JoEllen McBride, PhD
The Kepler telescope, despite technical issues, has observed over 100,000 stars in our galaxy. Its database is full of stars that show the tell-tale sign of an orbiting planet– a periodic and repeatable dimming of the starlight. But one stellar dimming sequence doesn’t follow the expected protocol and it has astronomers getting creative to explain why.
Tabby’s star, or more fondly, the WTF (Where’s the Flux?) star, is a yellow star slightly larger than our Sun located over 1200 light-years away in the constellation Cygnus the Swan. You can’t see it with your eyes but looking through a small 5-inch telescope you can see it just fine.
Kepler continuously observed the region of space where WTF lives from 2009 to 2013. Then in 2015, Citizen scientists analyzing the data noticed something very peculiar about WTF’s brightness. In March of 2011, the star dimmed by 22% of its original brightness, suggesting something big was passing in front of it. Then 700 days later in 2013, the star dimmed significantly again, but this time did so irregularly– suggesting that not just one but many large objects were passing in front of the star. This is where the science gets interesting.
When astronomers study the light from stars we create graphs that are called light curves. Light curves describe how the brightness of a star changes over a period of time. We choose a star, take images of it periodically and measure how bright it is. If the star’s brightness decreases, we will record a lower brightness value than in previous measurements.
Usually, when a star has planets orbiting it, the dimming will be periodic– tied to the orbit of the planet. So we will measure a smooth dip in the brightness of the star at regular intervals as the planet passes in front. What’s so spectacular about WTF’s brightness is that there is a single, smooth dip in brightness followed 700 days later by irregular but large decreases that lasted for 100 days before the brightness returned back to normal levels.
After ruling out issues with the Kepler telescope and the variability of WTF, the lead scientists considered more celestial explanations for the irregular dimming. Debris from a violent collision like the one that formed our Moon would probably create enough large particles to recreate the dimming– but the likelihood of us catching such a one-off event is extremely small. A large conglomerate of comet fragments also seemed like a reasonable and likely cause. But we’ve never observed this before so can only make educated guesses as to what that light curve would look like.
Other scientists have jumped in on the task of explaining these dips with suggestions ranging from weird internal variations with the WTF star itself to unfinished alien megastructures. But recently, a group of researchers has proposed an explanation that’s a little more familiar and easily testable.
Follow the Gravity Train
To understand their proposal, we need to discuss a little-known fact (at least, I didn’t know this) about our solar system’s largest planet, Jupiter. All massive bodies in our solar system exert a gravitational force on other massive bodies. If we think of space as a bed sheet held taut at its corners and place a bowling ball at the center, the ball would create a pit or well in the sheet due to the mass of the ball. If we then place a baseball somewhere else on the sheet, the sheet will also bend due to the mass of the baseball. The larger well in the sheet due to the bowling ball will overlap in some places with the well in the sheet due to the baseball. This is sort of how gravitational forces interact with each other.
But space is a bit more complicated. The interaction of the gravitational forces of two massive bodies ends up creating what are known as Lagrange points. In our sheet analogy, these would appear as five additional wells created at specific locations around the bowling ball-baseball system. In space, these points orbit the more massive body at the same speed as the smaller body. Any objects living at these points are stuck following the smaller body around the larger one, never catching up or falling behind.
In the case of the Sun-Jupiter system, there are three Lagrange points that lie along Jupiter’s orbit and are home to thousands of asteroids. The two large ”Trojan” swarms are located on either side of Jupiter in its orbit around the Sun and the smaller “Hilda” swarm is always located on the opposite side of the Sun from Jupiter.
There is evidence for Trojan-type regions in other exoplanet systems and planet formation theory shows that these regions can exist long after planets form in solar systems. So this makes their detection more probable than one-off events like planetary collisions or never observed events like swarms of comet fragments.
Researchers in Spain took a known idea and made it bigger to explain the weird dimming of the WTF star. Their proposal suggests the first, smoother dimming event is due to a large, ringed planet– almost five time larger than Jupiter. This large planet would also have larger Trojan swarms which would explain the irregular dips in brightness 700 days later. Since the Jovian system has two Trojan regions, the astronomers expect there to be another irregular dimming episode again in February 2021 which would correspond to the second Trojan region. Then two years later in 2023, the giant ringed planet should pass in front of the star again, starting the approximately 12 year cycle over.
Their hypothesis even accounts for a smaller May 2017 dimming event which occurred at the same time their theoretical planet would have been passing behind the WTF star. If this system is similar to Jupiter, the dimming could be explained by a Hilda-like swarm of asteroids which would dim the star but not as significantly as the Trojan swarm.
You should still hold some reservations about this prediction though. The number of asteroids needed to produce such a large dimming is huge– like the same mass as Jupiter huge. No one has a clue if this sort of configuration would even be stable. The team is working on a computer model for the system and plans on releasing those results in a forthcoming paper. But the key to a successful hypothesis is that it is easily testable and the Trojan hypothesis gives us something to look forward to in 2021. We only have to wait 4 years to see if these researchers are right or if we need to go back to the drawing board to figure out what’s going on with the WTF star.