Exiled exomoon may be dimming the "alien megastructure" star
Tabby’s Star is one of the weirdest stars ever found. It flickers and fades over a few days, before flaring back up to its usual brightness in a completely unpredictable pattern. Now astronomers from Columbia University have a new theory – and no, it’s not an alien megastructure.
Tabby’s Star, officially known as KIC 8462852 and unofficially as the “alien megastructure” star, has been observed for well over a century, but only started grabbing headlines in 2015. That was when astrophysicist Tabetha Boyajian noticed its irregular dimming, sometimes as much as 22 percent. This could last days or weeks before clearing, with more dimming episodes following later at random intervals.
Most of the time, when astronomers see stars dim, it’s because a planet is passing in front of it. This is the main method by which exoplanets are discovered – but of course, precise orbits mean those dimming events are very predictable. That’s not the case with Tabby’s Star, meaning a planet isn’t to blame.
Instead, explanations suggested by astronomers include swarms of comets, fragments of shattered planets, problems with observatory equipment, or an alien megastructure encompassing the star to drain its resources. As fun as that last one would be, it’s pretty unlikely.
The Columbia team’s new theory is a bit more down-to-Earth. According to the researchers, the star may have captured a moon that was orbiting one of its planets. After the exoplanet was destroyed, this exomoon could have been pulled into a new orbit around the star, turning it into a hypothesized object called a “ploonet.”
Now closer to the star, this icy, dusty ex-exomoon would have started receiving more radiation than usual, melting away its outer layers. Those layers would be shed to space, leaving a trail behind it like a comet. Eventually the larger chunks and smaller grains could form a ring around the star, which would block light at sporadic intervals and to different depths.
“The exomoon is like a comet of ice that is evaporating and spewing off these rocks into space,” says Brian Metzger, principal investigator on the study. “Eventually the exomoon will completely evaporate, but it will take millions of years for the moon to be melted and consumed by the star. We’re so lucky to see this evaporation event happen.”
The researchers ran simulations of systems with three or four bodies in them, and found that the scenario is possible in as many as 10 percent of cases where an exoplanet is destroyed.
Although the story neatly explains away many of the weird observations around Tabby’s Star, the debate is far from settled. After all, we haven’t yet even directly detected any exomoons at all yet, although our solar system tells us that they should be more common than exoplanets – and we’ve found more than 4,000 of those.
The next step would be to find more evidence of exomoons, and hopefully, ones that have been stolen by their host stars but not yet destroyed.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.
Source: Columbia University