New Ideas for the Mysterious Tabby’s Star: a Gigantic Planet or a Planet With Rings

KIC 8462852 (aka. Tabby’s Star) captured the world’s attention back in September of 2015 when it was found to be experiencing a mysterious drop in brightness. A week ago (on May 18th), it was announced that the star was at it again, which prompted observatories from all around the world to train their telescopes on the star so they could observe the dimming as it happened.

And just like before, this mysterious behavior has fueled speculation as to what could be causing it. Previously, ideas ranged from transiting comets and a consumed planet to alien megastructures. But with the latest studies to be produced on the subject, the light curve of the star has been respectively attributed to the presence of a debris disk and Trojan asteroids in the system and a ring system in the outer Solar System.

The first study, titled “KIC 8462852: Will the Trojans return in 2021?“, was written by a team from the University of Valencia, the Institute of Physics of Cantabria (IFCA) and the Astrophysical Institute of Andalusia (IAA). The paper was recently submitted to the Monthly Notices of the Royal Astronomical Society, and presents the argument that the dimming of Tabby’s Star can be explained by the presence of stable debris.

Diagram of the KIC 8462852 system, showing the possible influence of Trojans on its brightness. Credit: F. Ballesteros et al.

Led by Fernando J. Ballesteros, the team used data obtained by the Kepler mission to create a model of the system that could account for all the dips in brightness. These include the up to 20% drop that was observed in 2015 and the non-periodic repetitions and asymmetric dips that followed. From this, they determined that a ringed body and Trojan asteroids that share its orbit could explain the first large dip and the subsequent period of dips.

This explanation not only offers an entirely natural account of what could be causing the star to dim, but also offers a prediction that (if true) would confirm their theory. As they state in their paper:

“Whereas most of the scenarios that have already been discussed by other authors invoke the presence of astronomical objects that have never been directly observed, from the comet clouds in Boyajian et al. (2016) to the Dyson sphere in Wright et al. (2016), our model requires the presence of relatively familiar objects, namely a large planet with orbiting rings and a cloud of Trojan asteroids. Moreover, our model allows us to make a definite prediction: the leading Trojan cloud should induce a new period of irregularities in the light curve approximately in 2021.”

Interestingly, Jason Wright – an associate professor from Penn State University and the one who proposed the alien megastructure theory – chimed in on this paper. And it only seems fair, since the team refer to his work in their study! As he indicated on his website, AstroWright, the theory does have several strong points, but does not account for certain observations.

Artist’s impression of an orbiting swarm of dusty comet fragments around Tabby’s Star. Credit: NASA/JPL-Caltech

As he states, the dips observed from Tabby’s Star are quite steep, which is something natural phenomena cannot easily account for. Their study also does not address things like secular dimming, or the upper limits of IR and millimeter wave-observations. But perhaps most glaring, according to Wright, is the mass that would be required to create the kind of dimming that has been seen:

“They need a lot of asteroids: they don’t actually say how much, but the number they do give is huge: over a Jupiter mass of them!  It’s not clear to me how stable such a swarm could be co-orbital to an actual planet.  Part of the reason Jupiter’s Trojan asteroids work as they do is that they don’t really perturb Jupiter. Also, how do you keep a Jupiter mass of material from collapsing or falling into the planet?  Also, where would you get a Jupiter mass of rock?!”

The second paper, titled “Tabetha’s Rings”, was also recently submitted to MNRAS. Written by Professor Jonathon Katz of the Department of Physics and McDonnell Center for the Space Sciences at Washington University, the paper argues that the dips observed from Tabby’s Star could be caused by matter in the Solar System – specifically, a ringed object that lies between Kepler’s line of sight and KIC 8462852.

Based on the interval between dips, and the orbit and line of sight of the Kepler mission, Katz calculated what the distance of this possible ring would be, and provides estimates on the size and distribution of particulate matter within it as well. As he wrote in his study, a 600 m large object would be able to obscure all light coming from the Tabby’s Star, although only briefly.

