Has space junk trashed the alien theory? NASA/JPL-Caltech

Step aside, alien megastructure. It seems that the behaviour of an oddity known as Tabby’s star – which some have speculated may be caused by a massive extraterrestrial construction project – might in reality be explained by interstellar junk.

KIC 8462852, as it is more officially called, was spotted by NASA’s Kepler space telescope, which recently spent four years carefully watching the same patch of sky and looking for any stars that dipped in brightness at regular intervals. These dips, which can be as large as 1 per cent, happen when an exoplanet crosses in front of a star.

But Tabby’s star dimmed randomly, and by as much as 20 per cent, leaving astronomers dumbfounded – and leading to the speculation that an alien megastructure was responsible for the signal.


Now Valeri Makarov at the US Naval Observatory in Washington DC and Alexey Goldin at Teza Technologies in Chicago, Illinois, have taken a closer look at the smallest of the star’s dips in light – only to find that Tabby’s star isn’t responsible for them. Whenever the small dips occur, there is also a slight jump in the position of the light, leading them to conclude that it is actually a different star along the line of sight that is dimming.

Tabby’s star is still responsible for the largest dips in light, however. If more than one star is dimming, say Makarov and Goldin, then the culprit for the erratic behaviour can’t be an asteroid belt, debris from a smash-up between two larger bodies, or an alien megastructure around Tabby’s star. In fact, it can’t be anything around Tabby’s star alone. Instead, it has to be something lying between us and them.

Interstellar comets

The pair speculate that this something may be a swarm of “interstellar comets”. These objects are created in star-forming regions, says Makarov, and when those objects move apart over time, all the stars and comets become scattered.

“So maybe interstellar space is full of free-floating comets,” he says. “But try to find them! They are dark and cold, and travelling from nowhere to nowhere, basically, forever.”

“If this is right, then that conclusion is dead on,” says Jason Wright at Pennsylvania State University, who postulated the alien megastructure in the first place – although he notes that comets are usually defined by their proximity to a star, so calling them “interstellar comets” doesn’t make sense. A second dimming star automatically leads to an interstellar explanation, he says.

Wright himself had lately come to a similar conclusion. The latest evidence shows that not only does Tabby’s star dim sporadically, but it also appears to have been gradually fading for a century. Wright thinks that only an interstellar cloud passing in front of the star could explain such long-term dimming.

But, he concedes, “we don’t know of any such cloud along the line of sight”, and observing one directly will be tricky. Instead, Makarov thinks our best chance is to wait a decade or so until the blended, second star has moved away from Tabby’s star and we can catch the intruder red-handed.

Journal reference: https://arxiv.org/abs/1609.04032