There’s clearly something mysterious going on. We don’t know what it is or how it works and we’re as close to figuring it out as we were in 2009.
Sandhya Ramesh is a science writer focusing on astronomy and earth science.
In May of 2009, several hundred people around the world noticed a star in a galaxy far, far away. It looked like nothing they’d ever seen before. They thronged to discussion forums on the web for the citizen science project called Planet Hunters and frantically discussed what they were seeing. They’d all been working on data obtained by the Kepler Space Telescope. The star, later called KIC 8462852, was blinking in a very strange way.
Hunting for planets outside the Solar System is an active field of astronomy today. The first exoplanet was discovered in 1992. Only 25 years later, there are more than 3,500 confirmed planets orbiting stars other than our own. NASA’s Kepler Space Telescope alone was responsible for finding over 2,000. It did so by watching the stars in a patch of the sky for weeks, months or even years at a time. When a planet that goes around one of these stars crosses against the face of its star, it casts a small but definite shadow in the starlight. These shadows are recorded and can be used to calculate the size of the planet. The larger the planet, the more light it blocks.
The resulting database is hundreds of terabytes large, so astronomers use finely tuned computer-programs to sift through it looking for the telltale signs of an exoplanet. The programs are more efficient and can process information much faster than a team of astronomers ever could. But they aren’t perfect either. Sometimes, the programs miss things that they haven’t been taught to look for. So enter citizen science.
Students, homemakers, artists, grandparents all contribute to citizen science projects in myriad ways. There are projects that help look through camera trap pictures to identify animals. Projects where you and I can look at images of the Martian surface to identify wind patterns, at scanned manuscripts and digitise text, even create a database of wildlife. The human brain is very good at recognising patterns – so much so that we even see patterns where there might not be any.
Planet Hunters is a citizen science project to help identify dips in the brightness of stars that computers could have missed. As of August 2017, over 300,000 citizen scientists access it and comb through Kepler data every day.
In June 2009, astronomer Tabetha Boyajian was alerted by a number of citizen scientists that a particular star about 1,500 lightyears away was being consistently flagged by users as “interesting” and “bizarre.” Then a postdoc at the University of Yale, Boyajian and her colleague Debra Fischer, who founded Planet Hunters, were hooked. The star they were looking at did look like an anomaly. Its brightness initially dipped by a tiny 0.5% – which was normal – but it lasted for four days, which is totally not normal. Typically, planets that cross the path of stars like these tend to have dips that last only a few hours.
That was the first dip in brightness they noticed in the data, in early 2009. Following this, a few weeks later, the brightness of KIC 8462852 dipped for almost a full week. Boyajian and Fischer spent days poring over the data and running through numbers, but the behaviour of KIC 8462852 couldn’t be explained away. The instruments were working fine, the computers were working fine, the data was clear. The dips were real – and inexplicable. And not only were there long periods of dimming, they were also weird kinds of dimming.
When a planet passes in front of a star, because of its near spherical shape, it creates a symmetric dip as it crosses against the disc of the star. The way the brightness dropped on a graph was also the way it picked up (see image below).
But the dips Kepler spotted with 8462852 were asymmetric.
Planets also orbit their stars in fixed periods of time, and they reappear after exactly the same amount of time during each dimming event. But whatever was moving around KIC 8462852 was causing the starlight to dim unpredictably, randomly.
Citizen scientists and astronomers continued to study the star, which had returned to its regular brightness.
Two years later, in March 2011, Kepler recorded a new dip in the brightness of KIC 8462852 and sent astronomers into a tizzy. A large planet the size of Jupiter records about 1% dip in its star’s brightness, at most. But the brightness of KIC 8462852 had dipped by nearly 15%. Like in 2009, the dip was again asymmetric, and lasted a week before slipping back to normal. This pattern repeated for a few days and then the star went ‘silent’ again.
In February 2013, and later in April, KIC 8462852 woke up once more. This time, it threw up a very irregular series of dips. While previous patterns involved simple dips in brightnesses at random intervals, the new data implied that its brightness was shifting in both intensity and, now, duration. The dip patterns were complicated as well. Astronomers spotted smaller dips within bigger dips, and even smaller ones within them. This fractal flux of its brightness was repeated across many days, as well as in the form of irregular recursions within the same dataset, which itself was becoming quite complex. At one point, the star’s brightness dipped by a stunning 22%.
