From Dyson swarm to comet apocalypse—it's been 10 years since the discovery of Tabby’s
star and we finally have a leading contender to explaining the strangest known star in the galaxy.
The galaxy swarms with worlds. Almost every one of its hundreds of billions of stars boasts a
system of multiple planets. Some may be much like the Earth. We know this because the Kepler
mission has clocked the miniscule but extremely regular darkening of thousands of stars due
to their planets passing in front of them. It’s the regularity of these transits that
tells us that the shadow is caused by the repeated orbit of a planet.
So what do we make of a light curve like t his? You’ve probably seen this before—it’s the light
curve of KIC 8462852—better known as Tabby's Star after its discoverer,
Tabitha Boyajian. These dips are not due to planets. They’re not regular in either
repetition or depth. Sometimes up to 20% of the star’s light is somehow blackened.
For a while, Tabby's Star became one of astronomy's greatest mysteries. What could
possibly be causing these shadows? It inspired explanations ranging from swarms of comets to
giant ring systems—and even a brief fascination with the possibility of alien engineering.
The galaxy’s strangest star dropped out of th e headlines in the decade since, but astronomers
continued to study it intensely. Observation after observation, some hypotheses were ruled out and
others bolstered. And just last month a new paper came out that may have found a hidden object in
orbit around Tabby’s star. Not the thing making the shadows. The thing making the shadow-makers.
The Kepler mission worked so well because planets are supremely predictable,
and so are the shadows they cast. They’re very round objects following pretty round orbits.
We see tiny but consistent dips in their host star’s light exactly once per orbit when they
pass in front — for the small fraction of stars whose planetary systems happen to be alined to
allow such a transit from our perspective. The time between dips gives the orbital period,
the duration of the dip gives the speed of motion, the depth of the dip and the duration of the dip’s
onset gives the size of the planet; and together this is enough to pretty thoroughly characterize
the world. All that from its shadows—its very clean and distinctive shadows. But shadows over
Tabby’s star are far from clean and distinctive, and are certainly not from simple planets.
Now Kepler also worked really well because it’s supremely specialized. A camera designed
to detect the miniscule dimming of stars out to a few thousand light years, with a field of view to
monitor over 150,000 stars over its 5 year primary mission. But specialization comes at a cost—and
one of the costs was that its pictures are all black and white. More technically,
it had a single filter and so had no way to collect spectral or colour information.
That’s OK, because when a planet passes in front of a star, it blocks all light equally
and spectral information isn’t that useful. A planet is as opaque to blue light as it is
to red light. We call such colour-independent dimming “achromatic” or “grey”. The same should
be true of any solid, opaque object causing the shadow. That includes a planetary ring system,
whose icy particles are large enough to block all light they intercept, and certainly most
technological structures that we can imagine. But not everything casts a grey shadow. Some
“absorption” is chromatic—the darkening depends on the wavelength of the light. For example, Earth’s
atmosphere blocks short wavelength light more easily—blue more easily than green more easily
than red. That scattered blue light gives us our blue daytime sky, and the increased path through
the atmosphere during sunset increasingly saps shorter wavelengths, reddening the setting sun.
We also see this effect out there in the universe. The most common type of chromatic absorption is
due to tiny particulates—heavy elements blasted out by dying stars that have congealed into tiny
grains. In other words, by dust. Obscuration by dust was another leading contender for the
shadows over Tabby’s star, and here at last we have a real way to test a hypothesis. Planets,
giant ring systems, Dyson swarms, should produce gray absorption. Dust
should produce chromatic absorption . In 2018 the Boyajian team published new
observations of the star, in which they showed that the dimming is indeed stronger at shorter
wavelengths, just as you’d expect from giant clouds of dust passing in front of the star.
Yep, dust. After years of speculation, you’d be forgiven if you were disappointed at the
apparent ordinaryness of this explanation. But there’s a reason we never, ever see
dust absorption around stars like Tabby’s. In fact, astronomers were as perplexed as ever.
That’s because dust can never just sit there near an active star—the pressure of the stellar
radiation will quickly sweep it away or destroy the tiny grains. Now we do see thick dust around
some stars. Very young stars are often surrounded by leftover material from their formation. But
that dust is quickly blasted away early in the star’s life. And very old stars can eject dust in
their final phases. But that dust only persists because it’s constantly being replenished.
Tabby’s star is neither young nor old. It’s a regular main-sequence star about 40% more massive
than the Sun, and is in the prime of its life. The leftover dust of its birth should be long gone,
and it isn’t producing dust itself. And yet it's been showing these strange,
varying dust shadows for years. So where does the stuff come from? That’s now the
mystery. The dust must be replenished from somewhere. But from where, and how?
Let’s return to good ol’ Sol for some inspiration. Our solar system does have a source of new dust.
From comets. Way out beyond Neptune are debris fields that stretch a third of the way to the
nearest stars. There, frozen chunks of material leftover from the Sun’s formation drift through
the darkness on vast orbits that can take millions of years. But, now and then, one of these objects
will find its way to the inner solar system. There it becomes a comet. It begins to melt in the
increasing solar radiation and sheds a long tail of water vapor and … guess what … dust. When it
was trapped in ice, that dust had been protected from radiation, but once freed it forms a streak
of material—the cometary tail—that persists only until it’s dispersed by solar radiation.
This happens only rarely. Objects out in the vast Oort cloud can be gravitationally nudged
by passing stars to fall into the solar system. Those in the more nearby Kuiper
belt are slowly nudged until they have close encounters with gas giants that can smack them
further inwards. On their new elliptical orbits, these comets zip through the inner solar system
and then spend between decades and gigayears in the outer reaches. It’s very rare to find
more than one comet in the inner solar system. So if we want to explain Tabby’s star’s situation,
we need a lot more comets than this. Swarms of them. We need some mechanism that’s constantly
flinging comets at the star. Now maybe alien spacecraft are peppering the inner
planets to soften them up for an invasion, or maybe to seed the barren planets with enough
water to begin terraforming. But—much more likely— is there's a massive, invisible body
that’s causing gravitational disruption on a much larger scale than we see in our system.
