KIC 8462852, or the WTF (Where's the Flux?) star, is an F-type main sequence star about 1,480 ly away.  It's a little larger and more massive than the sun, and a few times brighter.  Age is uncertain, but probably older rather than younger.  

Kepler observations over the last few years reveal very strange large and aperiodic flux variations (up to 20%) - of the general form predicted by some ETI megastructure models.  However there doesn't appear to be any excess infrared.

The star's fluctuations were discovered by the PlanetHunters team.  In the WTF paper they review a large number of unlikely natural explanations and settle on an unusual comet swarm as the most likely scenario.

Abstract of the WTF paper:

Over the duration of the Kepler mission, KIC 8462852 was observed to undergo irregularly shaped, aperiodic dips in flux down to below the 20% level. The dipping activity can last for between 5 and 80 days. We characterize the object with high-resolution spectroscopy, spectral energy distribution fitting, and Fourier analyses of the Kepler light curve. We determine that KIC 8462852 is a main-sequence F3 V/IV star, with a rotation period ~0.88 d, that exhibits no significant IR excess. In this paper, we describe various scenarios to explain the mysterious events in the Kepler light curve, most of which have problems explaining the data in hand. By considering the observational constraints on dust clumps orbiting a normal main-sequence star, we conclude that the scenario most consistent with the data is the passage of a family of exocomet fragments, all of which are associated with a single previous breakup event. We discuss the necessity of future observations to help interpret the system.

 

From "Comets or Aliens?", on the Planet Hunters blog: " However, so  far over 100 professional scientists have had a look at the lightcurves and not managed to come up with a working solution."

In a another recent paper Jason Wright et al discusses the WTF star in more detail and critiques the comet theory.

The Search for Extraterrestial Civilizations with Large Energy Supplies. IV: the Signatures and Information Content of Transiting Megastructures:

Arnold (2005), Forgan (2013), and Korpela et al. (2015) noted that planet-sized artificial structures could be discovered with Kepler as they transit their host star. We present a general discussion of transiting megastructures, and enumerate ten potential ways their anomalous silhouettes, orbits, and transmission properties would distinguish them from exoplanets. We also enumerate the natural sources of such signatures. 
Several anomalous objects, such as KIC 12557548 and CoRoT-29, have variability in depth consistent with Arnold's prediction and/or an asymmetric shape consistent with Forgan's model. Since well motivated physical models have so far provided natural explanations for these signals, the ETI hypothesis is not warranted for these objects, but they still serve as useful examples of how nonstandard transit signatures might be identified and interpreted in a SETI context. Boyajian et al. 2015 recently announced KIC 8462852, an object with a bizarre light curve consistent with a "swarm" of megastructures. We suggest this is an outstanding SETI target. 
We develop the normalized information content statistic M to quantify the information content in a signal embedded in a discrete series of bounded measurements, such as variable transit depths, and show that it can be used to distinguish among constant sources, interstellar beacons, and naturally stochastic or artificial, information-rich signals. We apply this formalism to KIC 12557548 and a specific form of beacon suggested by Arnold to illustrate its utility.

Jason Wright discusses WTF here on his blog.

Big reddit discussion on r/askscience here.

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I know not that much about astronomy, but I found this thread on Reddit and frankly I agree that the original paper does not spend enough time (the last two paragraphs of section 4.2) discussing the twin star hypothesis. Could anybody with more experience/knowledge explain why this is not plausible in this case? Is the main objection to this hypothesis that it is not clear how such a system would form without a disk of matter present?

I'm not an astronomy expert either, but see section 2.6: "Limits on a companion". That reddit thread didn't seem to realize that section 2.6 actually lays out the case against an undetected binary based on doppler shifts.

The 20% dip requires a large occluder, about as large as the F-class star itself - or more likely even larger assuming it isn't perfectly opaque/cohesive. A star that big should be detectable unless it is close enough to be indistinguishable, because stars only get that big due to fusion overcoming gravity. The doppler analysis in section 2.6 rules out any large heavy close companion.

The other sections in general place geometric limits on the location of the occluder object(s) - within a range of 3 to 8 AUs.

Also, the hidden binary hypothesis doesn't much help explain the odd fractal complexity of the dip around 1540, which suggests a number of objects. The simpler single dip pattern at around 800 fits the profile of a single occluder clump about the size of the star or larger (partial occlusion), which of course is itself also weird. The second occlusion event at 1540 could be caused by the 800 clump plus additional smaller objects/clumps. For comparison, a more typical occlusion by a jupiter sized object creates a plateau shaped dip pattern of < 1%, like this.

