3-4% dimming of the star over that period. And the black curve is the high frequency measurements. The big vertical lines, those are the dips I showed you earlier. So now we are looking at all four years, so they become very narrow shape. You can see all the little excursions, but there's also this long-term trend that they have picked out. The gray curve is their best guess as to the long-term light curve of this object combining long-term and short-term photometry.

Tabby's star faded throughout the Kepler mission, and so this doesn't really confirm exactly what Bradley Schaefer saw, because this-- the plates stopped in 1990, and this all happened more recently. On the other hand, they looked and they saw the same thing, just in different time. So I think this strongly confirms Bradley Schaefer's measurements that suggest the star really is 20 to 25% dimmer than it used to be.

At this point, hypotheses like "giant comets" are starting to look even more contrived. Why would comets over a century get dimmer? What's going on? We are all back at the drawing board, trying to figure something out. We're also wondering like, can megastructures do that? Is that what happens? Some people have started facetiously offered that perhaps this is a Dyson sphere under construction, and you see lots of material getting built in just 100 years they have blotted out 20% of the star light. That seems kind of fast to me, but who knows. I think a more natural explanation, if you wanted to invoke an alien egastructure hypothesis, is that to say there's a swarm and those solar panels are in orbit around Tabby's star, and some parts of the swarm are more dense than other parts. If they are out orbiting 10 astronomical units or 20 astronomical units, then they move slower and you will slowly see denser parts of that swarm orbit into view. And that would naturally make the star get brighter and dimmer as the density moves around the star. So if I had to invoke a megastructure to explain this, that would be the one I would pick that seems consistent. You have lots of panels in different shapes, different sizes, big ones make big dips and the small ones make little dips, and then the whole swarm is kind of like a translucent screen that makes it. That explanation, comets, a lot of other explanations, and they are all having a lot of trouble. And that's because we go back to the waste heat. Anything, it doesn't have to be a computer or solar panel, it could just be dust. Anything that's around a star that intercepts the star light is going to heat up. Once it heats up, it has to give off infrared radiation. Once it gives off infrared radiation, we should be able to detect it. And we don't. This was not something that popped up. When we looked at the WISE data set, it did not have a lot of infrared radiation. And in fact, there was a paper recently where they looked at millimeter radiation.

Constraints on the dust around KIC 8462852

And they didn't see anything. So this is spectral energy distribution, like the one I showed earlier. Grey line there, that's what we expect the star to have. Sure enough, on the left, at optical wavelengths its brightness is, the black dots are right where you expect. The triangles on the left, the four triangles, are upper limits, non-detections, cannot be brighter than this level. So the first one on the green line, that's 20 microns, from WISE. And the 3 on the right are millimeter wavelength observations made with the millimeter array telescope. And those curves represent how much you should expect if you have cold dust of certain mass. And so they measured things in Earth masses, but for the purposes of this talk, I will be translating things into fractions of star light names. I've translated these measurements into slightly different units here. The black curve, is what would happen if 20% of the star light, ... .. was being collected by a swarm of stuff, whether dust or solar panels or anything, and it was all coming out at 65 kelvin which is pretty cold... if you make it colder, that bump moves to the right a little bit; and that's ruled out by a factor of 100. And 20% of the starlight is not beig processed... in fact the purple line is if only 0.2% of the star light is being processed by anything, and that's barely consistent with the curve. So what this says is that whatever's blocking the star light cannot be isotropic. It's not surrounding the whole star. It must be along our line of sight. So you can do that if it's at a distance of some kind. And that hopefully will help us constrain what the heck is going on. But I think it's, it's almost a fatal blow that it's a spherical swarm of megastructure surrouding it.

The way out is that, of course, the alien civilization is doing something with the radiation, that is, it's allowed, with the laws of thermodynamics, to use some fraction of the energy in a low-entropy way and only dump the last bits out of high entropy. But keep it. So what do you with energy if you can't keep it? If you don't radiate it away? You could turn it into mass. But that seems hard. It could radiate it away at low entropy as communication. They could use it as Kardashev originally proposed, as radio. So if that's right, then maybe there's something to decode in radio waves.

Allen telescope array has already pointed at it and done a pretty deep stare. And they had a nice SETI institute paper about their non-detection of radios. When they looked at the radio frequencies at the sensitivity they had, they did not see any strong radio transmission coming out. I should also mention that if they were to do that, at 20% the maximum efficiency, they would have to be at.. 90%... so this would be just barely at the edge of possibility according to thermodynamics. They are collecting 20% of the starlight, then of that 20% they are allowed to beam out 99% of it, and the other 1% has to come out in the infrared, and that's the purple line, which is just barely consistent with what we have seen.

So anyway, if that's what's going on, and it's an even longer shot today than it was in the first place.. then hopefully we will be able to see something with the Green Bank Telescope when we head out in October and get a nice big data set on it. While we are there, I'm trying to see if we could look for stuff along the same line of sight that might be able to explain what's going on. I would say that we have no good explanations right now for what's going on with Tabby's star. We've been sort of crossing off of our list of bad explanations as we get more data. We still have a couple more ideas that we're working on. What could really be going on? For now it's still a mystery. I think one of the things that Tabby's star teaches us is about the value of this non-communication SETI and what it offers.