This idea of a hidden gravitational disruptor is reasonably convincing,
and the idea has been around for a while, however a dark mass orbiting a distant star is next to
impossible to see. So how do we test this? In 2019, the Transiting Exoplanet Survey
Satellite—TESS__watched Tabby’s star for 55 days to study the strange, irregular dips. It happened
to record a single, isolated, and unusually symmetric dip during that period. At the time,
it was thought to be a slightly odd dust dip. But over the following few years,
researchers failed to produce convincing models that could explain the dip in terms of dust.
Then, just this year, a team of researchers led by Cristina Madurga-Favieres put forward
a new proposal. Perhaps this dip is not dust but, rather, the dust maker. Perhaps it was exactly
the type of planetary transit event that Kepler and TESS were built to discover—in this case,
the massive body hypothesized to be flinging comets at Tabby’s star.
Let’s take a closer look at the intriguing shadow. It blocks only about one percent of
the star’s light, is just over 20 hours long, and seems highly symmetric. This
is what planetary transits look like. In fact, had Kepler first seen a simple dip like this,
rather than these crazy ones, researchers would have immediately guessed “planet”. It was only
in the context of the strange dimming that this wasn’t immediately flagged as a likely planet.
But when we learned that the crazy dimming is probably due to dust,
the existance of a gravitational disruptor to rain comets down on Tabby’s star became a prediction.
Which is why this dip g ot a second look. The challenge now is that we can’t confirm
that a planet caused this shadow. At least not yet. Normally, to confirm a planet via
the transit method we need to see three transits. Two full revolutions since the first one. If those
two transit separations take the same amount of time, then it’s a smoking gun for an orbiting
planet. Here we only have one transit, so it’s only tentatively a planet until two more cycles.
But not content with just waiting—potentially years—to see if the dip repeated, the team looked
backwards in time at old data from Kepler, TESS, and a suite of other monitoring programs for signs
of past dips. They found none consistent with a planet, but because of that they were able
to rule out many possible orbits. In fact they were left with a few orbital periods clustered
between 1000 and 1300 days as being consistent with no past transit observation. Factoring in
the 20-ish day duration of the transit to get the actual speed, this gives an orbital period
of 3.3 years and an orbital radius of something like 2.5 times Earth’s—placing it between Mars
and Jupiter in our solar system. From the 1.1% dimming it causes, it should be around 70% larger
than Jupiter in size. That’s a huge planet. Earlier we were talking about comets coming
from the Kuiper belt and Oort cloud. This star is 43% more massive than the Sun and probably
has a commensurately larger Oort cloud. Too big to feel the gravity of a planet that’s so close
to the star. However, it’s also possible that this system has a much tighter ring of icy debris
similar to our Kuiper belt. If this super-Jupiter is massive enough, perhaps it disrupted outer
planet formation leaving a rich field of debris for it to throw at the inner system.
But for that it would want to be very massive. And as it happens, the next
stage of this analysis can not only verify the planet’s presence, but also measure its mass.
After transits, the next most productive way to find planets is the radial velocity method. A
planet doesn't actually orbit its star—it orbits its mutual center of mass with its star. And
that star orbits that same mutual center of mass. Because stars are much more massive than planets,
this mutual orbital point is much, much closer to the star—often actually inside
of it. But stars do still move—or at least wobble—in little circles due to each of their
planets. That motion can be detected by watching for the tiny shift in wavelength
of the star’s light due to the doppler effect. Now the team used both old and new spectroscopic
data to find evidence of that wobble. It’s especially difficult for Tabby’s star because
the star itself is rapidly rotating, which causes additional Doppler smearing of its light. However,
the team were able to find a tentative Doppler signature that’s consistent with the orbital
parameters they’d found. And the mod el that best fit the radial velocity
analysis gives , a ridiculously massive planet—10 times that of Jupiter. That’s
encouraging if we want some gravitational beast that’s able to disrupt outer orbits.
The radial velocity detection is quite tentative. The confidence is 2.3 sigma,
which means there’s a 1 in 50 chance that the radial velocity signal was a conspiracy
of random noise. It doesn’t yet qualify as a sure detection, and so more radial velocity
data is needed. But we also need more independent approaches—and we’re about to have the capability
to measure this stellar wobble in a way that was never before possible. Come December this year,
we’ll be able to measure the literal change in position of stars as they move in tiny circles
due to their planets. The Gaia spacecraft was designed to make the most precise position
measurements ever achieved. It operated from 2014 until March this year. And we’ve already learned
an insane amount about the positions and motions of Milky Way stars, but perhaps the most ambitious
and most difficult measurement it will make is of what we call the “astrometric wobble” of a
star due to its planets. And in December, the Gaia team will have completed the highest level of data
processing for the first 5.5 years of the survey and will release precision astrometry for that
period. They expect to discover 1000s of exoplanet systems this way. The Tabby’s star team claim
that its astrometric wobble due to its tentative super-Jupiter will be visible in this data. If so,
the mystery of Tabby’s star will be largely put to rest. The star is being eclipsed by evaporating
swarms of comets, flung inwards by a colossally large planet. And if the planet’s not there? Well,
we keep on thinking, and watching, and wondering about the strangest star in local spacetime.
a time of massive geologic upheaval full of incredible creatures, which all culminates in the
greatest mass extinction in the planet’s history. Check out the link in the description to watch.