Space

There’s a new explanation for mysterious Tabby’s star: a melting ploonet

We’re yet to make a conclusive detection of an exomoon. But if such moons are out there, orbiting planets outside the Solar System, one of them could be responsible for the peculiarities of KIC 8462852 – AKA Tabby’s star.

New research suggests that the strange brightening and dimming fluctuations of the star’s light that have been observed for years (and back-traced from archival data) could be the result of a disintegrating exomoon in orbit around the star.

Such a wayward moon – recently nicknamed a ploonet – would be shedding dust and chunks of rock that move between us and Tabby’s Star in a coalescing disc.

Tabby’s star, a yellow-white dwarf star located around 1,280 light-years away, was discovered in 2015, and since then it’s been a real head-scratcher. Its dimming is completely random. The depth of the dimming varies, too – it’s dimmed by up to 22 percent, and last year was caught dimming by just 5 percent.

This behaviour pretty much rules out planets; when an exoplanet passes between a star and Earth as it orbits, it will dim the star by a tiny amount – 1 percent or less – at regular intervals.

In addition, the star’s overall brightness seems to be fading over time; between 1890 and 1989, archival data revealed, it faded by 0.193 magnitude.

Follow-up observations have revealed that some wavelengths are blocked more than others, which wouldn’t be the case with an opaque solid object (like, for instance, an alien megastructure).

The star is thought to be too old for any remnants of a stellar accretion disc to still be orbiting; in any case, analysis has ruled out a high abundance of close material orbiting or falling into the star.

Some sort of dust or a swarm of comets that absorbs some wavelengths more effectively seems to be the most plausible explanation, but it would have to be an insane amount of dust or comets.

Where could that material come from? According to researchers at Columbia University, there could be a fitting answer: an orphaned exomoon, separated from its planet and slowly melting, shedding dust and debris that ends up orbiting the star in a clumpy cloud.

“The exomoon is like a comet of ice that is evaporating and spewing off these rocks into space,” said astrophysicist Brian Metzger of 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.”

Ploonets are one hypothetical outcome when an exomoon gets separated from the exoplanet it orbits. Earlier this year, scientists described a scenario in which these exomoons get kicked off orbit around gas giants by gravitational interactions as the exoplanet migrates inwards towards the star.

Some of these exomoons would crash into the star, or the planet, or get kicked out of the star system entirely; but in their scenario, just under half ended up in orbit around the star. These are the ploonets.

According to Metzger’s team’s model, ploonets could also be the result  of an exoplanet being destroyed by an interaction with the star. Around 90 percent of the time, this would destroy the exomoon; but 10 percent of the time, it survives to orbit the star.

This leaves the ploonet vulnerable to the star’s destructive radiation, the team inferred. This radiation would strip away the ploonet’s outer layers, sending dust clouds out into stellar orbit. This could explain the intermittent and irregular dimming.

And, if the material was slowly being spooled and evened out into a disc orbiting the star – like clumps of wool being spun into even yarn – this could explain the long-term fading, too.

Although other stars with unusual dimming patterns have been discovered, none of them have been quite like Tabby’s star. If we do happen to spot another star behaving similarly, it could help verify their hypothesis, the researchers said.

“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.”

The research has been published in the Monthly Notices of the Royal Astronomical Society.

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