Astronomers have now identified nearly 5000 extrasolar planets—planets which orbit stars
outside of our solar system—with thousands more candidates awaiting confirmation.
We are finding so many new exoplanets that astronomers now estimate that there is probably
at least one planet for every star in the galaxy.
Exoplanets may be fairly common in the universe, but astronomers recently found something in
orbit around a distant star that is truly baffling.
Whatever it is, this new extrasolar discovery has so far defied any explanation.
Welcome to Matter.
What exactly have astronomers found in orbit around this star?
And is it actually evidence of an advanced alien civilization?
Most of the time, finding exoplanets is a very slow, painstaking process.
Astronomers will sift through countless gigabytes of data, measuring the observed luminosity
of stars in the sky over time.
If they’re lucky, they’ll see one of these stars suddenly drop in brightness, and then
go back to normal a short while later.
This brief dip in luminosity is a hint that there is something else—like a planet—that
is blocking out some of the light from the star.
This isn’t always enough to confirm the existence of a planet, though.
Sometimes the dip is too small to accurately see through the noise.
Errors in the data, anomalous activity within the star itself, and a number of other factors
can also lead to false positives and inaccurate detections.
Because of this, astronomers usually like to wait for the planet candidate to complete
at least one more orbit, so that they have multiple transit events to work with.
Observing multiple transits of the same planet increases the chances of actually confirming
its existence and allows the scientists to make more accurate calculations of the planet’s
physical and orbital parameters.
To date, around 80% of all extrasolar planet discoveries have been made using this transit
method.
Although it is by far the most effective method for exoplanet detection that we have right
now, it isn’t easy.
For most extrasolar transit events, the parent star is only observed to dim by a tiny fraction
of a percent.
Extremely sensitive telescopes and equipment are required, or the transit won’t be seen
at all.
But in 2015, astronomers were sorting through data from the now-defunct Kepler Space Telescope.
What they found was something extraordinary indeed.
Instead of the little 1% blip that is typical of exoplanet transits, on March 5, 2011 the
star KIC 8462852—nicknamed Tabby’s Star—suddenly dropped in brightness by an unprecedented
15%.
In February of 2013, an even larger 22% dip was observed.
A third transit was detected only a month and a half later, which blocked out 8% of
the star’s light.
Normally, subsequent transits of the same exoplanet usually have the same depth and
occur at very predictable times according to the planet’s orbit.
But the transits of Tabby’s Star seem completely random, with no discernible pattern for the
transit depths nor transit timing.
Astronomers tried to predict another deep transit of Tabby’s Star in February of 2015,
but unfortunately the Kepler Space Telescope ceased operating before then—there was no
way to confirm nor refute their prediction.
Tabby’s Star remains truly one of the greatest astronomical mysteries of our time.
So far, the true cause of these sudden drops in brightness is still mostly unexplained.
Whatever it is that is obscuring the light from Tabby’s Star would need to obscure
at least half of the star’s visible disk in order to produce the observed reductions
in brightness.
The only thing that is certain is that it is far too large to be a planet.
One of the most exciting theories—popularized by countless news sources soon after the discovery
was announced—was that the dimming was caused by some sort of enormous megastructure, built
by an extraterrestrial civilization.
It could be something like a Dyson swarm—a structure that is designed to nearly completely
envelop an entire star, thereby being able to exploit a large percentage of the star’s
power output.
The possibility is certainly intriguing—could the dimming of Tabby’s Star be the first
conclusive evidence for an alien civilization?
This question, of course, piqued the interest of the SETI Institute.
They have conducted multiple surveys of the space around Tabby’s Star using the world’s
most powerful radio telescopes.
So far, none of these surveys have revealed any signals that might be attributed to alien
technologies.
While not necessarily dismissing it outright, many astronomers have also treated this theory
with an eye of healthy skepticism, instead favoring more probable explanations.
While Tabby’s Star probably isn’t home to any aliens, it still presents a fascinating
mystery.
Some other theories include a circumstellar ring of dust, a large cloud of comets being
torn apart, or even a planet being consumed by the star.
But none of the theories that astronomers have come up with have been able to entirely
explain everything about the strange transits.
Astronomers diligently kept a close eye on Tabby’s Star for several years, hoping to
catch a glimpse of something that might help them solve the mystery.
In 2017, they finally got lucky.
Dr. Tabby S. Boyajian—the star’s namesake—and her colleagues began to see their star dim
slightly.
They quickly got the word out to their fellow astronomers around the world, and dozens of
observatories were turned to watch the event.
The transit lasted for around three days, and showed a 2% fluctuation of the brightness
of Tabby’s Star.
Observations were made using wavelengths across the electromagnetic spectrum, in an attempt
to gather as much information as possible during the fortuitous event.
Six additional fluctuations were observed over the following year, with the greatest
being at least a 5% drop in brightness.
After analyzing the data, Dr. Boyajian’s team made a significant breakthrough—the
dimming of Tabby’s Star is wavelength-dependent, meaning that whatever it is, it blocks out
certain wavelengths of light and lets some others through.
This does cast even more doubt on the idea that the transits could be caused by an alien
structure.
Dr. Boyajian said that “If a solid, opaque object like a megastructure was passing in
front of the star, it would block out light equally at all colors.
This is contrary to what we observe.”
So, it probably isn’t aliens.
But what is it, exactly?
Nobody is quite sure, but the transits from 2017 and 2018 provided some helpful insight.
The fact that Tabby’s Star dims unequally across different wavelengths is consistent
with space dust.
Tiny dust particles—even smaller than the microscopic particles in cigarette smoke—might
clump together in large clouds surrounding the star.
These clouds would absorb some wavelengths of light while others are scattered through.
However, that doesn’t explain where all that dust comes from.
One additional theory is that the star is orbited by a gas giant planet with exceptionally
large, fluctuating rings.
Another suggests that the planet could be followed in its orbit by a swarm of asteroids.
Spectroscopic analysis of the star shows that there is no material close to the star, and
that all of the dust is very cold and far away.
This likely rules out the theory that the transits are caused by an exploding planet.
More recently, in 2019, a new study posits that the fluctuations of Tabby’s Star might
be caused by orphan exomoons.
The study utilized numerical simulations of the migration of gas giant exoplanets and
their moons shortly after formation.
Approximately 50% of the simulated scenarios show that tidal interactions can eject a newly-formed
exomoon from orbit around its parent planet, the moon becoming a small planet on its own.
If the exomoons are rich in volatile compounds—such as water ice, methane, or ammonia—they would
be dramatically affected by the solar radiation, especially during the chaotic period of their
migration and ejection.
Their sun’s radiation may cause much of the ejected exomoons’ volatile surface material
to sublimate and drift into space, forming a diffuse, gaseous envelope around it.
The simulations showed that these hypothetical orphaned moons and their evaporating surfaces
might produce light signatures similar to those observed with Tabby’s Star.
While the mystery of Tabby’s Star has not yet been definitively solved, astronomers
have found some major clues along the way.
No, it does not seem likely that the strange transits are evidence of an interstellar civilization.
But that doesn’t mean that this isn’t a significant astronomical discovery.
Tabby’s Star is an anomaly, which means there is something about planetary formation
that we don’t understand.
This is an exciting opportunity for astronomers to learn something new about our universe
that we didn’t know before.
Being able to unravel this puzzle will give us a better understanding of how stars, planets,
and moons form, and how they interact and evolve over time.
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