The occluder clumps appear to be something about as large as the star, but of less mass, and they can't be very hot. They outline a full set of observational constraints in 4.4.1 which doesn't leave much room for known objects. The only things that seem to fit the full profile are swarms of stuff/debris/clouds bound to small objects, moving in tight formations (thus the swarm of disintegrating comets theory).

There is another oddity that the paper briefly mentions but does't even attempt to explain. There is a clear but small high frequency modulation in the light curve which they explain as the star's rotation period of 0.8 days. There is another larger longer term modulation on the order of 10-20 days which is present most of the time but strangely disappears in at least one time period:

We also report on the presence of a possible 10 – 20 day period (Figure 2), which, when present, is visible by eye in the light curve. We illustrate this in Figure 4, showing zoomed in regions of the Kepler light curve. The star’s 0.88 d period is evident in each section as the high-frequency flux variations. The panel second from the bottom ‘(c)’) shows no low-frequency (10 – 20 day) variations, but the rest do. We have no current hypothesis to explain this signal.

Assuming this isn't aliens, then perhaps this will turn out to be the stellar equivalent of ball lightning.

What's the probability that this is caused by aliens?
[pollid:1072]

The probability that any specific observation is caused by aliens is so heavily influenced by priors/models that it's more interesting to consider conditionals such as P(aliens caused WTF | aliens exist) or P(WTF observations | aliens caused WTF).

What's the probability that this is caused by aliens?

What is your own probability estimate? I am not sure I can accurately distinguish between below 1/100,000 and between 1/10,000 and 1/100,000. With probabilities this small I am not sure that any estimate is useful

Another way of saying "below 1/100,000 chance of aliens" is "above 99.999% chance of natural causes". That seams awefully certain of the unlikelyness of aliens. I'm pretty sure it's not aliens, but I'm not that confident. I'd happily lose a dollar in that bet, if someone wanted to wager $100,000 against it.

it would tell you how much more evidence you would need to begin taking it seriously. That said, agree that it's not very useful.

Can you put in an "I'd just like to see the results" option?

My guess is somewhere between .2% and 2%, depending on how many stars have actually been examined, a figure I don't know.

KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989

Bradley E. Schaefer (Submitted on 13 Jan 2016)

KIC 8462852 has been dimming for a century. The comet explanation is very unlikely.

I just posted a comment on facebook that I'm going to lazily copy here:

At this point I have no idea what's going on and I'm basically just waiting for astrophysicists to weigh in. All I can say is that this is fascinating and I can't wait for more data to come in.

Two specific things I'm confused about:

  1. Apparently other astronomers already looked at this data and didn't notice anything amiss. Schaefer quotes them as saying "the star did not do anything spectacular over the past 100 years." But as far as I can tell the only relevant difference between their work and Schaefer's is that he grouped the data into five year bins and they didn't. And sure, binning is great and all, and it makes trends easier to spot. But it's not magic. It can't manufacture statistical significance out of thin air. If the binned data has a significant trend then the unbinned data should as well. So I don't get why the first paper didn't find a dimming trend (unless they were just eyeballing the data and didn't even bother to do a linear fit, but why would they do that?). I mean, in the end Schaefer's plot looks pretty convincing, so I don't think this throws his work into doubt. But it still seems weird.

  2. Any explanation for this has to kind of walk a tightrope walk - you need something that blocks out a significant amount of light to account for the data, but thermodynamics is pretty insistent that any light you absorb has to come out as infrared eventually. So if you posit something that blocks out too much light you run up against the problem of there being no infrared excess. The nice thing about the megastructure hypothesis was that it could explain the dips while still being small enough to not produce an infrared excess.

Now, though, we have to explain not just dips but progressive dimming. And yeah, progressive dimming certainly sounds consistent with a dyson swarm being built. But dyson swarms large enough to dim an entire star seem like the kind of thing that would definitely produce an infrared excess. And in fact it seems like any explanation for that much dimming would require an infrared excess, which we don't see.

I guess it all depends on the magnitude of the dimming, though. If it's not that much dimming, I guess there could be an intermediate-sized dyson swarm (or weird astrophysical phenomenon, it doesn't matter, they should all produce infrared) that was big enough to cause the dimming but not big enough to produce noticeable infrared excess.

For now I remain confused and fascinated.

I am not happy about it because of probability of SETI-atack. What if it is a beacon to catch our attention? http://lesswrong.com/lw/gzv/risks_of_downloading_alien_ai_via_seti_search/

That's less likely than that it's something aliens constructed to make their lives better.

Frankly, both of those suggestions sound about equally ridiculous to me. But then again, it may just be scope insensitivity because of how minute both likelihoods are to begin with.