In October 2015, Boyajian and her team published a paper with her team, titled ‘Where’s The Flux’, that set off a media frenzy and brought the star within the astronomy world’s spotlight. Ever since, KIC 8462852 has been called Tabby’s star, Boyajian’s star and – quite aptly – the WTF star.
Things that cause stars to dim have very specific signatures depending on their physical properties. This kind of data is usually found in the light that the star emits. If it was a cloud of dust blocking it, it’d be blocking more blue light than anything else. If something more solid and ample was in the way, it would be blocking light of all wavelengths. Solid objects would also heat up and glow in the infrared data. Another kind of heating, so to speak, can also reveal what the object is made of. Some elements absorb certain frequencies of starlight, which would show up as blanks in the recorded data. This barcode of sorts can be used to ID groups of elements.
Using their spectroscopic tools, astronomers were able to deduce that Tabby’s star was a middle-aged star burning just like our 4.5-billion-year-old Sun (in technical parlance, an F-type main-sequence star).
In light of all this information, Boyajian and her team offered a few scenarios in 2015 that could explain what might be causing the mysterious dimming.
On top of the list – and to no one’s surprise – was a protoplanetary disc. A star system forms when a giant cloud of gas collapses under the weight of its own gravity and starts to spin around. When that happens, all the material spreads out like a disc, spinning faster. It is densest at the centre, where the system’s host star will eventually form, while the light stuff – gases and some rocks – are flung outward. This is the protoplanetary disc. It’s where planets are born. Clumps of material collide here and there, forming bigger clumps that then snowball into planetesimals. The gases that are expelled further out of the disc form the distinctly spectacular yet bottomless gas giants.
The protoplanetary disc revolves around a star for millions of years before a stable planetary system can come to be. In this time, the disc can cause irregular dips in the star’s brightness (as seen by a distant observer).
But there was one big hole in this theory: protoplanetary discs, by virtue of being ‘proto’, exist only around newborn stars. Tabby’s star isn’t newborn.
Moreover, dust absorbs heat. When it becomes even mildly hot, say to the temperature of the human body, it starts glowing in the eyes of an infrared telescope. But NASA’s Spitzer telescope saw no such thing. It also ruled out ideas like two planets colliding and trashing their orbits with rocks or a planet being blown apart in an intergalactic space battle. In fact, it seems there’s just no debris around Tabby’s star.
Next idea: Maybe the star itself grew in size and brightness and then shrank back down? There are several such stars we know today. Astronomers call them variable stars. The largest star in the observable universe, UY Scuti, is an example. They’re observed when a young star’s brightness and mass suddenly spike when a large amount of material falls from a planetary disc into the star. But this idea was also knocked down by the same hammer: the star isn’t young and there’s no protoplanetary disc.
The Boyajian et al paper concluded that a family of passing comets would be the most acceptable explanation. But it’s also unlikely: to cause the levels of dimming that astronomers were seeing with Tabby’s star, a locusts’ swarm of giant comets would have had to be passing by. If they existed, they would likely have been formed when a rocky body about 100 km wide was shattered in a single explosion. Comets also don’t glow in the infrared; the ice on them absorbs the heat and sublimates into space. As improbable as it sounds, this was the only idea that couldn’t be knocked down right away.
However, none of these ideas – as trusting as they were in natural explanations – caught the public imagination as much as two words that emerged in a conversation between Boyajian and an astronomer at Pennsylvania State University a year earlier.
One of the things we look for when we look for intelligent civilisations out there is a synthetic sign of their presence. We look for things they might’ve built or signals their machines might’ve emitted. We assume that if there are aliens out there, they’ve been around for longer than we have and lead more sophisticated lives. We think they might’ve exhausted all the fuel available on their planet and are now looking elsewhere. For such a guzzler of a civilisation, what better power source to focus on than an entire star?
In a paper published in 1960, the physicist Freeman Dyson described a megastructure since called a Dyson sphere. It would be a supermassive shell built around a star, its insides lined with solar panels and other devices. All the energy that the star had to give, the sphere would devour. At various stages of construction, this megastructure would be like a ring equipped with living areas and power stations, then a ‘bubble’ in the form of multiple rings, and then the sphere itself.
In 2014, John Wright, the Penn State astronomer was fascinated by large structures that advanced civilisations could build to harvest energy. He suggested looking for a Dyson sphere around Tabby’s star. This is why it’s also called the ‘alien megastructure’ star.
Like with the disc and variable star ideas, there was a hole in this one as well. A Dyson sphere, by virtue of fielding the total energy output of an F-type star, would have to be heating up. But the corresponding heat signature couldn’t be found in the data.
