27th October 2015
Could alien megastructures be orbiting a distant star?
In recent weeks, the world of astronomy has been abuzz with news about a star known as KIC 8462852. Highly unusual light fluctuations detected in this system appear to have defied all logical explanations, leading to speculation about a possible extraterrestrial presence.
KIC 8462852 – also known as Tabby's star – was discovered through Planet Hunters, a citizen science program launched by Yale University in 2010, which allows users to analyse data from the NASA Kepler Space Telescope. An F-type main-sequence star, it is located in the constellation Cygnus about 1,480 ly from Earth.
Last month, a team of astronomers posted a study on arXiv, in which they analysed the unusual light fluctuations of Tabby's star. By observing changes in the brightness levels of stars, it is possible to calculate the size, orbits and other characteristics of exoplanets passing in front of them. In the case of Tabby's star, Kepler observed small, frequent, non-periodic dips in brightness – along with two massive recorded dips in brightness appearing to occur roughly every 750 days. The magnitude of these changes and their irregularity has been of great interest to astronomers and are consistent with many small masses orbiting the star in "tight formation".
The first major dip, seen in March 2011, reduced the star's brightness by 15%, while the other (in February 2013) by 22%. In comparison, a planet the size of Jupiter would only obscure a star of this size by 1%, indicating that whatever is blocking light during these major dips is not a planet – but rather something covering up to half the width of the star. Due to the failure of two of Kepler's reaction wheels, the star's predicted 750-day dip in April 2015 was not recorded; further observations are planned for May 2017.
Based on the spectral and main sequence type of Tabby's star, its changes in brightness could not be attributed to intrinsic variability. A few hypotheses have been proposed involving material orbiting the star and blocking its light, but none of these fully explain the observed data.
One explanation is that a cloud of disintegrating comets or asteroids is drifting around the star in an elliptical orbit. In this scenario, the gravity from a nearby star may have disrupted the objects from a surrounding Oort cloud, making them fall in towards the star – similar to what will happen in the year 1,400,000 AD with our own Sun and Gliese 710. Evidence to support this includes the fact that a companion red dwarf star exists 132 billion km (885 AU) away from Tabby's star. However, the notion that disturbed Oort cloud comets orbiting elliptically close to the star could exist in high enough numbers to obscure 22% of the star's luminosity makes this theory seem unlikely.
Other proposed explanations include instrument/data errors, a variable B(e) star, interstellar dust, a young companion star with a protoplanetary disk or a series of giant planets with very large ring structures.
High resolution spectroscopy and imaging observations have also been made, as well as spectral energy distribution analyses using the Nordic Optical Telescope in the Canary Islands. A massive collision scenario would create warm dust that glows in infrared wavelengths – but there is no observed excess infrared energy, ruling out massive planetary collision debris. Other researchers think the planetary debris field explanation is unlikely because of the very low probability that Kepler would ever witness such an event, given the rarity of collisions on that scale.
A far more profound and exciting possibility has been suggested by Penn State University astrophysicist Jason Wright. He and colleagues in another recent paper speculate that the objects eclipsing the star could be parts of a megastructure built by aliens, such as a Dyson sphere or Dyson swarm. These concepts were popularised by Freeman Dyson in his 1960 paper, "Search for Artificial Stellar Sources of Infrared Radiation" and would in theory be designed to function as colossal solar panels, harvesting the vast amounts of energy required for an advanced civilisation.
"When [they] showed me the data, I was fascinated by how crazy it looked," Wright says in an interview with The Atlantic. "Aliens should always be the very last hypothesis you consider, but this looked like something you would expect an alien civilisation to build."
Tabby's star is an outstanding target for SETI, because natural explanations have yet to fully explain the dimming phenomenon. The SETI Institute is currently using the Allen Telescope Array to search for non-natural radio signals from the direction of KIC 8462852 and hopes to announce its results within the next week. This effort is looking for both narrow-band signals (similar to traditional SETI experiments), as well as broader transmissions that might be generated by a powerful spacecraft.
Additional follow-up observations are being proposed separately by other astronomers, potentially using the ground-based Green Bank Telescope in January 2016 (the world's largest fully steerable radio telescope), the Very Large Array Radio Telescope, and future orbital telescopes such as WFIRST, TESS, and PLATO.