Sky and Telescope - June 2017 - 21

From Kepler's laws (the man, not the spacecraft), we know
that objects in small orbits move quickly, so they would transit the star quickly, too. If an object were on a circular orbit, it
would have to be at least a few times the Earth-Sun distance
from Tabby's star to create a transit lasting a few days. If it
were on an elliptical track, the object may lie farther out than
the orbit of Pluto. At these distances, material would be pretty
cold, so it's possible that it could evade infrared detection.
Objects within the star's system are credible based on what
we know about our solar system, which has a large cloud of
material at these distances called the Kuiper Belt. Lots of
other stars have also been found hosting Kuiper Belt analogs,
some of them considerably more massive than our own.
Alternatively, a large cluster of asteroids might transit
the star. We think Tabby's star could be roughly the same
age as the Sun was during our system's period of Late Heavy
Bombardment, so it's conceivable that a family of asteroids
might have scattered through the young system. Astronomers
have created computer models of asteroid or comet clusters
transiting Tabby's star, and they match some of the short-term
observations reasonably well.
Recently, Brian Metzger (Columbia University) and colleagues proposed that this scenario could be augmented
to explain the long-term dimming, too. If Tabby's star had
recently (within the past 10,000 years) swallowed a planetsized companion, the star may have brightened during a time
period when we weren't watching. The long-term dimming
we've observed might then be a slow return to normal. Remnant clouds of planetary fragments and gas orbiting the star
could cause the shorter dips.
An argument against this scenario's plausibility is that
we'd expect such material to still radiate in the infrared.
Still, we can't quite rule out the possibility. Perhaps there's
just enough material to cause the dips but not enough to be
detected by infrared telescopes.
Thinking farther outside the box, some astronomers
have suggested that material around the star may have a
less natural origin: The dips could indicate the presence of
an alien megastructure, perhaps a Dyson sphere designed to
extract energy from the star. Needless to say, this scenario
has received considerable media attention. Let's investigate it
the same way we investigated the others.
It's workable. An alien megastructure can explain the
observations easily enough - almost too easily, since scientists
can tweak the megastructure model to match the data. Therein
lies the problem with this idea: There are no data on what alien
megastructures "should" look like. However it appears, though,
it's hard to imagine how a giant structure near the star could
avoid heating up and radiating brightly in the infrared.

dust in that space, too. Interstellar dust preferentially scatters
bluer light while letting redder light through. From the shape
of the star's spectrum, we know that about a quarter of the
starlight never makes it to our solar system.
Interstellar dust and gas isn't distributed smoothly
throughout space; there's signiﬁcant structure on every
measured scale, from hundreds of light-years to tens of
astronomical units (a.u.). We don't have a lot of evidence for
structure on smaller scales, if only because our instruments
aren't sensitive enough yet to measure it. So perhaps small
clumps of dusty material are moving along our line of sight,
blocking the light from Tabby's star every now and then. The
star's long-term dimming may in turn be due to the slow
motion of additional interstellar material into our line of
sight, which scatters more and more photons.
This scenario is plausible, but it has its faults, too. For
example, we might expect other stars, especially those at large
distances, to display similar dips. Yet none of the other Kepler
stars displays similar changes.
Still, interstellar dust clouds provide an excellent match to
observations. The timescales of the short dips seem to make
sense: Based on the speeds at which material moves through
our galaxy's disk, we would expect interstellar clumps spanning less than 1 a.u. to transit the star over several days in an
irregular way. This model would predict no periodicity to the
dips, nor any consistency in their depths, just as observed.
The hardest detail for this model to explain is the extreme
depth of some short dips. A 20% decrease in starlight over a
period of days requires a fairly compact clump of dense mate-

N ASA / SDO

Through Deep Space
After leaving the star's system, the photon will encounter
material between the stars. Although this space is a near vacuum, the spectrum of Tabby's star reveals two interstellar gas
clouds that lie between the star and us. The spectrum reveals

p GIANT SUNSPOT This sunspot, imaged on the surface of the Sun
on October 23, 2014, was the largest spot of the current solar cycle,
spanning almost 80,000 miles. Spots on other stars may be bigger still,
but the size depends on the depth of the stellar convective zones.
s k y a n d t e l e s c o p e .c o m

* JUNE 2017

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Sky and Telescope - June 2017

Table of Contents for the Digital Edition of Sky and Telescope - June 2017