An international team of astronomers have spotted something mysterious blocking the light of a young star known as RIK-210, which lies roughly 472 light-years from Earth.
Normally, when a star's light dims, it's a hint that something is orbiting it, such as a planet – but RIK-210 doesn't have any of those things. And it's one of several weird stars we've found in recent months that seems to be experiencing an unexplained dimming pattern. So what's going on?
Astronomers frequently monitor the incoming light from distant stars in hope of discovering new planets orbiting them. This type of planet-hunting is aptly dubbed the transit method, because the dimming of starlight can suggest something is passing in front of the star, momentarily blocking its light from reaching Earth.
Most of the time, researchers can figure out what's causing this dimming through prolonged observation, and usually it falls into pretty straightforward categories, such as another planet or another star, but sometimes things are bit stranger.
For example, back in 2015, astronomers noted the strange dimming of KIC 8462852, better known as Tabby's Star, which has yet to be explained, leaving a whole lot of hypotheses out on the table, including the far-fetched idea that it could be being orbited by an alien megastructure.
The newly discovered dimming effect has just as much mystery, with far less alien speculation.
The dimming was first spotted by researchers working on NASA's Kepler mission, which uses the Kepler space observatory to hunt down exoplanets by monitoring the light of many distant stars. This data was then handed off for further evaluation to the current team, which is led by Trevor David from the California Institute of Technology (Caltech).
As they report, RIK-210's light dims up to 15 percent every 5.67 Earth days, lasting just a short period of time. Also, this timeframe accurately falls in line with the star's rotation, meaning that as it rotates, the strange source of the dimming rotates with it.
From there, the team went about trying to find a possible reason for this. The first hypothesis they threw out was the existence of a companion brown dwarf, which was taken off the table after prolonged observation and imaging showed no signs of one.
"High spatial resolution imaging revealed the star is single, and radial velocity monitoring indicated that the dimming events cannot be due to an eclipsing stellar or brown dwarf companion," the team writes in their paper.
Further analysis also showed that the dimming couldn't be caused by a single spherical body, either, throwing out the notion that it could be a planet. They also claim that the star – which is roughly 5 to 10 million years old – also lacks a protoplanetary disc, a ring of dust that often forms around young stars.
"We argue it is unlikely the dimming events could be attributed to anything on the stellar surface based on the observed depths and durations," the team writes.
"Variable obscuration by a protoplanetary disk is unlikely on the basis that the star is not actively accreting and lacks the infrared excess associated with an inner disk."
So, what are we left with? Well, the team says that the most likely cause is a cloud inside the star's magnetosphere, the region around the star that is full of charged particles controlled by the star's magnetic field.
This hypothesis, though unverified, makes the most sense because a cloud of that nature would 'orbit' around the planet at the same speed as its spin.
To fully answer the question, though, the team says they will need to monitor the star for a longer period of time.
"Multi-band photometric monitoring can be used to test whether the dip depths are wavelength-dependent; solid-body transits are achromatic, while extinction by dust is less severe at redder wavelengths," the team writes of their future plans.
"Finally, spectroscopic monitoring while the star is known to be dimming can test whether there is enhanced absorption by a gaseous cloud transiting the star."
After they analyse the star further, we will hopefully gain a better understanding of what's causing the dim before we have another alien megastructure mystery on our hands.
Currently, the team's work is waiting for publication in a peer-reviewed journal. So, we will have to wait to see if their results are independently verified before a conclusion is drawn.
Until then, you can read their full paper online at arXiv.org, where it's been uploaded so other scientists can scrutinise it before they submit it for publication.