The Star That Keeps Dimming for No Known Reason

In 2015, astronomers analyzing data from NASA's Kepler Space Telescope discovered a star designated KIC 8462852, located about 1,470 light-years from Earth in the constellation Cygnus, that was exhibiting brightness fluctuations unlike anything that had been observed in over 150,000 stars surveyed by the Kepler mission, and the pattern of dimming was so unusual and irregular that it could not be explained by any known natural phenomena including planets orbiting the star, stellar pulsations, or dust clouds, leading some scientists to seriously propose that the dimming might be caused by artificial structures built by an advanced alien civilization, specifically something like a Dyson swarm of solar collectors orbiting the star to harvest its energy, though this explanation while exciting was considered a hypothesis of last resort only to be entertained after all natural explanations had been exhaustively ruled out. The star, which became known informally as Tabby's Star after astronomer Tabetha Boyajian who led the research team studying it, showed dimming events where its brightness dropped by up to 22 percent, far more than could be explained by a planet passing in front of it, which typically causes dimming of only a fraction of a percent, and the dimming events were irregular and aperiodic, meaning they did not repeat on any predictable schedule, and different dimming events had different characteristics with some showing gradual dimming over days and others showing more sudden brightness drops.

The discovery was made by participants in the Planet Hunters citizen science project who were examining Kepler light curves looking for the subtle regular dimming that indicates planets transiting in front of their host stars, and when they flagged KIC 8462852 as showing unusual brightness variations, professional astronomers began detailed analysis and quickly realized this star was behaving in ways that challenged conventional astronomical understanding, and the irregularity and depth of the dimming suggested something much larger and more complex than a planetary system was affecting the star's light. Initial hypotheses focused on natural phenomena including a swarm of comets passing in front of the star, which could create irregular dimming if the comets were fragmenting and creating variable amounts of dust and debris, or a ring system around a massive planet that happened to be oriented edge-on to our perspective creating unusual transit patterns, but detailed modeling suggested these scenarios had difficulties explaining all the observed characteristics of the dimming events.

THE ALIEN MEGASTRUCTURE HYPOTHESIS

The hypothesis that drew the most public attention and scientific controversy was proposed by astronomer Jason Wright and colleagues who suggested that the dimming pattern could be consistent with a Dyson swarm, a theoretical megastructure proposed by physicist Freeman Dyson in the 1960s where an advanced civilization might build massive solar collectors in orbit around their star to capture a significant fraction of its energy output for their civilization's use, and if such a structure were being built around KIC 8462852 and we happened to be viewing the star from an angle where these collectors passed in front of it, we might see irregular dimming as the various components of the partially constructed megastructure blocked the star's light. This hypothesis, while scientifically legitimate as a possibility to consider, was immediately controversial because it invoked an extraordinary explanation requiring the existence of an advanced alien civilization at a specific stage of technological development building a specific type of megastructure that happened to be oriented so we could detect it, and the principle of Occam's Razor suggests that natural explanations should be exhaustively explored before resorting to such exotic possibilities.

The alien megastructure hypothesis generated enormous media coverage and public interest, with Tabby's Star becoming one of the most famous astronomical objects in popular culture, and while most astronomers emphasized that this explanation was highly speculative and that natural causes were far more likely, the mere fact that serious scientists were willing to publish papers discussing the possibility of alien engineering as an explanation for an astronomical observation was remarkable and reflected both the genuine puzzlement about what could cause such dramatic and irregular brightness variations and the scientific principle that all hypotheses consistent with the data deserve consideration even if some are much less probable than others. Follow-up observations were conducted to search for other signatures that might support the alien megastructure hypothesis, including looking for waste heat in infrared wavelengths that would be produced if the star's energy was being captured and used by technology, and searching for radio signals that might indicate technological activity, but these searches found no supporting evidence, which while not ruling out the hypothesis made it seem less likely as other observations accumulated.

COMETARY EXPLANATION AND DUST ANALYSIS

Further analysis of Tabby's Star's light, including spectroscopic observations that examined which wavelengths of light were most affected by the dimming events, provided crucial evidence about what might be blocking the star's light, and researchers found that blue light was blocked more than red light, a pattern consistent with scattering by small particles like dust rather than blocking by solid objects like planets or megastructures, and this wavelength-dependent dimming suggested that whatever was causing the brightness drops involved clouds of small particles rather than large opaque structures, making the alien megastructure hypothesis significantly less likely and lending support to natural explanations involving dust or debris. The leading natural explanation that emerged from this evidence was that a family of comets or the remnants of a destroyed comet in a highly elliptical orbit around the star was periodically passing through our line of sight and creating the observed dimming, with the irregular pattern explained by the fact that different fragments of the broken comet had different orbits and different amounts of dust and gas, creating variable dimming events as these fragments transited the star at different times.

