Sky and Telescope - June 2017 - 22

Tabby's Star

rial; it could exist, but we don't know of any such clumps
toward the Kepler ﬁeld.
As in the case of previous scenarios, we can't conﬁrm this
one or rule it out with the available data; we can only say it's
reasonable and ﬁts the observations.

Into the Solar System
After 1,300 years of traveling through interstellar space, our
photon reaches the outer edges of our solar system. What if it
came across a cloud of material, perhaps originating from a
recent collision between a few comets?
This interesting idea isn't a great ﬁt to the data, nor is it as
likely as some of the other ideas. Tabby's star shines at a declination far above the ecliptic plane, where planets and most
comets live, and we don't have any evidence for comet collision
clouds in the outer solar system so far from the ecliptic.
Also, thanks to Kepler's orbital motion, any nearby object
in the solar system would appear to move quickly across the
sky relative to more distant stars, so it would have to be huge
(spanning an a.u.) to block Tabby's star throughout the year.
Any such object ought to affect other Kepler stars as well, and
it doesn't, making this scenario unlikely.
22

J U N E 2 0 1 7 * SK Y & TELESCOPE

Finally, our photon reaches the end of its journey, landing
on Kepler's CCD detector and marking its presence by exciting an electron. It's possible that some artiﬁcial effect in the
instrument caused both the short-term dips and long-term
dimming. Such an idea isn't completely implausible; instrumental defects do happen. For every planet that makes it into
the Kepler catalog, the automated planet-ﬁnding pipeline
identiﬁes 10 instrumental artifacts. Such defects might alter a
single grouping of pixels; however, Kepler rotates four times by
90° during each orbit around the Sun, in order to keep its solar
panels pointed sunward, and we observe Tabby's star's strange
behavior across multiple orientations of the telescope. So we
can all but rule out instrumental effects.
By the time a photon has landed on Kepler's detector, it has
spent more than 100,000 years working through the insides
of Tabby's star and traveled another 1,300 years through
interstellar space. What conclusions has it led us to? The most
likely scenario, it seems, is that a cloud of interstellar or circumstellar material crossed between Tabby's star and Earth.
It's the photons that we observe in the future that will
separate the chaff from the wheat.

Let the Experiment Begin
Our favorite scenario, interstellar material crossing between
us and Tabby's star, naturally explains both the short-term
dips and the long-term dimming. It makes testable predictions, too. First, the overall brightness of the star should
continue to change. Because interstellar material preferentially absorbs bluer photons, we would expect that the rate of

DICK LOCK E

p THE DUST BETWEEN Evidence exists for structure within interstellar
dust and gas on every scale that we can measure. This image, centered
on the Snake (Barnard 72) in Ophiuchus, shows dense concentrations of
dust and gas that block out the light from background stars. The Snake
is a few light-years across. The concentrations required to explain the
brightness variations of Tabby's star would have to be much smaller, less
than 1/10,000 of a light-year.

Sky and Telescope - June 2017

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