Could a ring-like structure in the outer Solar System be what’s causing the observed dimming of Tabby’s Star? Credits: NASA/JHUAPL/SwRI

What’s more, given the orbital motion of Kepler (and the Earth), the observed dips in brightness would require the existence of an obscuring cloud that extends along the ring a distance equal to the distance the telescope travels. Ultimately, this paper is more of a thought experiment than a definitive hypothesis, one which Katz acknowledges in his conclusions.

“The occurrence of deep dips in two epochs separated by about two Kepler-years is a hint that the phenomenon may be local rather than circumstellar,” he states. “This evidence is suggestive but not statistically compelling because the interval differs from an exact integer multiple of Kepler-years by a few percent. However, the difficulty of developing a compelling circumstellar model and the history of discovery of narrow planetary rings by stellar occultation justify investigation of possible explanations involving Solar System rings.”

Another interesting aspect of Katz’s study is the fact that it too makes predictions about future dimming events. In short, his hypothesis indicates that future dips may be observed from Earth at intervals that are just a year apart. But according to Wright, who commented on this paper as well, this seems like a miscalculation.

“Some of the implications are worked out, but some of the math seems wrong to me (he predicts that the dips will be visible every 365.25 days from earth, which ignores the orbital motion of that object),” he wrote. However, Wright also congratulates Katz for making this argument since it is similar to one he himself made a year ago (which Katz acknowledged in his paper).

Dyson Sphere by Eburacum45
Could this be what’s causing Tabby’s Star to experience drops in brightness, an alien megastructure? Credit: Eburacum45

Last summer (August 31st, 2016), when writing on the subject of what could be causing Tabby’s Stars observed dips in brightness, Wright considered the possibility that a Solar System Cloud might be responsible:

“If there is something between us and the star, then proper motion should change our line of sight through it… For the moment, let’s put the hypothetical cloud out at 10,000 AU. Parallax would make it appear to move by about 20 arcseconds, and its orbital motion would move it by about the same amount over 100 years. So if the cloud is 20″ across, it could be responsible for the long-term dimming.  This would also help explain the 1.96 Kepler year gap between the two dips (although not the lack of dips at 0.98 years): that’s the time it takes our line of sight from Kepler to return to about the same place, with ~1% taken off due to the cloud’s own orbital motion.”

However, Wright also pointed out the flaws in this theory, stating that such a ring could not account for all the observations made of Tabby’s Star, and that he and other astronomers were at a loss to explain how such a ring could have been caused. “Not only is Boyajian’s Star way above the ecliptic (but does that even matter at 10,000 AU?), but a 20″ cloud at 10,000 AU would be 1 AU across.  What could cause it?” he wrote.

In the end, we may never know what is behind KIC 8462852’s strange behavior. But our ongoing efforts to gather additional information are making increasingly educated guesses possible. As we eliminate more and more in the way of possibilities, we are getting closer to an explanation that actually fits.

Next generation telescopes will certainly help in this regard. And who knows? Someday we may actually be able to explore this system directly and see if any our theories were correct!

Further Reading: AstroWright, MNRAS, (2)

Hubble Images Three Debris Disks Around G-type Stars

An image of the circum-stellar disk around HD 207129. The three circled objects are background objects and part of the disk. Image: Hubble Space Telescope, Glenn Schneider et al 2016.

A team using the Hubble Space Telescope has imaged circumstellar disk structures (CDSs) around three stars similar to our Sun. The stars are all G-type solar analogs, and the disks themselves share similarities with our Solar System’s own Kuiper Belt. Studying these CDSs will help us better understand their ring-like structure, and the formation of solar systems.

The team behind the study was led by Glenn Schneider of the Seward Observatory at the University of Arizona. They used the Hubble’s Space Telescope Imaging Spectrograph to capture the images. The stars in the study are HD 207917, HD 207129, and HD 202628.

