A few years ago, astronomers noticed a star that was dimming in a strange pattern, leading to speculation that an "alien megastructure" might be orbiting it. More plausible explanations were later put forward, with the most likely culprit being a cluster of comets. But now scientists have discovered an even weirder star system that appears to be dimming completely at random, and none of the usual explanations seem to fit.

EPIC 249706694 (or HD 139139) is a binary system, meaning it's composed of two stars gravitationally locked together. During the second Kepler mission, the system was observed for 87 days straight, and in that time its light was found to dim 28 times. Normally, these kinds of dips in light indicate that a planet is passing in front of the star, but if so the dimming usually follows a strict schedule – for example, aliens looking at the Sun from the right angle would see the Earth block out its light once every 365 days, like clockwork.

But in this case, the dips had no regularity to them at all. The international team describing the star system calls it "The Random Transiter," quipping that "their arrival times could just as well have been produced by a random number generator." The level of dimming was also seen to be about the same each time.

If this sounds a bit familiar, that might be because of the similar story of "Tabby's Star," whose irregular dimming grabbed headlines in 2015. One particularly intriguing study suggested that these dips fit the pattern that might be expected from an "alien megastructure" orbiting the star and harvesting its energy. As fun as that would be to believe, the more likely explanation is that a swarm of comets or asteroids is orbiting the star, periodically blocking out some of the starlight.

But EPIC 249706694 is even more fascinating. The team examined nine common scenarios that might normally explain irregular dimming, and found that none of them fit very comfortably.

The most common-sense idea is that the system is home to a series of planets, and the dips were these worlds passing in front of the star at different times. But the team found that this could only reasonably account for less than half the dips. For example, if a planet orbited the star every 20 days, it could account for four of the dimming events in that 87-day period. But in a normal system, that means that the next planet out from the star could only make three passes in that time.

To explain all the dips, there would have to be 19 planets around the star – far more than any known system – and they would all have to be close enough to have "years" of less than 90 days. Throw in that they'd also all have to be roughly the same size, and it's just not plausible.

Similarly, the team also ruled out other popular theories, such as a disintegrating planet, dusty asteroids, planets orbiting both stars, large "spots" on the stars as seen on our Sun, or that they naturally dim and brighten for other reasons.

That leaves the astronomers stumped as to what could be causing the dips. Before anybody jumps to the age-old aliens answer, the team is asking other astronomers to point different instruments at the strange star system to see if they can figure it out.

The research was published in the journal Monthly Notices of the Royal Astronomical Society.

Source: ArXiv (PDF)