The Kepler spacecraft was launched into heliocentric orbit in 2009. Its primary mission was to stare at a small area of the sky and monitor around 150,000 stars in its field of view (around twice the size of the bowl of the Big Dipper), watching for the subtle dimming of stars when planets orbiting them passed in front of their parent stars (a transit). Before its retirement in October, 2018, it had discovered 2,662 exoplanets (planets orbiting stars other than the Sun). It also saw some other, very curious things.
You may have heard about Tabby’s Star (KIC 8462852), a main sequence star which exhibits irregular deep dimmings which have, so far defied all attempts to explain them.
Kepler’s primary mission came to an end in 2012 when failure of on-board reaction wheels made it impossible to aim the telescope at its target in the sky. In 2014 an extended mission called K2 was begun, which used a clever method of using solar radiation pressure to orient the spacecraft, and this mission continued until its maneuvering fuel was exhausted, forcing its retirement.
The extended mission allowed observing other regions of the sky, and detected numerous additional exoplanets. It also saw some distinctly odd things.
On 2019-06-28, a preprint of a paper, “The Random Transiter – EPIC 249706694/HD 139139” was posted to the arXiv.org server. It discusses a star with the nomenclature in the title, which was observed over a period of 87 days during the Kepler extended mission.
Here is the light curve of the star over the period of observation, normalised to take out effects such as star spots and artefacts of data reduction. The mean flux from the star is 1.0, and the red line indicates the calculated noise floor: anything below it meets the criterion used in observations of other stars to indicate a planetary transit candidate.
What you expect for a star with one or more transiting planets is a series of dips as the planets pass in front of the star, occurring at regular intervals, since there are few phenomena in nature as regular as the orbits of planets. But this light curve is crazy. All of the standard tools used to detect periodicities in data sets find none. In fact, the authors note, “their arrival times could just as well have been produced by a random number generator”. And yet, with two exceptions, the dips are of comparable magnitude (200±80 parts per million in flux) and around the same shape, consistent with a transit of an opaque object.
Complicating the analysis is that the primary star, which is much like the Sun, has another star very close to it in the sky, which contributes to the same pixel in the sensor. It is not known whether the two stars are gravitationally bound into a double system or are a chance alignment and there is no way to know from the Kepler data whether the bright star or the dimmer nearby star is responsible for the dips. It may be possible to determine this from follow-up ground-based observations, but that question is not presently resolved.
In section 6 of the paper, the authors consider nine possible explanations for the observed random dips in flux. They essentially exclude all of the suggested instrumental and astrophysical causes except for hypothetical short-lived star spots which have never been observed in any of the hundreds of thousands of other stars studied by astronomers.
Almost every time we’ve looked at nature in a a new way: a different scale of spatial or temporal resolution, a new frequency band, or a broader scope of sampling, we’ve found things that “just don’t make any sense”. Isaac Asimov said,
The most exciting phrase to hear in science, the one that heralds the most discoveries, is not “Eureka!” but “That’s funny…”.
This is funny.