When it comes to the hunt for extraterrestrial life, perhaps the biggest dream we have isn’t to encounter a civilization like our own, but one far more advanced. Rather than seeking radio signals, electromagnetic beacons or chemical signatures, perhaps we could look for something far more advanced than anything we’ve done here. Perhaps looking for large-scale planet or solar system modifications — evidence that aliens had altered an entire world or the space around a star — would be the miracle signal we’ve been longing for. If you ever encountered a case where all the natural explanations you could come up with for your observations failed, the idea of Alien Megastructures might be the ultimate dream come true.
The Kepler spacecraft was remarkable for its ability to observe and monitor the brightness of more than 100,000 stars at once over timescales of multiple years. If a star brightened or dimmed, temporarily or permanently, once or periodically, we could determine all sorts of natural causes. Transiting planets that dipped behind or passed in front of their parent star were revealed; hundreds of eclipsing binary stars were uncovered; intrinsically varying stars were found. Practically all of the variations seen were regular, periodic and predictable. But there’s one star that stands out from all the others: KIC 8462852, known as Tabby’s star for the leader of the team studying it, Tabetha S. Boyajian. Its flux drops off by incredibly large amounts, larger than any conceivable planetary transit could cause, and it does so with no discernible period, with varying drops by varying amounts.
There are a few stars that have been discovered that do something similar, but all of them are part of a class of objects known as YSOs: Young Stellar Objects. The other stars are very young, having not finished forming yet. They all exhibit protoplanetary disks and give off large amounts of infrared radiation. In particular, their light curves — including the shapes of their dips — also differ from that of Tabby’s star. Out of over 100,000 stars imaged by Kepler, Tabby’s star is unique. For one, it has the hallmarks of an older star, one only slightly more massive than our Sun but that’s hundreds of millions years old, rather than just a few million like YSOs. No infrared radiation means no protoplanetary disk around it. Large, ringed planets give the wrong signal shape to block that much light. Colliding, massive planets have been disfavored by follow-up studies looking for warm dust as a result. Even dusty, swarming comets would have infrared radiation, but none beyond what’s localized in the central star system is seen.
And unlike all the other stars, Tabby’s star has controversially been slowly but consistently changing in brightness, with a net dimming, over the past 120 years. When all the seemingly natural, expected explanations fail, whatever is left, no matter how unlikely, must be the truth. Would that mean alien megastructures? Not so fast!
In the past 48 hours, two new, independent studies have come out, each one offering up a possible explanation that might be consistent with all of the data so far. And neither one invokes aliens. The first one theorizes that these unique variations come from dimming “outbursts” originating from the star itself.
The method involves looking, in gory detail, at all the small dips in brightness coming from the star itself as well as the large, aperiodic ones. The smaller dips are also non-periodic, but are thought to represent the underlying activity — likely caused by outbursts that then block the light — from the star. These are frequent, and they occur on all scales: at the 1% level, the 0.1% level, the 0.01% level, etc. By modeling the frequency and intensity of these outbursts, they can construct a mathematical model for the large outbursts, which they dub statistical avalanches. As Mohammed Sheikh, leader of the study, says,
“Once the light curve drops below the threshold, we consider such an event the start of an avalanche. While the light curve remains below the threshold the avalanche continues, and it stops when it increases again to a value above the threshold.”
Their analysis shows that the avalanche model is extremely consistent with what’s observed. In other words, this could just be a star that’s intensely active in some poorly understood way, giving off periodic massive outbursts that cause a dimming of the light. And the fact that stars like this are rare is what’s been fooling us all along.
The second paper hypothesizes a new class of object: a massive object surrounded by a dust cloud. This would sound insane except we already have objects that do this in our Solar System: comets. If they start a long distance from the star, start emitting a tail and a coma, and then transit in front of the star, they could cause these large flux dips. The signal would also be aperiodic. By simply setting up this hypothesis and fiddling with a few free parameters, they can easily fit all four of the largest flux dips fairly well, showing that there is another potential path to solving this problem other than aliens.
It’s only human nature to want the most fantastic of our dreams to come true. To discover an alien species much more advanced than our own would certainly meet that criteria, and would revolutionize how we view our place in the Universe. But before embracing anything along those lines, all the natural explanations need to be ruled out. This includes large-perihelion comets, which could block a large amount of the star’s light in a temporary, transient way, and the statistical avalanche theory, which might hold a clue. As physicist Steinn Sigur∂sson states:
The large brightness variations cannot be explained with simple astrophysical models. Normal middle-aged main sequence stars do not exhibit large spontaneous brightness variations. While other stars have been seen to dim by comparable amounts and durations, they were all much younger—in or near the star-forming phase. [Tabby’s] star would be a one-in-a-million anomaly.
It’s already a one-in-100,000+ anomaly, and everyone agrees that further follow-up observations are mandatory for uncovering the root cause. No matter what the solution, we’re bound to learn something new about the Universe. The revolution is at hand. The only question is whether it will be a revolution for stellar astrophysicists, planet hunters, or the entire human race.