Theoretical models of circumstellar disk dynamics suggest the presence of CDSs. Direct observation confirms their presence, though not many of these disks are within observational range. These new deep images of three solar analog CDSs are important. Studying the structure of these rings should lead to a better understanding of the formation of solar systems themselves.

A is the observed image of HD 207917. B is the best-fit debris ring model of the same star. Image: Hubble, G. Schneider et. al. 2016.
A is the observed image of HD 207917. B is the best-fit debris ring model of the same star. Image: Hubble, G. Schneider et. al. 2016.

Debris disks like these are separate from protoplanetary disks. Protoplanetary disks are a mixture of both gas and dust which exist around younger stars. They are the source material out of which planetesimals form. Those planetesimals then become planets.

Protoplanetary disks are much shorter-lived than CDSs. Whatever material is left over after planet formation is typically expelled from the host solar system by the star’s radiation pressure.

In circumstellar debris disks like the ones imaged in this study, the solar system is older, and the planets have already formed. CDSs like these have lasted this long by replenishing themselves. Collisions between larger bodies in the solar system create more debris. The resulting debris is continually ground down to smaller sizes by repeated collisions.

This process requires gravitational perturbation, either from planets in the system, or by binary stars. In fact, the presence of a CDSs is a strong hint that the solar system contains terrestrial planets.

A circumstellar disk of debris around a mature stellar system could indicate the presence of Earth-like planets. Credit: NASA/JPL
A circumstellar disk of debris around a mature stellar system could indicate the presence of Earth-like planets. Credit: NASA/JPL

The three disks in this study were viewed at intermediate inclinations. They scatter starlight, and are more easily observed than edge-on disks. Each of the three circumstellar disk structures possess “ring-like components that are more massive analogs of our solar system’s Edgeworth–Kuiper Belt,” according to the study.

The study authors expect that the images of these three disk structures will be studied in more detail, both by themselves and by others in future research. They also say that the James Webb Space Telescope will be a powerful tool for examining CDSs.

Read more: It’s Complicated: Hubble Survey Finds Unexpected Diversity in Dusty Discs Around Nearby Stars

Search for Planetary Nurseries in the Latest Citizen Science Project

Growing up, my sister played video games and I read books. Now that she has a one-year-old daughter we constantly argue over how her little girl should spend her time. Should she read books in order to increase her vocabulary and stretch her imagination? Or should she play video games in order to strengthen her hand-eye coordination and train her mind to find patterns?

I like to believe that I did so well in school because of my initial unadorned love for books. But I might be about to lose that argument as gamers prove their value in science and more specifically astronomy.

Take a quick look through Zooniverse and you’ll be amazed by the number of Citizen Science projects. You can explore the surface of the moon in Moon Zoo, determine how galaxies form in Galaxy Zoo and search for Earth-like planets in Planet Hunters.

In 2011 two citizen scientists made big news when they discovered two exoplanet candidates — demonstrating that human pattern recognition can easily compliment the powerful computer algorithms created by the Kepler team.

But now we’re introducing yet another Citizen Science project: Disk Detective.

Planets form and grow within dusty circling planes of gas that surround young stars. However, there are many outstanding questions and details within this process that still elude us. The best way to better understand how planets form is to directly image nearby planetary nurseries. But first we have to find them.

zooniverse

“Through Disk Detective, volunteers will help the astronomical community discover new planetary nurseries that will become future targets for NASA’s Hubble Space Telescope and its successor, the James Webb Space Telescope,” said the chief scientist for NASA Goddard’s Sciences and Exploration Directorate, James Garvin, in a press release.

NASA’s Wide-field Infrared Survey Explorer (WISE) scanned the entire sky at infrared wavelengths for a year. It took detailed measurements of more than 745 million objects.

Astronomers have used complex computer algorithms to search this vast amount of data for objects that glow bright in the infrared. But now they’re calling on your help. Not only do planetary nurseries glow in the infrared but so do galaxies, interstellar dust clouds and asteroids.

