The Strange Case of Tabby’s Star: An Ongoing Story?

by Steve Beyer on

Hayden Planetarium Blog

“Extraordinary claims require extraordinary evidence.”  —Carl Sagan


Tabby’s Star in the constellation Cygnus
Arrow points to location of Tabby’s Star in the constellation Cygnus.
Credit: Roberto Mura

A notable star in the constellation Cygnus has recently been the subject of considerable scrutiny. Although the star itself seems unremarkable, interest stems from observations provided by the Kepler planet hunting mission which reveals that light from the object exhibits unusual brightness dips that vary erratically. The recorded phenomena are different from systematic variations seen in over a thousand Milky Way objects where orbiting extrasolar planets periodically cross lines of sight between us and their host stars. Known by its Kepler Input Catalog designation, KIC 8462852 (KIC  2852 for short), it’s a normal spectral type F3 main sequence object with a rotation period of about 21 hours. The star is yellow-white, 1.5 times larger than the Sun, and nearly 1,500 light years from Earth.

The discovery team’s favored explanation for KIC 2852’s unusual variations is that they’re caused by an orbiting swarm of comets or comet fragments. However, an alternate conjecture was suggested that an advanced civilization’s orbiting megastructures might be causing the erratic light dips. This secondary concept prompted an unusually quick response, as investigators collaborated with Andrew Siemion, Director of the SETI institute at UC Berkeley last month to begin a two week preliminary search for evidence of extraterrestrial radio activity at KIC 2852.

Astronomer Tabetha S. Boyajian of Yale University was lead author of a paper submitted on September 11 to Monthly Notices of the Royal Astronomical Society describing perplexing observations of the object. Known informally as “Tabby’s Star,” KIC 2852 was tagged for analysis by her team because during the four year period in which it was observed beginning in 2009, sequences of “dipping” reduced the star’s brightness by as much as 22% over periods of between 5 and 90 days duration.

KIC 8462852 Light Curve
This graph shows the four year sequence of light intensities observed in star KIC 8462852 by the Kepler mission. The left hand scale, marked 0.75 to 1.00, indicates brightness of the star, with 1.00 being maximum brightness and 0.75 representing a 75% level.  Horizontal units note days after January 1, 2009 (Julian Day 2454833), marked at intervals of 0, 500, 1,000, and 1,500 days. Dramatic dips are seen in two episodes, one near day 793 and again during the 90 day interval from day 1,490 to day 1,580.
Credit: Tabetha S. Boyajian, et al.

The Kepler mission was designed to reveal extrasolar planets by noting minute reductions in target stars’ apparent brightness, declines which typically last several hours in the case of Earth-sized planets. These events are equivalent to the transit of Venus across the Sun’s disk witnessed by millions in June 2012. Planetary transits occur on well-defined schedules, causing minute reductions in stellar brightness. For example, transits of Venus and similarly sized extrasolar planets reduce star brightness by about one part in 10,000.  Transiting “Super Jupiters” typically decrease target star brightness by just a few percent.

Much larger incongruous dips of KIC 2852 were discovered by D. M. LaCourse while volunteering in the Planet Hunters project. He and other citizen scientists sift through voluminous Kepler observations seeking evidence of unusual stellar activity overlooked by automated searches. The research team also included 28 professional astronomers from many institutions including the University of Cambridge, Cornell’s Carl Sagan Institute, California Institute of Technology, the Niels Bohr Institute, and the Universities of Hawaii, Chicago, and Texas.

Detailed images from the 10-meter Keck II telescope in Hawaii show a red dwarf star near Tabby’s Star, separated by 885 astronomical units, about 22 times Pluto’s average distance from the Sun.  It’s not known if that star is gravitationally associated with KIC 2852 or just passing by.   

Several explanations for KIC 2852’s odd light dips have already been ruled out by Boyajian’s team.