If it is alien structure, what purpose it have most probably? In your opinion?

Two things come to mind, providing energy or highly directional interstellar communication.

It seems to me that if the structure was built by aliens to provide energy, that would be an example of Nancy's "something aliens constructed to make their lives better", wouldn't it?

That is almost true, but don't give us information about their final goals. But also: building directional energy weapon?

The star is 1,480 light years away. Any aliens there will never affect us.

Us personally? That depends on when they started their attack.

If they are a really old and hostile civ, then they already would have found and destroyed us. Since that is not what we observe, they are either not hostile or are relatively young.

Assume for a moment the worst case - they are hostile and young. Due to observation selection effects, most observers detect either no alien civs or alien civs around their same age.

Say it takes 1000 years to go from early space civ (us today), to megastructure civ (them), and say 100 years from early space to get large space telescopes/sensors sufficient to detect most earth size planets within a few 1,000 lyrs. So they are only about 1000 years ahead of us, but 1480 years away.

They would build interstellar sensors around 1100 AD, at which point they would image us from 380 BC. They'd see a biosphere, but hopefully not yet any evidence for civilization, as that shouldn't really be detectable until the industrial era - about 2200 years later.

To detect earth civ circa 380 BC or earlier they'd probably need images from a probe near the planet or at least near the sun, which implies a minimum of ~3000 year round trip time.

In general, I think one can construct an argument that we should expect to have roughly order ~D time until any contact/invasion, where D is the distance in lyrs between us and the alien civ.

Except that a SETI-type attack such as the one described in Hoyle's A for Andromeda (mentioned in the link provided by turchin above) would not necessarily target us specifically; it could be launched by aliens with no knowledge of our existence.

They could send information in form of radiowaves and it could be description of unfriendly AI

What probability do you assign to this happening? How many conjunctions are involved in this scenario?

I estimate total probability of human extinction because of SETI attack in 1 per cent. But much smaller in case of this star. There are several needed conjunctions: 1.ET exist but are very far from each other, so communication is wining over travel. 1 milion light years or more.

  1. Strong AI is possible.

ET exist but are very far from each other, so communication is wining over travel. 1 milion light years or more.

So, you see a SETI attack as primarily useful for intergalactic colonization/expansion? It seems to me that a SETI attack would be an efficient way for a civilization to expand within a galaxy as well. Why do you think it would be useful only at such great distances?

Can you explain why you see a SETI attack as so high? If you are civilization doing this not only does it require extremely hostile motivations but also a) making everyone aware of where you are (making you a potential target) and b) being able to make extremely subtle aspects of an AI that apparently looks non-hostile and c) is something which declares your own deep hostility to anyone who notices it.

It results from exponential growth of victims in case of successful SETI-attack. The new victims will send the signal further. It is like virus behaviour. a) we know only location of the previous victim, not the starter. b) not so difficult if sender is more advanced civilization and AI may looks like a helper or gateaway of space Internet. с) the same as a. We don't know who started it. We could see only victims.

Anthropic selection effects make hostile expansive aliens improbable.

Assume that life is plentiful and hostile civs are common. If that was the case then most observers such as ourselves would find themselves on unusually early planets. Instead our planet is somewhate late in the order of all habitable planets to form in our galaxy, and is roughly in the middle for all habitable planets in the universe.

But in fact last research said that our planet is in only first 8 per cent of all habitable planets in the Universe, so probably the opposite is true and future universe is full planet-colonizing alien civilizations.

But if SETI attack is only way of space colonization, no starships, in this case before-radio civilizations will be distributed randomly.

But in fact last research said that our planet is in only first 8 per cent of all habitable planets in the Universe, so probably the opposite is true and future universe is full planet-colonizing alien civilizations.

As I pointed out in the other thread, first 8% is not early at all - it is firmly in the middle, statistically speaking. Early would be first 0.000001%. 8% is much closer to the middle than uncertainty in the estimate - well within one std.

But if SETI attack is only way of space colonization, no starships, in this case before-radio civilizations will be distributed randomly.

Well yeah if there are SETI attacks but no colonization, then sure there could be lots of civs like ours that get snuffed out, and that scenario is still compatible with our observations. I think it's unlikely though for the same reasons any SETI broadcasts are unlikely, as SETI broadcasts don't have much of a purpose, and the scope is narrow. (attack or communication only makes sense between civs at similar development points, and such temporal encounters are unlikely - the number of civs in the galaxy that are within +- 100 years of our dev level is probably < 1, even if the total number of civs in the galaxy is large)

I think that once a civilization fail victim of a SETI atack, it could broadcast for millions or even billions years, spending almost all its resources on it. It will build Dyson sphere and use all it energy to broadcast on maximum possible distance.