While there’s likely to be thousands of planetary nurseries glowing bright in the data, we have to separate them from everything else. And the only way to do this is to inspect every single image by eye — a monumental challenge for any astronomer — hence the invention of Disk Detective.

Brief animations allow the user to help classify the object based on relatively simple criteria, such as whether or not the object is round or if there are multiple objects.

“Disk Detective’s simple and engaging interface allows volunteers from all over the world to participate in cutting-edge astronomy research that wouldn’t even be possible without their efforts,” said Laura Whyte, director of Citizen Science at the Adler Planetarium in Chicago, Ill.

The project is hoping to find two types of developing planetary environments, distinguished by their age. The first, known as a young stellar object disk is, well, young. It’s less than 5 million years old and contains large quantities of gas. The second, known as a debris disk, is older than 5 million years. It contains no gas but instead belts of rocky or icy debris similar to our very own asteroid and Kupier belts.

So what are you waiting for? Head to Disk Detective and help astronomers understand how complex worlds form in dusty disks of gas. The book will be there when you get back.

The original press release may be found here.

Astronomy Without A Telescope – Alien Mining

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Recently, some researchers speculated on what types of observational data from distant planetary systems might indicate the presence of an alien civilization, determined that asteroid mining was likely to be worth looking for – but ended up concluding that most of the effects of such activity would be difficult to distinguish from natural phenomena.

And in any case, aren’t we just anthropomorphizing by assuming that intelligent alien activity will be anything like human activity?

Currently – apart from a radio, or other wavelength, transmission carrying artificial and presumably intelligent content – it’s thought that indicators of the presence of an alien civilization might include:
• Atmospheric pollutants, like chlorofluorocarbons – which, unlike methane or molecular oxygen, are clearly manufactured rather than just biogenically produced
• Propulsion signatures – remember how the Vulcans detected humanity in First Contact (or at least they decided we were worth visiting after all, despite all the I Love Lucy re-runs)
Stellar engineering – where a star’s lifetime is artificially extended to maintain the habitable zone of its planetary system
Dyson spheres – or at least their more plausible off-shoots, such as Dyson swarms.

And perhaps add to this list – asteroid mining, which would potentially create a lot of dust and debris around a star on a scale that might be detectable from Earth.

There is a lot of current interest in debris disks around other stars, which are detectable when they are heated up by the star they surround and then radiate that heat in the infra-red and sub-millimeter wavelengths. For mainstream science, debris disk observations may offer another way to detect exoplanets, which might produce clumping patterns in the dust through gravitational resonance. Indeed it may turn out that the presence of a debris disk strongly correlates with the existence of rocky terrestrial planets in that system.

But now going off the mainstream… presuming that we can eventually build up a representative database of debris disk characteristics, including their density, granularity and chemistry derived from photometric and spectroscopic analysis, it might become possible to identify anomalous debris disks that could indicate alien mining activities.

Some recent astronomy pareidolia. Not an alien mining operation on Mercury, but a chunk of solidified ejecta commonly found in the center of many impact craters. Credit: NASA.

For example, we might see a significant deficiency in a characteristic element (say, iron or platinum) because the aliens had extracted these elements – or we might see an unusually fine granularity in the disk because the aliens had ground everything down to fine particles before extracting what they wanted.

But surely it’s equally plausible to propose that if the aliens are technologically advanced enough to undertake asteroid mining, they would also do it with efficient techniques that would not leave any debris behind.

The gravity of Earth makes it easy enough to just blow up big chunks of rock to get at what you want since all the debris just falls back to the ground and you can sort through it later for secondary extraction.

Following this approach with an asteroid would produce a floating debris field that might represent a risk to spacecraft, as well as leaving you without any secondary extraction opportunities. Better to mine under a protective canopy or just send in some self-replicating nanobots, which can separate out an enriched chunk of the desired material and leave the remainder intact.

If you’re going to play the alien card, you might as well go all in.

