Could comet clusters, or something other-worldly, explain this cosmic flicker?
January 10, 2017
With more than 100 billion stars in the Milky Way galaxy, there are plenty of celestial objects for scientists to survey. Every once in a while, astronomers see something puzzling, prompting explanations that may seem even more out there: planetary collisions, comet clusters or even aliens.
Astronomers have proposed all three of those possibilities to explain some strange shadows in a star more than one thousand light years away. From our point of view on Earth, the brightness of the star, named KIC 8462852, dips by up to 20 percent at irregular intervals.
Brightness dips are not unusual; in fact, a search for brightness dips are how astronomers first noticed KIC 8462852. When objects like planets, comets or clouds of dust pass in front of a distant star, we see that as a slight dimming, like a mini-eclipse. NASA’s Kepler mission surveys the skies for these shadows in an attempt to locate distant planets.
Like Earth, distant planets rotate around their stars at constant rates, so telescopes here on Earth see those planets by detecting regular flickers in the brightness of the stars.
A citizen science group called Planet Hunters, founded at Yale University, combs over data from the Kepler mission to find the specific flickers that signify a planet. In 2011, some of those Planet Hunters saw KIC 8462852 and noticed that not only did the flickers come in at seemingly random intervals, but more of the starlight was blocked during the flickers than is typically blocked by planets. In fact, 15-20 percent of KIC 8462852’s light was blocked, when a planet the size of Jupiter would only block about one percent of the light.
Given this unusual shadow, Yale University astronomer Tabetha Boyajian began investigating other possibilities for what could cause the flickering. She published her findings in the open access journal arxiv.org.
Beginning with the star itself, KIC 8462852 is not the type of star to wax and wane in brightness on its own, and nearby stars do not have the capacity to cause changes in KIC 8462852’s brightness.
Next on the list of possibilities were cosmic crashes. If one large object did not cause the flickers, many small objects could be the culprits. When planets or asteroids crash into each other they create clouds of dust and smaller rocks. The problem with asteroids is that you would expect to see the starlight dim for long periods of time as dust clouds popped up from frequent collisions. Planet collisions on the other hand can create gigantic clouds of dust, but the problem is that those dust clouds orbit the star at constant rates just like planets do, which does not explain the random intervals between flickers. Further, planetary collisions are extremely rare, and it would be very lucky to catch one in the act.
Every scenario seemed to have a few issues, but Boyajian found a possible solution in comets. Comets travel in elliptical, egg shaped orbits and many comets can be gravitationally trapped into very similar orbits, creating a comet family. The comets fly at different points on the orbital path, forming a patchy ring. They come in at random intervals, which would match the random flickers. Further, comets in large groups could block out a sufficiently large fraction of KIC 8462852’s light.
A further study of KIC 8462852 will be needed to determine whether the comet cluster theory checks out. But for now, scientists from the Search for Extra Terrestrial Intelligence (SETI) Institute have another theory: alien structures.
Obviously, humans have no proven experience with aliens, but when scientists speculate what a distant civilization might look like, they consider problems we have here on Earth, including energy. Stars are among the largest energy sources that we know of, and an advanced civilization would likely harness that energy.
One way to do that would be to set up a large number of solar energy stations in orbit around the sun. It turns out that a fleet of solar panels placed at the right distance from a star, could block that star's light from our perspective. Also, those installations could theoretically pass by at random intervals.
Scientists at SETI have actually been searching for a star that flickers like KIC 8462852 to show the possibility of an alien civilization, but they are not quickly jumping to the conclusion that they have found an alien civilization. SETI actually has a protocol for evaluating candidates, and eliminating all other possibilities is a key step. The flickers could be cosmic crashes or swirling comets or even fragments of alien technology. What's important to remember, though, is that at 1,400 light years away, whatever caused the flickers caused them more than a millennium ago, and could be long gone by now.
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.