I think that once a civilization fail victim of a SETI atack, it could broadcast for millions or even billions years, spending almost all its resources on it.

The SETI attack civ can't touch civs more elder than it - they could prevent the SETI attack civ before it even forms. So it can only attack civs that come after.

But there are probably much easier attacks available for an elder civ. Dyson sphere broadcasting is a rather ludicrous waste of energy - the same energy budget could allow observation and then simulation of all other bio worlds at a lower level of dev. For a small energy budget the elder civ could launch small probes which could then intervene on each planet as necessary, arbitrarily influencing later civ development - accomplishing anything and more than any SETI attack (constrained to pure photons).

Dyson sphere broadcasting is a rather ludicrous waste of energy - the same energy budget could allow observation and then simulation of all other bio worlds at a lower level of dev.

How do you figure that? A SETI attack can be launched simply by transmitting a signal. How is that a waste of energy compared with vehicular travel?

A realistic SETI attack would need to transmit a very large quantity of information broadcast over very large distances, where the cost is quadratic with distance.

With a physical attack the 'energy' payload can be very dense and targeted, and it can use adaptive computations to decompress based on local information, harness energy from the local star, do physical manipulations, etc.

There is related analysis showing that sending particles or physical objects at high speeds can beat photons for interstellar communication in terms of energy efficiency.

With a physical attack the 'energy' payload can be very dense and targeted, and it can use adaptive computations to decompress based on local information, harness energy from the local star, do physical manipulations, etc.

This is an interesting point. However, while it is true that a physical attack can be precisely targeted, it is also true that it must be precisely targeted. One of the advantages of a SETI type attack is that it can be launched with no specific target in mind at all. The quadratic drop-off in signal strength that you allude to comes with a corresponding cubic increase in coverage space/volume, provided you run the signal continuously.

There is related analysis showing that sending particles or physical objects at high speeds can beat photons for interstellar communication in terms of energy efficiency.

This is an interesting point. However it would seem to apply to a targeted, point-to-point communication whereas a signal would not need to be precisely targeted.

One of the advantages of a SETI type attack is that it can be launched with no specific target in mind at all.

That isn't a practical advantage - by the time the attack civ is capable of launching a SETI attack they will know where the other stars are located, and they will probably also know which ones are likely habitable. More elder civs may exist in the interstellar medium or elsewhere, but that hardly matters because they are immune anyway.

If civilization are rare in space, like one in a million light years, observation would be difficult. And if probe speed is 0.5 с, SETI attack will cover 8 times more volume of space during the same time interval. So SETI attack usefulness depends of two unknowns - civilizations density and probe speed. EY by the way said that he believes that StrongAI will be able to expand on almost speed of light so there is no need of SETI attack.

EY by the way said that he believes that StrongAI will be able to expand on almost speed of light so there is no need of SETI attack.

The fact that EY said that strong AI can expand almost at the speed of light does not rule out SETI attacks being useful/efficient to an expansionist AI; perhaps the SETI attack is one of the mechanisms the strong AI uses to expand.

If civilization are rare in space, like one in a million light years, observation would be difficult.

This doesn't help SETI attack, as the energy cost increases with dist squared.

A SETI attack may be faster in terms of latency, but that doesn't matter unless the variance in development time is very low. Realistically the temporal dist is probably spread out over say a billion years or so. The difference between 1c and say 0.1c is just the difference between 100k years and 1,000k year latency and doesn't matter much.

The 1c attack only matters for the tiny fraction of civs that are on the brink of elder status at the time at which the attacking civ achieves said tech level.

A SETI attack may be faster in terms of latency, but that doesn't matter unless the variance in development time is very low.

I think that you are understating the importance of latency. Earlier, in this post you pointed out some of the difficulties in a civilization 1480 ly away launching an attack on Earth's civilization due to the distances involved. It seems to me that a SETI attack would eliminate these difficulties; the attack could be launched with no knowledge of Earth or Earth's civilization whatsoever, so the attack would not have to wait 1400 years for our radio signals to bring us to the attention of a (hypothetical) advanced civilization. And, a SETI-type attack would not require expensive and time consuming vehicular scouting/probe missions.

Earlier, in this post you pointed out some of the difficulties in a civilization 1480 ly away launching an attack on Earth's civilization due to the distances involved.

I started that with the unrealistic assumption that the attacking civ is close to us in age. In that specific scenario the latency does matter and a photon attack has a potential niche. But that scenario is rare.