This mysterious star could be consuming a stolen exomoon

For more than a century, Tabby’s Star has intrigued astronomers with its unusual dimming, sometimes over days or weeks, before returning to its normal brightness. Now, new research suggests that the mysterious star is chowing down on a moon stolen away from a planet.

More than 1,000 light-years away, the star, also known as KIC 8462852, is 50% bigger than our sun and 1,000 degrees hotter. It’s the only star to ever be observed exhibiting this strange behavior.

It’s now largely known as Tabby’s Star, named for Tabetha Boyajian, a Louisiana State University assistant professor of physics and astronomy.

Boyajian has been researching and authoring papers about the star since citizen scientists participating in the Planet Hunters project identified its odd behavior to her science team in 2015. A colleague, Penn State Department of Astronomy and Astrophysics Assistant Professor Jason Wright, called it “Tabby’s Star” in an interview, and the name caught on.

Tabby’s Star brightens and dims

For no obvious reason, Tabby’s Star has been dimming and brightening in strange and unpredictable ways. It has dimmed for a few days or a week at a time. And then there’s the fact that it’s grown fainter over the past century.

Tabby’s is an F star, which is supposed to maintain constant brightness. Its long and short fluctuations in light didn’t make sense.

The logical conclusion: Something had to be blocking it. Suggestions from fellow astronomers included a mass of planets, or even an alien megastructure. Boyajin herself published a research paper last year in Astrophysical Journal Letters suggesting space dust.

“Dust is most likely the reason why the star’s light appears to dim and brighten,” Boyajian said. “The new data shows that different colors of light are being blocked at different intensities. Therefore, whatever is passing between us and the star is not opaque, as would be expected from a planet or alien megastructure.”

But astronomers weren’t sure how the dust was produced or why it was in front of the star.

A new study published Tuesday in the Monthly Notices of the Royal Astronomical Society suggests that there is a thick disk of debris building up and orbiting the star. This would lead to long-term dimming, the researchers said.

The debris is being ripped away from an exomoon, or a moon that once orbited a planet outside of our solar system.

The star also pilfered a moon

Astronomers believe that Tabby’s Star stole an exomoon from a nearby planet that no longer exists and pulled it into orbit. The star’s strong radiation has lashed the exomoon’s outer layer, which is comprised of ice, gas and rock.

Smaller particles create dust clouds that are eventually blown out into space. The large particles go into orbit in a dust disk around the star. Eventually, the particles reach a peak temperature and melt.

This explains both the short and long-term dimming of the star.

But how did the planet lose its moon? Exoplanets can have destructive relationships with their host stars, colliding with them or ultimately being destroyed by an intense interaction. This leaves the exomoon alone and at the mercy of the star. It could be pulled into orbit, ejected from the star system or even collide with the star.

In this case, the exomoon will likely evaporate after millions of years of being stripped down by the star’s radiation.

“The exomoon is like a comet of ice that is evaporating and spewing off these rocks into space,” said Brian Metzger, study author and associate professor of astrophysics at Columbia University. “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.”

Now, the researchers want to find other stars that show unexpected brightness dips because similar situations could be playing out across different solar systems.

That would also likely lead to the confirmation of exomoons. Recent studies have suggested where they might be, but astronomers have not confirmed a discovery of one yet. Because they are small and faint, exomoons are incredibly difficult to detect with current telescopes.

“We don’t really have any evidence that moons exist outside of our solar system, but a moon being thrown off into its host star can’t be that uncommon,” Metzger said. “This is a contribution to the broadening of our knowledge of the exotic happenings in other solar systems that we wouldn’t have known 20 or 30 years ago.”