The comet hypothesis gained additional support from observations of the star's long-term brightness trends, which showed that in addition to the dramatic short-term dimming events, the star had been gradually dimming over the entire four-year period that Kepler observed it and indeed had dimmed by about 0.9 percent over the century prior based on analysis of photographic plates from historical astronomical surveys, and this long-term dimming suggested that the star was developing an increasingly dense cloud of dust around it, consistent with cometary activity where fragmented comets were releasing dust that accumulated in the system over time. However, some researchers noted that the amount of dust required to explain the century-long dimming was extraordinarily large, equivalent to a comet or moon hundreds of kilometers in diameter being completely pulverized, and while such events are possible they would be rare and it seems somewhat coincidental that we would happen to be observing this star during the relatively brief period after such a catastrophic disruption.

THE ONGOING MONITORING AND NEW MYSTERIES

In 2017, a coordinated global observation campaign involving both professional and amateur astronomers monitored Tabby's Star continuously and successfully captured new dimming events in real-time, allowing detailed spectroscopic and photometric analysis during the actual dimming rather than relying only on archived Kepler data, and these observations confirmed the wavelength-dependent dimming pattern suggesting dust as the cause, and also revealed additional complexity including that different dimming events had different color signatures suggesting different properties of the dust clouds, possibly indicating multiple populations of particles with different sizes or compositions creating the observed effects. The ability to observe dimming events as they happened also allowed scientists to search for simultaneous changes in other wavelengths including infrared and radio emissions that might provide clues about the nature of the dimming source, and while no dramatic findings emerged from these multi-wavelength observations, they did further constrain the possible explanations and provided data that future modeling efforts could use to test various scenarios.

Analysis of archival data from other telescopes found additional mysterious aspects of Tabby's Star's behavior, including evidence that the star had experienced even more dramatic dimming events in historical data from the 1890s, with brightness drops potentially as large as 30 percent, far exceeding anything observed by Kepler, and if this historical dimming was real and not an artifact of measurement error in the old photographic plates, it suggested that whatever is affecting the star's brightness has been operating for over a century and might be getting more extreme over time, or alternatively might be a periodic phenomenon with a very long cycle that we are just beginning to understand. The verification of historical dimming is complicated by the limitations of old astronomical observations which were much less precise than modern digital photometry, and different researchers analyzing the same historical plates have come to somewhat different conclusions about whether the dimming is real or within the error bars of the measurements, creating ongoing debate about this aspect of the mystery.

THE BROADER IMPLICATIONS FOR EXOPLANET DETECTION

The study of Tabby's Star has had significant implications for the field of exoplanet detection and characterization, revealing that stars can have brightness variations from causes other than planets and that these variations can be complex and difficult to interpret, and this realization has led to more careful analysis of other unusual stars found in Kepler data and to the recognition that stellar systems can be much more dynamically active than previously appreciated, with debris, dust, and cometary activity creating observable effects that need to be distinguished from planetary signals. The discovery of other stars showing somewhat similar though less extreme irregular dimming patterns, including a star called EPIC 204278916 that showed asymmetric dimming events thought to be caused by an irregular or ringed planet, has demonstrated that Tabby's Star while unusual is not completely unique and that the universe contains a variety of stellar phenomena that produce brightness variations we are only beginning to catalog and understand.

The lesson from Tabby's Star about being open to multiple hypotheses while maintaining scientific rigor about the quality of evidence required to support extraordinary claims has been valuable for astronomy as a field, and the fact that the alien megastructure hypothesis was seriously considered and investigated rather than dismissed out of hand while simultaneously being held to high evidentiary standards demonstrates mature scientific thinking about how to approach genuinely puzzling observations that do not fit existing frameworks. The star continues to be monitored by professional and amateur astronomers, and future observations may either confirm the cometary explanation by detecting additional evidence of dust clouds and debris, or may reveal new complexities that require refining our models, and the possibility, however remote, that some component of the dimming pattern might ultimately have a more exotic explanation cannot be entirely excluded until we have a comprehensive model that explains all observed characteristics of this remarkable star's behavior.

The mystery of Tabby's Star represents the frontier of our understanding of stellar systems and the variety of phenomena that can affect stellar brightness, and whether the final explanation proves to be cometary debris, some other natural phenomenon we have not yet fully characterized, or in the extraordinarily unlikely event actually does involve artificial structures of some kind, the scientific journey of investigating this star has already provided valuable insights into how science approaches the unknown, how we distinguish between probable and improbable explanations, and how even in an age of advanced astronomy and comprehensive surveys of the sky, the universe can still present us with phenomena that challenge our understanding and require years of careful observation and analysis to explain. The star continues shining 1,470 light-years away, its light reaching us now showing what was happening there during the Middle Ages on Earth, and the photons carrying information about its mysterious dimming will keep arriving year after year, providing an ongoing opportunity to solve one of the most intriguing astronomical puzzles of the early 21st century.