Forget about alien mega-structures: new observations explain the behavior of the star Tabbi with dust alone

The artistic image of KIC 8462852, whose brightness over the past few years has changed in an unusual way

The scientific hunt for the planets in the XXI century really unfolded, and in the first place is the transit method . When the planet passes in front of its parent star, if we look from our point of view, the part of the star’s light disappears for a while. Such transits provide planetary hunters with a fruitful method of searching for worlds in orbits alongside other stars. Today we already know thousands of stars with their own planets, and most of them have been discovered by the transit method.

When developing a mission that focuses on the discovery of planets, we can expect that with its help we will discover many strange things. However, astronomers were not at all prepared to detect such a curiosity as the star Tabbi , whose glow is muted very strongly and completely irregularly. After several years of speculation on this topic, during which versions from comet storms to alien megastructures were expressed, scientists finally solved this riddle . Dust was appointed the culprit, although it behaves in a somewhat unusual way.


Left - infrared, right - ultraviolet radiation of Tabbi star. No evidence of a multitude of explanations for luminosity failures for “natural” reasons.
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For several years, the Kepler mission has managed to explore more than 100,000 stars. Of the hundreds of thousands of stars, one has stood out for its interesting behavior. KIC 8462852, known as the Tabbi star, or the Boyadjian star (after the discoverer of this behavior), or as the WTF? (where is the flux? [The wordplay is the same abbreviation for where is the flux, “where the glow is,” and what the fuck is “what the fuck” / comment perev.]) has a completely unique combination of properties. She is at the same time:

• demonstrates a serious drop in luminosity, up to 22% (for most of the stars of the planet, their luminosity decreases by no more than 1%),
• tarnishes slowly on periods of several decades, but sometimes it becomes brighter (which is not done by any other similar stars),
• the overall brightness fluctuates during luminosity decreases (and not just gradually decreases, and then increases, as is the case with planets),
• does not give infrared radiation (which is shown by all other stars with a serious drop in luminosity).

This gave rise to a complex puzzle.


Already received a large number of images of protoplanetary systems. The SPHERE infrared telescope is considered a work of art in this area, regularly issuing images with a resolution of about 10 ", or less than 0.003 degrees per pixel. KIC 8462852 does not have the usual properties or infrared radiation.

It could not be a planet, because a planet cannot be so large as to block so much light from a star. Even if one imagines a planet with a huge system of rings, such as giant Saturn, dips in luminosity will be periodic and smooth, with a certain plateau on the graph. But this contradicts the data collected.


An artistic depiction of the ring system of a young giant planet or brown dwarf J1407b orbiting another star. Worlds with extremely highly developed rings can lead to a strong decrease in the visible light of a star, but such reductions should be periodic, which was not observed.

It could be a very young star with planetesimals , a protoplanetary disk and an extremely dusty environment. All the stars we have seen with a strong drop in luminosity fall into this category.

But Tabbi's star is too old to have a protoplanetary disk: it is many hundreds of millions of years old. And most importantly, it does not give infrared radiation, which should be a star with a protoplanetary disk. Therefore, this star was originally called WTF, "where is the glow."


Artistic image of a young star, surrounded by a protoplanetary disk. Many of the properties of protoplanetary disks around sun-like stars are unknown, but they all have characteristic infrared radiation. Tabby has no such thing.

This could be several successive events related to the flight of comets, during which they emit large amounts of dust, falling into the inner part of the star system. Relatively recently, it was shown how this can explain the observed luminosity dips.


Illustration of a comet storm around a star close to us, This Crow . The comets variant is one of the explanations for reducing the luminosity of the Tabbi star, which was also rejected after receiving an astronomical spectrum of high quality.

But there is another phenomenon that does not explain the proposed solution: the long-term decrease in the luminosity of a star. A star is called a “Tabbi star” not because this particular scientist discovered it, but because it began a scientific investigation of interesting and important behavior that has not been seen before.

But this star has been known for more than a century, and we are seeing a long-term decrease in luminosity, which this model cannot explain. Planet dust blows away in a few months; in order to block the star's light for a hundred years, it would take almost continuously to bombard it with comets. It would have required the presence of many comets in similar orbits, and we do not imagine how to obtain such a configuration.


Harvard light curve for the star KIC 8462852, and a comparison with two other stars whose apparent brightness has not changed.

What possible explanations remained? One of the popular ideas was that around this star, some technologically advanced civilization builds a megastructure that periodically (or aperiodically) covers most of the star's light. The structure eventually comes to an end, and blocks more and more light. The fact that over the past hundred years the light from the star has faded so much could be explained by the progress of construction.

The idea is intriguing, albeit non-standard.


Partially obscure the light of the star could be an unfinished alien megastructure, which the Gaia space telescope could in principle detect. However, in the vicinity of KIC 8462852 it is not at all this - this option rejects the study of the spectrum.

But, thanks to the huge amount of follow-up observations, we know that this is not the case. The reason is that the alien megastructure would be completely opaque to light: he could not pass through it. How could not pass through planets, moons, or any other solid objects.

From 19,000 images taken in the last three years, made in four wavelength ranges, from blue to infrared light, we learned that in all cases of dimming the light, blue light is first of all blocked: from short-term drops to long-term tarnishing. And only one thing can block blue light while passing red: particles of a certain size dust.


