Sky and Telescope - June 2017 - 23

JOHN M A RTINE Z / CC 0 1.0

* Find more information on AAVSO's campaign to observe
Tabby's star here: https://is.gd/AAVSOTabbysStar.

change would be different at different wavelengths. The scattering of blue photons would also create different shapes for
the short-term dips when measured at various wavelengths, a
prediction that we can test if another one happens.
Finally, we would expect the always-present spectral lines,
created when interstellar material absorbs the star's light, to
change over time. While the lines would look obviously different during a dip, they also ought to change slowly as the
long-term dimming continues or reverses.
But interstellar material isn't the only viable scenario. At
this point we can't rule out clouds of material orbiting Tabby's star, nor can we rule out the recent ingestion of a planet
to explain the long-term dimming. Under these conditions,
we would expect the short-term dips to eventually show some
periodic behavior and, like the interstellar material scenario,
they ought to look different at different wavelengths. However, in this case, we wouldn't expect any signiﬁcant changes
in the interstellar absorption lines during or between dips.
If the star truly is brighter than it should be because it
consumed a planet, then we might also expect geometric
(parallax) measurements of the star's distance, as will be
provided by the Gaia mission, to show that the star is farther
away than we think it is.
The remaining scenario under consideration is the one
that we can't easily describe as likely or unlikely: an alien
megastructure. The real test comes down to what this idea
predicts that other ideas do not.
If we were seeing the rotation of a gigantic structure, then
we'd expect the star's dips to appear the same at every wavelength, as opposed to the color-dependent dips of circumstellar or interstellar dust, because the giant, solid structures
would block all of the star's photons. We would also expect
no changes in the shape or strength of the interstellar features observed in the star's spectrum. If any of these turn
out not to be true, then natural hypotheses would provide a
better ﬁt to the data.
On the other hand, the detection of radio signals, laser
pulses, or other signs of alien intelligence from this area of the
sky would support the idea of an extraterrestrial megastructure. So far, though, SETI researchers haven't found anything.
Other groups are looking, too, including the Las Cumbres
Observatory's global telescope network (see box at right),
as well as members of the American Association of Variable
Star Observers, who are patiently watching and waiting for
another dip that may or may not arrive.
Astronomers are also monitoring the shape and strength
of the interstellar absorption features with spectroscopic
observations, looking for telltale changes that would suggest
pockets of denser interstellar material. Future data releases
from the Gaia mission will improve not only our estimate of
the star's distance, but also the measurement of the amount
of obscuring material that lies between us and the star.

Additionally, the Transiting Exoplanet Survey Satellite
(TESS), in many ways Kepler's successor, will search much of
the sky for transiting planets. Scheduled for a March 2018
launch, TESS will observe each ﬁeld, including the Kepler
ﬁeld, for four weeks at a time. If Tabby's star were to dim
during this time, TESS would detect it. Moreover, TESS will
monitor 500,000 nearby stars during its two-year mission,
providing plenty of opportunity to detect analogs to Tabby's
star - if they exist - across the Milky Way.
Within a few years, we may understand what makes this
star's behavior so out of this world.
¢ BENJAMIN MONTET is a NASA Carl Sagan Fellow at the
University of Chicago, where he studies planetary systems and
their host stars.
¢ TABETHA BOYAJIAN is an Assistant Professor of Astronomy
at Louisiana State University.

Las Cumbres Observatory

Tabby's Star: Kickstarter Campaign
º We can make a strong case to continue monitoring KIC
8462852, but actually doing so isn't trivial. For starters,
the Kepler space telescope has moved on to another
mission, and we can no longer use its excellent photometric capabilities to keep an eye on the star. That leaves
us with ground-based observatories, which are limited
by the Sun's daily appearance. For a typical astronomical
case, such a limitation wouldn't be an issue, but we can't
predict when and what this star will do next. We need
around-the-clock coverage so we don't miss a thing.
Fortunately, there's a private facility uniquely suited to our
needs: the Las Cumbres Observatory (S&T: Oct. 2012,
p. 36), a global network of robotic telescopes. To help
acquire the data, we launched a crowd-funding campaign via Kickstarter. Thanks to the public's enthusiastic
response, we raised enough funds to monitor the star for
the next couple of years. We're especially excited to see
what happens in the next several months - stay tuned!
¢ TABETHA BOYAJIAN

s k y a n d t e l e s c o p e .c o m

* JUNE 2017

https://is.gd/AAVSOTabbysStar http://www.skyandtelescope.com

Sky and Telescope - June 2017

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