Tabby's Star - The Dark Corners of the Impossible
(Originally posted October 28, 2016 on Blogger)
Starting this week, scientists at the Berkeley SETI Research Center will be turning their attention to a very unique star in our own Milky Way galaxy, and they’ll be using the Green Bank radio telescope in West Virginia to do it. It’s part of a bold project called the ‘Breakthrough Listen project’, which will dedicate eight hours a night, for 3 nights to detect possible signals from an alien civilization.
The star in focus is KIC 8462852, or more endearingly called “Tabby’s star”. Tabby’s star gets its moniker from the Planet Hunters project manager, Tabetha Boyajian. The Planet Hunters project is a citizen science collaborative, through which data from the Kepler spacecraft is made public, and the eyeballs of tens of thousands of people are utilized to spot ‘dips’ in light curves of distant stars.
The Kepler spacecraft’s unique mission was to survey a single portion of our Milky Way galaxy in order to discover planets around distant stars. Equipped with star trackers, a solar array, high-gain antenna, and a photometer with a 12-degree field of view, Kepler was sent into orbit where it spent 4 years focused on that single group of stars. The data it collected was sent back to Earth and eventually made available online to citizen scientists from around the world (The Planet Hunters).
The data came in the form of light curves on graphs where the X-axis represent time, and the Y-axis represents star brightness. The light curves are a collection of white dots on a black background for ease of viewing for hours at a time. Each dot represents a single observation of that star's brightness. With an observation done every 30 minutes over the course of 4 years, you'll have a lot of dots!
A star without planets might show these dots remaining more-or-less at the same level of brightness through time. But stars that host planets dim when a planet passes between it and Kepler because it blocks a small portion of that star's light. These events are called transits. The dip in brightness is small, perhaps a single percent drop at most, but on a scaled graph this small dimming is noticeable and is represented by dots much lower along the Y-axis than what is otherwise normal for that star. Here is an example of a light curve showing a pattern of transits from a confirmed planet:
As of this writing, the Kepler mission has confirmed a total of 2,330 planets just in that tiny portion of our own Milky Way! A small portion of it... in a single galaxy.. in a Universe of billions of galaxies... each galaxy containing hundreds of billions, if not trillions of stars! Imagine how many planets must exist out there! It's mind boggling! Any planet not a part of our own solar system is referred to as an exoplanet.
Exoplanets come in all sorts of sizes, densities, composition, and orbits. From hot-superEarths to cold ice giants, it's clear that our galaxy is teeming with more solar systems than Star Wars and Star Trek universes combined! But the type we're all most interested in are the rocky, Earth-sized planets in the habitable zone of their respective host stars.
The habitable zone is a distance range within which liquid water could exist. It doesn't necessarily mean that life exists should a planet's orbit be within the zone, as there are many other factors necessary for life; such as a protective, breathable atmosphere for one.
As mentioned above, if a star hosts a planet, its transit will result in a dip in that star's brightness as observed by Kepler. This dimming is about 1% of the star's total brightness. Also, these dips are exhibit a pattern, occurring at predictable intervals through time; indicative of a planet's repetitious orbit around its host star (see graph above).
But Tabby's star is unique in this regard. Very unique.
There isn't just a single pattern of dips at nice neat intervals, each dimming the starlight by a single percent. There are all sorts of dips--irregular dips--in its light curve, and big dips; One sequence of dips that occurs after day 1,500 of observation dimmed the starlight by 22%!
Two small dips around day 1540 of the Kepler mission... 2 small dips at day 1,205 & 1,540 that differ in duration by a factor of 3, and in dimming degree by a factor of 5! 15% dimming of starlight between days 788 to 795... distinctive symmetric triple dips... (source: https://arxiv.org/abs/1509.03622)
This star has something or some THINGS very weird orbiting around it!
Some scientists have postulated that it could be some sort of disc of dust and gas surrounding the star; a protoplanetary disc from which planets coalesce. Something some young stars (and their young solar systems) might have. But Tabby's star "falls outside the region where empirically-calibrated age diagnoses such as chromospheric activity, or stellar rotation period can be used", according to Boyajian et al. So it will be difficult trying to determining if Tabby's star is a younger star or not. Younger, as in less than 100 million years old... young by star standards.
Worse still for the protoplanetary disc hypothesis, is that Tabby's star is outside a region known for having young disc stars. Young disc population stars have low velocity dispersion, and occupy this special region within velocity space. Imagine them in a big box... Tabby's star is outside that box.
So perhaps it's an older star. Some discs of dust and gas can continue to exist around older stars. Known as circumstellar discs, perhaps Tabby's star hosts some sort of really eccentric, warped, multi-disc bonanza around it.
