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Extraordinary claims require extraordinary evidence, and it is my job, my responsibility as an astronomer, to remind people that the alien hypothesis should always be a last resort. Now I want to tell you a story about that. It involves data from a NASA mission, ordinary people, and one of the most extraordinary stars in our galaxy.
We began in 2009 with the launch of NASA's Kepler Mission. Kepler's main scientific objective was to find planets outside of our solar system. It did this by staring at a single field in the sky, this one with all the tiny boxes, and in this one field it monitored the brightness of over 150,000 stars continuously for four years, taking a data point every 30 minutes. It was looking for what astronomers call a transit. This is when a planet's orbit is aligned in our line of sight just so that the planet crosses in front of a star, and when this happens it blocks out a tiny bit of starlight, which you can see as a dip in this curve. The team at NASA had developed very sophisticated computers to search for transits in all the Kepler data. At the same time, with the first data release, astronomers at Yale were wondering an interesting thing: what if computers miss something? So we launched the citizen science project called Planet Hunters to have people look at the same data. The human brain has an amazing ability for pattern recognition, sometimes even better than a computer.
However, there was a lot of skepticism around this. My colleague Deborah Fischer, founder of the Planet Hunters project, said that people of the time were saying, "You're crazy, there's no way that a computer will miss a signal." And so it was the classic human versus machine gamble, and if we found one planet we would be thrilled. When I joined the team four years ago, we had already found a couple, and today, with the help of over 300,000 science enthusiasts, we have found dozens, and we've also found one of the most mysterious stars in our galaxy.
To understand this, let me show you what a normal transit in Kepler data looks like. On this graph on the left-hand side you have the amount of light, and on the bottom is time. The white line is the light just from the star, what astronomers call a light curve. Now, when a planet transits a star, it blocks out a bit of this light, and the depth of this transit reflects the size of the object itself. For example, let's take Jupiter. Planets don't get much bigger than Jupiter, and Jupiter will make a 1% drop in a star's brightness. Earth, on the other hand, is 11 times smaller than Jupiter, and its signal is barely visible in the data.
So, back to our mystery. A few years ago, Planet Hunters were sifting through data looking for transits, and they spotted a mysterious signal coming from the star KIC 8462852. The observations in May of 2009 were the first they spotted, and they started talking about this in the discussion forums. They said an object like Jupiter will make a drop like this in the star's light, but they were also saying it was giant. You see, transits normally only last for a few hours, and this one lasted for almost a week. They were also saying that it looked asymmetric, meaning that instead of the clean U-shaped dip that we saw with Jupiter, it had the strange slope that you can see on the left side. This seemed to indicate that whatever was getting in the way and blocking the starlight was not circular like a planet.
There were a few more dips that happened, but for a couple of years it was pretty quiet. Then, in March of 2011, we see this: the star's light drops by a whole 15%. This is huge compared to a planet, which would only make a 1% drop. We describe this feature as both smooth and clean. It is also asymmetric, having a gradual dimming that lasts almost a week, and then it snaps right back up to normal in just a matter of days. Again, after this not much happens until February of 2013, when things just start to get really crazy. There is a huge complex of dips in the light curve that appear, and they last for about a hundred days, all the way up into the end of the Kepler mission. These dips have variable shapes: some are very sharp and some are broad. They also have variable durations: some last just for a day or two, and some for more than a week. There are also up-and-down trends within some of these dips, almost like several independent events were superimposed on top of each other. At this time, the star drops in its brightness over 20%. This means that whatever is blocking its light has an area of over a thousand times the area of our planet Earth. This is truly remarkable.
When the citizen scientists saw this, they notified the science team that they had found something weird enough that it might be worth following up. When the science team looked at it, we were like, "Yeah, there's probably just something wrong with the data." But we looked really, really, really hard, and the data were good. So what was happening had to be astrophysical, meaning that something in space was getting in the way and blocking starlight. At this point we set out to learn everything we could about the star to see if we could find any clues to what was going on. The citizen scientists who helped us make this discovery joined along for the ride, watching science in action firsthand.