  1. Checks of optical, electronic, and mechanical integrity of Kepler’s hardware, as well as reviews of data analysis processes, suggest technical problems were not a factor; leaving attribution of the strange light variations to some astrophysical cause.
  2. Lack of Doppler radial velocity shifts in the target star’s spectrum show it’s unlikely a planet with a mass greater than 8 times that of Jupiter could be present.
  3. The possibility of the target star being intrinsically variable was ruled out due to absence of properties associated with known classes of variable stars.
  4. Presence of a circumstellar disk of obscuring material was ruled out due to lack of excessive infrared heat from the area of the target star, unlike what is detected radiating from known circumstellar dust belts.
  5. Discrete orbiting clumps of dust were also ruled out, also due to the presence of normal IR radiation levels detected from KIC 2852.
  6. A scenario involving swarms of dust produced by the collision of two planet sized bodies, occurring after the first Kepler observations and before the sets of dip anomalies were recorded was similarly discounted because of the normal levels of IR emissions.

The team’s favored hypothesis is that the star’s apparent light fluctuations are caused by intervention of an orbiting swarm of comets or comet fragments. Those suspected impediments may have twirled inward from KIC 2852’s equivalent of the Oort cloud, a region covering 20,000 to 100,000 astronomical units from the Sun, where billions of comet nuclei orbit at the outer limits of the solar system. Migration of cometary objects may have been induced to the immediate vicinity of Tabby’s Star by gravitational influence of the red dwarf seen in the Keck image—if it’s a rapidly passing interloper to the system. It’s a solid hypothesis based on analysis of available data, but another concept is also being conjectured.

In 1964, Russian astrophysicist Nikolai Kardashev proposed three sequential steps of development a civilization might attain as it adds technological sophistication increasing its ability to harness energy resources.

  • Level I: Humans have nearly reached the stage where we can make use of energy resources equal to the total amount of solar energy reaching our planet.
  • Level II: Typical of a civilization able to harness the entire energy output of its local star. That might be accomplished by shredding a terrestrial type planet then using its material to construct a “Dyson sphere” encapsulating their star, trapping and using its energy rather than letting it dissipate toward interstellar space.
  • Level III: A civilization able to harness all energy resources of its home galaxy.

An alternate to the comet swarm hypothesis is the suggestion what Kepler observed at KIC 2852 might have been caused by transiting alien megastructures, constructed long ago (we see the system as it was almost 1,500 years ago) by some alien presence—a type II Kardashev society.

In a related paper submitted October 15 to the Astrophysical Journal, Principal Investigator Jason T. Wright of Penn State University and colleagues detailed ways in which observations of light variations, orbit characteristics, and transmission properties might distinguish between planet-sized artificial artifacts and natural planets. They applied their criteria to another Kepler mission star having an unusual light profile and showed those anomalies appear to have a natural origin.

Wright’s team also reviewed astronomical literature noting that among motivations for alien built megastructures might be a desire for them to serve as beacons—signs of intelligent origin. Wright reviewed six hypothesized aspects of transiting alien megastructures and ten visual signatures that might distinguish such artifacts from exoplanets.

As a reality check, Wright cautions against leaping to an “alien” explanation for highly unusual astronomical observations, cautioning it’s best to “initially ascribe all anomalies to natural sources, and only entertain the ETI (Extraterrestrial Intelligence) hypothesis in cases where even the most contrived natural explanations fail to adequately explain the data.”

However, Wright suggests KIC 8462 provides a good excuse for practicing a SETI radio search, while also striving to find a natural explanation for its unusual light variations.

On October 16, the SETI institute aimed its Allen Telescope Array, located at the Hat Creek Radio Observatory northeast of San Francisco, to probe KIC 2852 over a range of radio frequencies. On November 5, Dr. Rick Fienberg, Press Officer for the American Astronomical Society forwarded a report from the Center for SETI Research commenting on first weeks of its study. The accompanying statement reflected scientific caveat, noting the SETI statement was being forwarded for information purposes but endorsement by the AAS was not to be implied.

Summing up their initial search, the SETI institute reported: “so far no evidence of deliberately produced radio signals has been found in the direction of KIC 8462852”. But, they added, “We expect that this star will be the object of additional observations for years to come.” 

Observing these unusual, perplexing events raises many questions among scientists and the public alike. The role of the scientist is to rule out all possible options and weigh the evidence on various theories, including those that are unconventional. Many observed phenomena take years or sometimes decades to fully understand, and this odd observation, too, may lie in that category. With more data and analysis, astronomers will converge on a sensible scenario that passes multiple tests and stands, as theories do, to be the best explanation for what we observe.