On the left - a photo in visible light, on the right - in infrared; Barnard 68, a dust-rich globule, is visible in the image. Infrared light is less blocked because dust particles of small size cannot interact with light with a large wavelength.

So it must be dust. Whatever the reason for reducing the amount of transmitted light, as well as long-term tarnishing, dust must form the basis of this phenomenon. The dips of luminosity, obtained by Kepler, and the “secular dimming” are caused by the same phenomenon. As stated in the new job :

This color attenuation indicates the presence of dust particles up to ~ 0.1 microns in size. Such dust should have been blown off by the star's radiation pressure rather quickly, therefore dust clouds should have formed within a few months. Modern infrared observations were made with a dust coating of 12.4% ± 1.3%, and they correspond to the muting of light by starfate.

Here's what the evidence points to: dust. But still the situation remains somewhat mysterious.


An illustration of a complex, dust-filled region around a star that has been superimposed with recent data from Tabota Boyajyan, demonstrating recent dips in the glow. Dust can not be on the surface of the star as shown in the picture.

As a result, the star Tabbi combines features, the simultaneous presence of which we could not expect:

• The picture coincides with the one that should be in the presence of a large amount of circumstellar dust, which usually speaks of the extreme youth of a star that is at the stages of formation.
• The star itself is brighter, hotter and more massive than the sun: it radiates four times more light than the sun.
• The star is old: it is hundreds of millions of years old, and in all respects it stably burns, being in the main sequence.

In other words, the dust visible by us should remain only a few months, taking into account the properties of the star. So, the star has some way to replenish dust. As far as we know, there are two sensible possibilities: either the star has an outer dust ring with dense dust clouds or is constantly bombarded by asteroids, or the star has some external object blocking its light.


Currently, the leading idea is the existence of a disk of dust and fragments around the star. Then an incredible coincidence would have occurred, in which its plane would ideally coincide so perfectly with our line of sight — this would be a remarkable, but unlikely event. Even with a probability of 1%, it would be strange that we did not observe this in the remaining 99% of stars.

The decrease in the apparent brightness of the star from the 1890s, apparently, continues today, in 2018, but it goes unevenly. In addition, there are constantly periods of temporary decrease in luminosity, lasting for months, as well as shorter dips, lasting no more than a day. This is definitely due to the presence of dust particles, probably about 100 nm in size. The percentage of blocked light at different wavelengths confirms this and rejects other hypotheses.

But where does this dust come from? To reduce the number of options, scientists have calculated the amounts of dust needed to explain the gradual dimming of the visible light of a star over the past 100 years, as well as for short-term dips. To block, based on the intersection of the dust plane of our line of sight, you need a mass of dust equivalent to the moon.


Initially, to explain what was happening with Tabby's star, a scenario with fragments of a comet was considered. But instead, several comet-like objects with long periods of revolution and massive dust halos can lead to these temporary transitive luminosity drops, but this will require a huge mass of matter that is not in the form of opaque objects.

But that's not all. Previous studies have suggested that there may be a large amount of interstellar dust in space, which is confirmed by the data.

This theory can replace or supplement the variant with circumstellar dust. The disk of material around the star is the minimum necessary thing to explain. A huge amount of dust may not be in the plane that we observe, but outside it, possibly forming a halo. We do not know this, but we know that if it exists, then it should not be too close to the star, otherwise it would radiate in the infrared. Comets must also emit infrared radiation; James Webb's telescope is likely to be able to find out if comets are responsible for temporarily lowering the luminosity of a star.


A disk of dust and debris that revolves around either the star itself or around its planets should emit infrared radiation — but this is not observed. However, our observations can be explained by the presence of a disk (or halo) located further from the star.

Finally, there is another theory candidate for explaining observations: dust can be the result of stellar "indigestion."

If this star tried to digest the planet, a gas giant of, say, Uranus size, this could explain everything. A long-time fall on a star in a spiral of a planet or several planetary bodies, which occurred, perhaps hundreds or thousands of years ago, could have led to a temporary increase in brightness, from which the star returns to its normal, stable state. And the observed dips in luminosity can be explained by planetary fragments of previous cataclysms, or by evaporation and emission of gas from smaller bodies.


Artistic image HD 189733 b, hot Jupiter, so close to his star that its atmosphere boils and breaks out into space. If KIC 8462852 had recently swallowed a gas giant, it could, in principle, "burp" dust particles, which could cause the observed muting of the light.

Regardless of what mechanism works here, we can be sure of one thing: the reason for the tarnishing of the star Tabby lies in the dust. This is a normal particulate dust containing particles up to 100 nm in size, or less than the wavelength of visible light. The same dust, which causes short, day-long or less brightness drops, also causes tarnishing that lasts for months, and has led to a general fading of the star over the past hundred years. All this is due to normal, normal dust.

The following question remains the big and open question: where did this dust come from? It is not there because this star is young or still forming, and the most serious limitations are imposed on the presence of an invisible companion. It cannot fully be interstellar. Was any planet digested by a star? Is there anything else going on there? The only way to find out is to conduct more scientific research, and better quality. One thing is clear: even if somewhere there are alien megastructures, they do not exist there.