Space + Flight

03
Sep

‘Impossible’ Triple Star System Is a Mystery Astronomers Can’t Explain

It should have been a straightforward analysis of a triple star system. The Kepler space telescope observed KIC 2856960 for four straight years, giving astronomers seemingly ample data. But when three astronomers in the United Kingdom started digging into the numbers, they discovered the system was anything but ordinary.

Tom Marsh, David Armstrong, and Phillip Carter first pulled the Kepler data from the server the night of May 21, 2012. They began by looking at signals that hinted at two small dwarf stars that were very close to each other—the smaller appeared to orbit the larger about once every 6 hours. Marsh and team called this the binary. Then there appeared to be a third, the triple, which orbited every 204 days. Other astronomers who had studied the system had come to this conclusion, too.

Gliese 667C system
Astronomers first thought that KIC 2856960 would be a triple star system like Gliese 667C, imagined here in an illustration.

But then Marsh and colleagues started digging deeper. When sifting through the light curve data—the intensity of light over time as recorded by Kepler—they noted periodic dips in intensity. This isn’t unusual in multiple-star systems. Light from one star will dim or change if an orbiting body passes between it and the telescope.

The dips, however, didn’t match with their other calculations. The team felt confident in their understanding of the fast-orbiting, dwarf-star binary, but the appearance of the dip every 204 days was throwing off their results. Based on the dips in the light curves, they couldn’t arrive at realistic stellar masses.

Marsh and colleagues lay the dilemma out plainly in their paper, which was published to arXiv yesterday:

Not only do we not find a precision set of masses and radii for the component stars, we do not find any physically-consistent set of masses and radii that comes close to explaining KIC 2856960’s light curve.

There is one possibility, they note, but it’s so narrow as to be nearly implausible. KIC 2856960 could be a quadruple star system, either where two binaries orbit each other or where one binary is orbited by two independent stars (imagine a solar system-looking arrangement, but with fiery stars). But for a quadruple star system to fit the data, the additional star’s orbit, no matter its configuration, would have to be almost impossibly precise. Either the second binary or the two farther orbiters would have to be in such synchrony as to mimic a single star orbiting the main binary every 204 days.

For now, Marsh and his colleagues are calling this “the impossible triple star.” To decipher the puzzle, more data is needed, they say. Unfortunately, with Kepler handicapped by its broken reaction wheels, it may be some time before that happens. Perhaps the Transiting Exoplanet Survey Satellite, or TESS, will be able to shed some light after it launches in 2017. Until then, the true nature of KIC 2856960 will likely remain an astronomical mystery.