Wright had been working on a paper about detecting transiting alien megastructures with the Kepler telescope. He theorised that the telescope was powerful enough to distinguish between artificial structures like giant solar panels, ringworlds, large spaceborne beacons looking for other aliens, etc., from exoplanets. He’d written a blog post about it in 2013. When he was in the process of turning it into a journal article, Boyajian visited Penn State for a lecture and shared then-unpublished data of the dimmings with Wright.
By the next year, Wright was captivated enough by the strangeness of the data to book time on the Green Bank Telescope, operated by the Berkeley SETI Research Centre, to observe Tabby’s star. (SETI stands for the Search for Extraterrestrial Intelligence.)
At the same time, media hysteria was fuelled by a paper Wright and his team finished writing in December 2015, about how megastructures would look, using Tabby’s star as an example.
It didn’t take long for Wright’s measured comments to become sensationalised. Once every two or three weeks in 2016, a headline somewhere in the world screamed “alien megastructure”. But while alien megastructures could exist in theory, astronomers weren’t sold on it. They demanded observational proof – as was their right.
They still don’t have it.
In the time since Wright made his suggestions, his peers have forwarded another theory – this one thankfully more natural again. Astronomers from the Universities of Valencia and Cantabria, both in Spain, asked: What if the cause of the dimming was one large, massive planet with orbiting companions called trojans?
Trojans are rocky bodies that orbit a star in the same orbit as a planet but at a different point. So for every planet, there are two areas in its orbit (before and after) where smaller bodies can park themselves. In some of these areas, the gravitational effects of the star and the planet would cancel out, leaving the body not moving in the orbit as much as moving with the orbit itself. In other words, this object – a trojan – would be at the same relative position as seen from the star or the planet.
The Spaniards simulated models of ringed planets and trojans. “We aim at offering a relatively natural solution, invoking only phenomena that have been previously observed, although perhaps in larger or more massive versions,” they wrote in a June 2017 preprint paper.
What if there really was a planet five times the size of Jupiter with numerous trojans on either side? According to the model simulated by the scientists, first some Trojans pass in between the star and us, causing intermittent and irregular dimming. Then, when this planet passes in front of its star, we first see the rings blocking the starlight dimly, followed by the planet creating a huge dent in the brightness, and then the rest of the rings pass by. Then, after 700 days, we see the trailing trojans. This, the scientists say, could explain the relatively smooth dip caused by a planet and then the series of sudden jagged, nested dips caused by the trojans.
If the model is anything to go by, the supergiant planet will have an orbital period of 12 years. And as things currently stand, those interested can expect to see the effect of trojans (assuming they’re there) on the light from Tabby’s star in 2021, followed by that of the planet itself two years later.
The idea of a ringed planet is particularly captivating to scientists, it seems. Another pre-print paper earlier this month made the same claim: maybe a planet with rings can explain the phenomenon…
However, a ringed planet would still provide a consistent dimming pattern – something we haven’t yet seen.
Starting late 2015, the SETI Institute began looking for signs of extraterrestrial intelligence from the direction of Tabby’s star. Specifically, the institute pointed the Allen Telescope Array in California in the star’s general direction hoping to pick up on radio waves encoded with information that would betray the hand of an alien intelligence. The odds of actually picking up on a signal were poor. Tabby’s star is 1,500 lightyears away. Anything we detect now should’ve been emitted that many years ago. Moreover, if the Tabbians were an advanced civilisation, beaming radio signals into arbitrary patches of sky wouldn’t have made much sense. They would’ve observed us first and then sent out something intended specifically for us – a 3,000-year process because that’s how long radiation would take to travel back and forth.
Three-thousand years ago – i.e. in 985 BCE – the Vedic tribes had just started settling on the plains of the Ganges, and Confucianism and Hellenism were just starting to develop. If at all these beings were able to detect life on our planet, they would have observed us in the Iron Age. First, they’d have had to calculate how long we would take to become proficient at detecting radio signals, then they’d have to beam signals at us for years together so that one day, someday, we’d finally turn our telescopic ears in their direction. Essentially, the chances are too low.
The Allen array didn’t detect anything during its observations. Astronomers at SETI also were on the lookout for radio signals. A smaller effort even looked for flashes of lasers from around the star.
In all this time, Tabby’s Star has been steadily dimming, with nothing blocking its light as far as we can tell. It’s fading away and we have no idea why.