Artifact SETI-- if we see something, we're really hard-pressed to say it's not natural. Without some kind of prime numbers or something like that, there's no reason to say it's just dust or you haven't thought of the right natural phenomena yet. Communication SETI looks for obviously intelligent signals, but you have to cast this impossibly wide net. You don't know where to look, you don't know when to look, you don't know what frequencies to use, it's just this huge sea of possibility and you're going through one glass at a time. These are complementary approaches. Artifact SETI approach can find the anomalies, and then hand them off to the communication SETI folks to enrich their target lists and give them better targets to look at. So at least we know where to look. I think that together the two types of SETI are pretty powerful and I'm glad to have contributd in that way. And that's the story of Tabby's star.

Okay, the floor is open for questions.

Q: I was wondering if the waste heat could be of some collector that radiated at a certain direction.

A: Right. Could you do non-isotropic radiation? It's less efficient to go non-isotropic. So you can't hit your maximum thermodynamic efficiency that way, but there are good reasons to do it. I mean, our spacecraft preferentially radiate in certain directions... to control torques and things like that. So they could be radiating away from our line of sight. If it's an isotropic swarm, there's no reason for them to not point at us, almost like they were hiding from us. If it was a disk, though, that would explain why we don't see any heat. For one reason, it's not isotropic, they are only absorbing some of it, and that gives you a natural direction out of this that they might have chosen. So, right, if it's non-isotropic, that would have lowered the waste heat. The upper limits we have only rule out, I think, the isotropic swarm around the star.

Q: I have another question about Tabby's star. If we take the idea that the plate study... and the new study.. are accurate, and it has been declining for 125 years, we think we know what kind of star it is and what stage of life it is, what is the upper limit for how bright it was before we started looking?

A: This is complicated by the fact that we don't know the distance to this star. Say it's something like 500 lightyears away or something like that. But, we're basing that on the assumption that it's not any more distinguished than we already see. Based on the colors in the star, 35% dimmer than it would be if there was no intermediate material, and that's where we get that distance. If it's in fact 80% dimmer than it used to be, then that would mean it's actually quite close, and we are being fooled by its dimness, so it's degenerate distance. Without a distance, we can't break that. But if we had a distance, then we would know the total amount of dimming going on. That would also tell us what's doing the dimming, because dust has characteritsic reddening, and we have an upper limit on that from how much .. dust.. fortunately it's on the gaia target list, and so gaia will give us the distance if it's astrometric solution is clean, that's the caveat. In just a few months, they will give us the distance. And that could be very surprising, or they could tell us yes it's all dust. 125 years is nothing in astronomical time. That's right. It's very fast. Stars have characteristic time lines on which they can adjust their brightness levels. That's millions of years. Anything happening in 100 years, is not just your slow thermal adjustment time on a star, it's something much faster.

Q: So Jason, does it turn out there's any fast adence data on Tabby's star for ingress or egress?

A: I don't think there's any.. case data on it. A lot of people wonder why we weren't throwing more resources at it during the Kepler mission. Why wait until after the data has stopped to look more intensely? You have to remember there's a lot of weird stuff in the Kepler data. It took Boyajian years to convince herself that this was a real anomaly.

Q: When JWST comes online, are you anticipating any contribution it could make due to the fact that it is an infrared observatory, is there anything it could contribute to this that you are expecting to see?

A: Definitely. JWST has sensitive spectrographs and cameras that work out to our theory. So we will be able to push out these limits way down, hopefully trace the star down, these will push down the upper limits much further. I haven't done the sensitivity calculations yet, but yes, it's going to be much better.

Q: So you don't know the points.. especially...

A: Yeah, they are from scoop 2 and the SNA.

Q: Better sensitivity..?

A: A better sensitivity? No.

Q: How much the stars dimmed over 4 years, 4 years was 3%, if you take it out, the powers, then it's more than 20%....

A: The original analysis by Bradley... he didn't claim this was monotonic. You can find lots of imaginative ways... it could be linear, maybe not. It's hard to tell. The consignment analysis showed that it's clearly not monotonic. It's possibe that it has episodes where it has brightness as well.

Q: What's your sense about the accuracy of looking at the ... by human beings.. with eyeballs? As a musician with a sense of studying things through the telescopes with their eyeballs and being able to process data with his eyeballs and his brains.. I don't think it's non-valuable to do that, but do you .. the variability of the film technology?

A: This is an art. You can't just look at the dot and look at how dark it is and turn it into a number and say that's how bright the star is. What Bradley does is he looks at other stars to check. He lets the other stars tell him the relative values. That's what this check star is, this grey series of dots at the top. It's just one of several other stars he monitored just to see what the quality of the data was. All of his check stars had this nice steady ... there's also issues like, did they change the kind of plate they were using, and the focus, and it's decades of work on the plates, they train you on what you can trust and what you can't. It comes down to whether you trust his expertise or not, unfortunately.

Q: Assuming that the anomaly of brightness of the star is caused by a disk, .. if it's not aliens, what would a disk explanation be for this century-long dimming of the star?

A: Really, I don't know. I think we're going to have to talk about bizarre structure stuff like that. The interstellar medium stuff will say sure there's stuff, they will say "I'm surprised more stars don't do this". It is a rare phenomena. That's where I am, now.

Q: Very distant, procession, very distant object....

A: Remember, earth and tabby's star are all moving in the interstellar medium, so your line of sight is moving at 30 km/sec through the interstellar medium. If there were strong density fluctuations in that direction, it could sort of do it, except that's not a thing that stars do......

Q: When you were talking about searching through Keppler data for other Tabby's stars... there's a company called Numenta..

A: For free?

Q: I would hope.

A: That's a good price. At the planetarium, they have been using non-parametric techniques to look through the data, for things like Tabby's star and for other interesting things. We need more techniques like that to find anomalies.

Okay, thank you very much.