Back in 2015, astronomers noticed a star that was behaving in a very unusual way. Nicknamed “Tabby’s Star” after astronomer Tabetha Boyajian, the star exhibits strange and unexplained dips in its light output. This led many to speculate that Tabby’s Star was surrounded by some sort of artificial technological structure like a Dyson sphere constructed by an advanced alien civilization. Radio monitoring of Tabby’s Star didn’t turn up any alien chatter or anything else unusual, but it didn’t explain what was going on with the star either. Now astronomers have discovered another star with mysterious dips in its light output (behavior which usually suggests it’s being orbited by something) for which astronomers also have absolutely no explanation.
According to Scientific American, the star was discovered using the now-retired Kepler space telescope, an extremely precise exoplanet hunting space telescope. After analyzing Kepler’s data on the star, called HD 139139 (it doesn’t have a fancy name like Tabby’s Star yet), for more than a year, astronomers have no explanation for what might be causing its anomalous behavior. Andrew Vanderburg, an astronomer at the University of Texas at Austin says:
“We’ve never seen anything like this in Kepler, and Kepler’s looked at 500,000 stars.”
As an exoplanet hunting telescope, Kepler’s job was to look for periodic dips in brightness coming from stars. These dips in brightness usually indicate orbiting planets as they pass in front of the star. Data from these observations are fed through algorithms that look for repeated patterns, a good indicator exoplanets considering they should be spinning around the star in a consistent pattern. Some of the patterns though are too complex for the algorithms to handle, and are instead placed in the hands of volunteer civilian astronomers. It was one of these astronomers contacted Vanderburg and told him to check out HD 139139, a star the size of the sun about 350 light-years away.
So what exactly makes this star so strange? According to Vanderburg:
“When I got that e-mail, I looked more closely and said, ‘okay, this definitely looks like a multiplanet system. But I can’t find any [planets] that appear to line up.'”
The dips in light make the star seem like it should be a multiplanet system, but it can’t be. The star displayed 28 dips in light, each of which lasted between 45 minutes and 7.5 hours with no repeats. The team says it looked more like noise than an actual signal. The team thought it may be a glitch in the system, but after checking again, the data turned out to be real.
One explanation was that this star had at least 14 planets and perhaps as many as 28—more planets than any other known system. That would in itself be very strange, but in order for even that explanation to work all of those exoplanets would have to be the same size, only slightly larger than Earth.
Yet there’s another problem with that explanation, according to Vanderburg. based on the short duration of the light dips, the planets would need to be in extremely close orbit to the star. But that doesn’t match the lack of a repeated pattern in the light dips. For there to be no repeats in the 80 days the telescope was pointed at the star is essentially impossible if the planets were as close to the star as they would need to be. So it’s not planets.
Among the other proposed explanations are a second, invisible source of gravity pulling on the star, the dust cloud of a planet disintegrating in front of the star, and short-lived starspots (cooler versions of our suns sunspots that have never been observed). For various reasons, none of these explanations hold up either.
Vanderburg says there was one explanation left out of the recent paper he published on the star: an alien megastructure. Vanderburg says that explanation is extremely unlikely (which is the least surprising thing here), but astronomer Jason Wright, who led the investigation into Tabby’s Star, says HD 139139 will definitely be added to the list of places being investigated for alien technosignatures.
It’s unlikely any further investigations will happen for a while, though. The Kepler space telescope has been decommissioned and there are few other telescopes that can match its precision. So until another telescope is pointed at this mysterious star, all we’re left with is wild speculation. And the experts that are running the numbers and actually have the know-how to comment on it, of course.