First, somebody said, "Well, what if this star was actually very young and it still had the cloud of material it was born from surrounding it?" Then somebody else said, "What if the star had already formed planets, and two of these planets had collided, similar to the Earth-Moon forming event?" Both of these theories could explain part of the data, but the difficulties were that the star showed no signs of being young, and there was no glow from any of the material that would be heated up by the star's light. You would expect this if the star was young or if there was a collision and a lot of dust was produced. Somebody else said, "Well, how about a huge swarm of comets passing by the star in a very elliptical orbit?" It turns out this is actually consistent with our observations, but I agree it does feel a little contrived. You see, it would take hundreds of comets to reproduce what we're observing, and these are only the comets that happen to pass between us and the star. So in reality we're talking thousands to tens of thousands of comets. But of all the bad ideas we had, this one was the best, and so we went ahead and published our findings.
Let me tell you, this was one of the hardest papers I ever wrote. Scientists are meant to publish results, and this situation was far from that, so we decided to give it a catchy title. We called it "Where's the Flux?" I will let you work out the acronym.
This isn't the end of the story. Around the same time I was writing this paper, I met with a colleague of mine, Jason Wright, who was also writing a paper on Kepler data. He was saying that with Kepler's extreme precision it could actually detect alien megastructures around stars, but it hadn't. Then I showed them this weird data that our citizen scientists had found, and he said to me, "Oh crap, Tabby, now I have to rewrite my paper." So yes, the natural explanations were weak, and we were curious. We had to find a way to rule out aliens. Together we convinced a colleague of ours who works on SETI, the search for extraterrestrial intelligence, that this would be an extraordinary target to pursue. We wrote a proposal to observe the star with the world's largest radio telescope at the Green Bank Observatory. A couple of months later, news of this proposal leaked to the press, and now there are thousands of articles, over 10,000 articles, on this star alone. If you search Google Images, this is what you'll find.
Now you may be wondering, "Okay Tabby, how do aliens actually explain this light curve?" Imagine a civilization much more advanced than our own. In this hypothetical circumstance, the civilization would have exhausted the energy supply of their home planet, so where could they get more energy? Well, they have a host star, just like we have the Sun, and if they were able to capture more energy from this star, that would solve their energy needs. So they would go and build huge structures, these giant megastructures like ginormous solar panels, called Dyson spheres. The image above shows lots of artists' impressions of Dyson spheres. It's really hard to provide perspective on the vastness of these things, but you can think of it this way: the Earth-Moon distance is a quarter of a million miles, and the simplest element of one of these structures is 100 times that size. They're enormous. Now imagine one of these structures in motion around the star. You can see how it would produce anomalies in the data, such as uneven, unnatural-looking dips. But even alien megastructures cannot defy the laws of physics. Anything that uses a lot of energy is going to produce heat, and we don't observe this, but it could be something as simple as them radiating it away in another direction, just not at Earth.
Another idea, one of my personal favorites, is that we just witnessed an interplanetary space battle and the catastrophic destruction of a planet. I admit this would produce a lot of dust that we don't observe, but if we're already invoking aliens in this explanation, then who's to say they didn't efficiently clean up all this mess for recycling purposes? You can see how this quickly captures your imagination.
Well, there you have it. We're in a situation that could turn out to be a natural phenomenon we don't understand, or an alien technology we don't understand. Personally, as a scientist, my money is on the natural explanation, but don't get me wrong, I do think it would be awesome to find aliens. Either way, there is something new and really interesting to discover. So what happens next? We need to continue to observe the star to learn more about what's happening. But professional astronomers like me have limited resources for this kind of thing, and Kepler has moved on to a different mission. I'm happy to say that once again, citizen scientists have come in and saved the day. This time, amateur astronomers with their backyard telescopes stepped up immediately and started observing the star nightly at their own facilities, and I am so excited to see what they find. What's amazing to me is that this star would never have been found by computers, because we just weren't looking for something like this. What's more exciting is that there's more data to come. There are new missions coming up that are observing millions more stars all over the sky, and just think: what will it mean when we find another star like this, and what will it mean if we don't?
Thank you.