Further reading: Forgan and Elvis. Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence.

Some useful tips on asteroid mining can be found here.

Astronomy Without A Telescope – Our Unlikely Solar System

A circumstellar disk of debris around a matured stellar system may indicate that Earth-like planets lie within. LUVOIR will be able to see inside the disk to watch planets forming. Credit: NASA

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Recent modeling of Sun-like stars with planetary systems, found that a system with four rocky planets and four gas giants in stable orbits – and only a sparsely populated outer belt of planetesimals – has only a 15 to 25% likelihood of developing. While you might be skeptical about the validity of a model that puts our best known planetary system in the unlikely basket, there may be some truth in this finding.

This modeling has been informed by the current database of known exoplanets and otherwise based on some prima facie reasonable assumptions. Firstly, it is assumed that gas giants are unable to form within the frost line of a system – a line beyond which hydrogen compounds, like water, methane and ammonia would exist as ice. For our Solar System, this line is about 2.7 astronomical units from the Sun – which is roughly in the middle of the asteroid belt.

Gas giants are thought to only be able to form this far out as their formation requires a large volume of solid material (in the form of ices) which then become the cores of the gas giants. While there may be just as much rocky material like iron, nickel and silicon outside the frost line, these materials are not abundant enough to play a significant role in forming giant planets and any planetesimals they may form are either gobbled up by the giants or flung out of orbit.

However, within the frost line, rocky materials are the dominant basis for planet forming – since most light gas is blown out of the region by force of the stellar wind and other light compounds (such as H2O and CO2) are only sustained by accretion within forming planetesimals of heavier materials (such as iron, nickel and silicates). Appreciably-sized rocky planets would probably form in these regions within 10-100 million years after the star’s birth.

So, perhaps a little parochially, it is assumed that you start with a system of three regions – an inner terrestrial planet forming region, a gas giant forming region and an outer region of unbound planetesimals, where the star’s gravity is not sufficient to draw material in to engage in further accretion.

From this base, Raymond et al ran a set of 152 variations, from which a number of broad rules emerged. Firstly, it seems that the likelihood of sustaining terrestrial inner planets is very dependent on the stability of the gas giants’ orbits. Frequently, gravitational perturbations amongst the gas giants results in them adopting more eccentric elliptical orbits which then clears out all the terrestrial planets – or sends them crashing into the star. Only 40% of systems retained more than one terrestrial planet, 20% had just one and 40% had lost them all.

The Moon has retained a comprehensive record of the Late Heavy Bombardment from 4.1 to 3.8 billion years ago - resulting from a reconfiguration of the gas giants. As well as clearing out much of debris disk of the early Solar System, this reconfiguration flung material into the inner solar system to bombard the rocky planets.

Debris disks of hot and cold dust were found to be common phenomena in matured systems which did retain terrestrial planets. In all systems, primal dust is largely cleared out within the first few hundred million years – by radiation or by planets. But, where terrestrial planets are retained, there is a replenishment of this dust – presumably via collisional grinding of rocky planetesimals.

This finding is reflected in the paper’s title Debris disks as signposts of terrestrial planet formation. If this modeling work is an accurate reflection of reality, then debris disks are common in systems with stable gas giants – and hence persisting terrestrial planets – but are absent from systems with highly eccentric gas giant orbits, where the terrestrial planets have been cleared out.

Nonetheless, the Solar System appears as unusual in this schema. It is proposed that perturbations within our gas giants’ orbits, leading to the Late Heavy Bombardment, were indeed late with respect to how other systems usually behave. This has left us with an unusually high number of terrestrial planets which had formed before the gas giant reconfiguration began. And the lateness of the event, after all the collisions which built the terrestrial planets were finished, cleared out most of the debris disk that might have been there – apart from that faint hint of Zodiacal light that you might notice in a dark sky after sunset or before dawn.

Further reading: Raymond et al Debris disks as signposts of terrestrial planet formation.