Monitoring it round-the-clock is important if only because astronomers can pick up on the smallest clues. But for this purpose, government-run telescopes are usually overbooked. Even if they were available, their administrators don’t prefer taking on long-term observations. So Boyajian and her team have turned to private observatories – like the Las Cumbres Observatory Global Telescope (LCOGT) Network. Its scopes are placed strategically across the globe so astronomers can use them to look at an object (in the sky, not across the road) 24×7. And to pay for using them for a year, Boyajian raised $107,421 through a Kickstarter campaign in May 2016.
Fast forward to 2017.
All the attention has brought together a community of amateur and professional astronomers who are constantly observing, working on and thinking about Tabby’s star. The smallest of fluctuations in its brightness is meticulously recorded and shared.
On April 24, an alert went out from a robotic telescope at the Fairborn Observatory, Arizona, after it noticed a dip in the star’s brightness. It was a perfectly normal statistical fluctuation and the brightness returned to its former level in a week. But astronomers weren’t – aren’t – risking any complacency. Eternal vigilance is the official party line.
A similar event occurred on May 14 at the Mercator Telescope in Spain. Scientists there promptly contacted Boyajian, but the data she was shown sported just another statistical variation. “In the end, it ended up being an artefact – i.e. not real,” she told The Wire.
But only four days later, Fairborn Observatory and the LCOGT observed another dip, a prospective dimming event. An alert went out the next day. “Fairborn and LCO saw the same borderline decline,” Boyajian said. “We monitored the star closely that night and sounded off the alert when it was confirmed by two different observatories.”
She was excited and called Wright at 4 am on May 19. Fairborn’s message said the star had dimmed by 2%. Just in case, Wright also tweeted, “ALERT: [Tabby’s star] is dipping. This is not a drill. Astro tweeps on telescopes in the next 48 hours: spectra please!” In a few hours, a small army of astronomers around the world were looking intently at the distant orb – whose brightness, by the way, went on to dip by another 1%.
Dozens of telescopes, like the Great Canary Telescope, booked weeks – some, months – in advance came together to observe the star. The heavyweights joined in too, including the twin telescopes at the Keck Observatory (Hawaii), the LCOGT and the Multiple Mirror Telescope Observatory (Arizona). The Green Bank telescope continued to observe it. Even the Swift telescope, which studies ultraviolet radiation in the universe from its perch in space, was going “WTF”.
This wasn’t an unexpected event at all. Tabby’s 2015 paper had said it would happen: “A more robust prediction is that future dimming events should occur roughly every 750 days, with one in 2015 April” – missed because of a glitch on Kepler – “and another in 2017 May.”
But what was really unexpected was that the star’s brightness returned to normal in a few days, dimming again by almost 2% on June 13 and 14 and bouncing back yet again in days. On July 4, its brightness dipped by 0.5% before returning to normal. At the time this article was published, the star dimmed by another 3%.
The next major dimming event is expected to occur in mid-2019. Hopefully that’s enough time for astronomers to think up more plausible explanations.
To complicate things further, Tabby’s star has been showing signs of long-term dimming. It’s slowly fading away. Bradley Schaefer, an astronomer at Louisiana State University, decided to dig around for older records of Tabby’s Star from sky surveys. He found out that the star had been photographed over 1,200 times between 1890 and 1989. Going through this 100 years’ worth of data, he discovered that the star has dropped in its overall brightness by nearly 20%. There have been more studies about this phenomenon and all seem to confirm it. This long term dimming isn’t steady either. There have been times when Tabby’s star decided to perk up and be bright… before starting to fade again.
Astronomer Huan Meng from the University of Arizona recently led a study in conjunction with Boyajian where Tabby’s star was observed two new telescopes: Swift (which observes in the X-ray and ultraviolet wavelengths) and Spitzer (infrared). They found that all wavelengths showed consistent dimming. However, the rate of dimming was different in different wavelengths. Meng concludes that the most likely explanation for this is microscopic particles orbiting the star (i.e. a circumstellar disc). This accounts for both the star’s long-term fading and its drastic dimming events – but a solid conclusion still needs more research, accurate prediction and observing what’s actually happening. Only then can we fully understand this unique star.
There’s clearly something mysterious going on. We don’t know what it is or how it works and we’re as close to figuring it out as we were in 2009. But when we do – as we think we will – we stand to learn something new for sure. Humankind has never encountered a star as baffling as Tabby’s. As Boyajian put it, “What will it mean if we find another star like this? More importantly, what will it mean if we don’t?”
Note: This article earlier stated that in 985 BC, the Mauryas were ruling India. That was incorrect and the mistake has been fixed.