Astronomers found a star that dims so erratically, they have no explanation for it

Well, friends, we have another dimming star mystery on our hands, and it’s unlike any dimming star we’ve seen yet.

It’s called EPIC 249706694, a binary star located around 350 light-years away, and astronomers have identified 28 dips in its brightness. While the drops in light are mostly the same, there’s also a problem – the time between them seems totally random.

Astronomers led by MIT’s Kavli Institute for Astrophysics and Space Research have tested every scenario that could be plausible, and have come up blank so far.

EPIC 249706694 was studied in the second run of the Kepler planet-hunting telescope’s data collection mission. If you’ve been following along, you know how Kepler’s “transit method” works.

The telescope stares at a patch of the sky, recording the brightness levels of stars in its field of view. If any of the stars dip in said brightness, that’s a hint an exoplanet could be passing between us and the star.

But just a dip isn’t enough. It also has to have something called periodicity – that is, several dips, all of the same brightness, and all occurring at equal time intervals. This periodicity would indicate that the potential planet is travelling on a regular orbit.

Astronomers have discovered other weirdly dimming stars before. The infamous ‘alien megastructure’ star, KIC 8462852, has neither regular brightness nor periodicity, but its observed behaviour still leaves room for several explanations, including a comet swarm and a giant dust cloud.

Meanwhile, the light curve of EPIC 204376071, which has shown radical dips in brightness, does fit other observations of transits, even if the incredible depth of its dimming remains unexplained.

But there’s not a lot in our current understanding of the Universe that accounts for a star with dips with the same light curve, but no periodicity. Which means EPIC 249706694 is in a league of its own.

During its 15th campaign, the Kepler space telescope collected 87 days worth of observations of the star. During that time, it recorded 28 dips, all but two of which had the same depth – that is, the amount by which the star’s light dimmed.

“The unusual aspect of these dips … is that they exhibit no periodicity, and their arrival times could just as well have been produced by a random number generator,” the researchers wrote in their paper.

“We show that no more than four of the events can be part of a periodic sequence.”

According to the team’s calculations, the size of the object that would produce this particular dimming would have to be around twice the size of Earth for one star, and around Jupiter-sized for the other (remember, EPIC 249706694 is a binary star).

It could be a number of objects, but to have that many different planets producing the same light depth would be an extraordinary coincidence, even if the lack of periodicity hadn’t ruled it out. Even dust-emitting or disintegrating planets would have periodicity.

Another avenue the team explored was the presence of dust-emitting asteroids. These could produce random time intervals – but, again, an incredible coincidence would be required to produce uniform dips in brightness.

Planets orbiting an entire binary could potentially reduce periodicity, since the stars themselves are moving, and therefore not every planetary orbit would produce a transit. But after careful modelling, the team could not produce a simulation that resulted in more than five of the observed dips.

It’s also not likely to be the result of bits of accretion disc orbiting the star; for one, it’s too old to have that much accretion disc left, and the light curves are closer to those of planets than clumps of dust. And, again, you’d probably expect to see periodicity.

As the team noted, “all transit scenarios that we have been able to conjure up appear to fail.”

It’s possible that there is something about the star itself that causes the light to dim – perhaps short-lived starspots – but it’s definitely one that’s going to require a lot more scrutiny to figure out.

“The purpose of this paper is largely to bring this enigmatic object to the attention of the larger astrophysics community in the hope that (i) some time on larger telescopes, or ones with high photometric precision, might be devoted to its study, and (ii) some new ideas might be generated to explain the mysterious dips in flux,” the researchers wrote.

So… any ideas?

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

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