The debate over this star's age continues. But it isn't the only obstacle scientists need to get over. Other hypotheses put forth are hitting veritable brick walls too; Gravity darkening being one. Fast-spinning stars distort considerably such that they bulge at their equator. Earth does too, but it's nothing compared to some stars. This distortion causes the poles of a fast-spinning star to brighten, and its equatorial region to dim by comparison. This effect is called gravity darkening. But Tabby's star doesn't spin fast enough for this. Current estimates put its spin rate at 84 kilometers per second... give or take 4 km/sec.
Perhaps it's starspots (sunspots) & magnetic cycles, polar spots, stellar pulsations.. not likely, not likely, not likely... and to read more on why those ideas are unlikely, visit this blog by Jason Wright at Penn State: https://sites.psu.edu/astrowright/2016/09/03/what-could-be-going-on-with-boyajians-star-part-ix-intrinsic-variations/
Some scientists have said the dimming could be causes by a swarm of comets; a massive comet swarm the likes of which even our solar system likely didn't experience during the late heavy bombardment. But other scientists calculated that for there to be 22% decrease in the star's brightness, that comet swarm would have to be really, um, swarmy.... to the tune of 648,000 comets, each with at least 200km diameters! Those are comets about the size of Estonia... EACH! Not very plausible.
About all we know is something is blocking a substantial portion of light from Tabby's star, and whatever it is, it is NOT a planet. What we have in Tabby's star is an as-of-yet unexplained mystery as to how and why its light is dimmed in such drastic and peculiar ways. See it for yourselves! It's light curve is available to the public, and here's how you can pull it up:
- Go to http://exoplanetarchive.ipac.caltech.edu/.
- Go to the top menu bar: Data > Kepler Light curves.
- Put 8462852 into the box labeled Kepler Id.
- Click on View
- Now you have a list of Kepler Data sets
- Click on any ‘DV Time Series’ in the list of data sets before and it will open an interactive plot.
Now you're doing science!
With all these hypotheses being shot down left and right, scientists are scrambling to figure out what it is. One hypothesis is that there may be intelligent life from a planet orbiting Tabby's star; life far more intelligent than our species. One that may have constructed what we know in science fiction as a Dyson Sphere!
Building a mega structure, or a swarm of smaller structures around a star in order to harness its energy is far beyond our current technology, budget, and resource availability. Though we have some of the components that could perhaps be used as part of such a structure (orbiting satellites, solar sails, and our burgeoning nanotech), the resources and tech needed to harness and transmit so much energy, not to mention the prohibitive cost to maintain such a structure simply put a Dyson anything--save for a vacuum cleaner--beyond our reach.
But what if an alien species has advanced beyond our own? One that, and this is hard to imagine given human history, cooperates on a global scale, who evolved technologically unhindered by historiographic shadows such as our own Dark Ages? That evolved millions, perhaps tens of millions of years before Homo erectus learned to control fire. A species that has seen itself evolve through its written history, learned from its collective past, and continues to survive by utilizing resources from other planets, rather than to scavenge their own. One that lives on a relatively pristine planet, that has somehow managed to avoid those pesky extinction-level impactors that our planet has had to deal with all its 'life'. We are sort of surrounded by a veritable ammo dump in the form of the Oort cloud and asteroid belt. But with all this said, the odds that what is causing Tabby's dimming being an alien-built megastructure are so remote, many scientists see the idea having been postulated at all as almost facetious, if not outright so.
But woe be to the person who doesn't at least investigate the impossible. And this is the mindset at SETI. Just this week, they've already completed an 8-hour observation of Tabby's star using the Green Bank radio telescope in West Virginia, and have 2 more nights set for more searching... searching for that tell-tale radio signal that only an intelligent species could transmit. According to Andrew Siemion, director of the Berkeley SETI Research Center and co-director of Breakthrough Listen, "the Green Bank Telescope is the largest fully steerable radio telescope on the planet, and it’s the largest, most sensitive telescope that [is] capable of looking at Tabby’s star given its position in the sky." (source: news.berkeley.edu)
If they don't detect a signal does this mean there's no intelligent life living on a planet orbiting Tabby's star? Well, I would say yes, but of course have to posit the chance that the ways in which we communicate (utilizing various wavelengths along the longer-end of the electromagnetic spectrum), may be outmoded by highly-intelligent alien standards. Perhaps they use fully functional quantum computers, and have a way to communicate far more efficiently than, say, radiowaves. Maybe they know what dark matter is, perhaps they've discovered a unified theory of all fundamental forces, and perhaps one or more of those forces are unknown by us.
I have very low hopes, as does Siemion himself, that an alien species has constructed a Dyson sphere around Tabby's star. But, it's still intriguing to consider that regardless of what it is causing Tabby's odd light curve, there are still hundreds of billions of other stars out there, in our own galaxy, waiting to be investigated... one of them, just maybe, hosts life at least as intelligent as our own species... tabloid article authors and reality TV stars aside.
In the world of science, and most particularly in the world of cosmology, some of the best places to look for answers are in the dark corners of the impossible.