Now, that’s Funny

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.

Light curve of EPIC 249706694 / HD 139139

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.

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Author: John Walker

Founder of Ratburger.org, Autodesk, Inc., and Marinchip Systems. Author of The Hacker's Diet. Creator of www.fourmilab.ch.

7 thoughts on “Now, that’s Funny”

  1. Considering past occurences of something funny noticed, this new one is cause for rejoicing and speculation, even for persons such as myself who have preciously shallow understanding of this post.  So thanks.

     

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  2. As I imagine the transit photometry method of exoplanet detection, it must surely underestimate the number of such planets. It seems to me that this method would only detect those planets which transit stars in the same plane as the observer – i.e. only those solar systems whose ecliptic is in the same plane as our detector. Planets in any other plane would not transit the star from the perspective of our detector. Am I understanding this correctly?

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  3. civil westman:
    It seems to me that this method would only detect those planets which transit stars in the same plane as the observer – i.e. only those solar systems whose ecliptic is in the same plane as our detector. Planets in any other plane would not transit the star from the perspective of our detector. Am I understanding this correctly?

    Yes, that is correct.  Whether a planet is observed to transit its parent star depends upon the inclination of its orbit as observed from Earth, the radius of the orbit (the distance between the star and planet, which may vary if the orbit is elliptical with substantial eccentricity), and to a smaller degree the size of the planet compared to that of the star (a large planet may have grazing transits which partially obscure the star while a smaller one would miss entirely).

    The inclination of planetary orbits as seen from Earth is essentially random, although all planets orbiting a star will typically do so in approximately the same plane.  A large planet orbiting close to its primary (“hot Jupiter”) has around a 10% probability of transiting the star as seen from Earth, while an Earth-like planet orbiting a Sun-like star at Earth’s orbital radius has only a 0.47% chance of being observed to transit.  It is thus around twenty times as probable that we’ll see a hot Jupiter in transit than an Earth-like planet, which explains why the list of exoplanets discovered so far includes so many of the former kind.

    Knowing these probabilities of transits allows astronomers to estimate how many of each kind of planet exist, extrapolating from those which we have observed in transit to those which do not transit as seen from Earth.

    There is another kind of selection effect which results in our missing planets in orbits far from their star.  Due to many potential sources of false positive detections (as high as 40% in raw Kepler data), any potential transit observation must be confirmed by subsequent observations which establish the period of the planet’s orbit and demonstrate that the observed light dip is consistent and repeatable and not, for example, caused by a star spot or a background variable star contributing light to the same pixel in the detector.  Exoplanets with long orbital periods, thus, can only be confirmed if observed over a time period of two or more orbits.  Given that large-scale transit searches is a new field, this means that our observation campaigns so far would have failed to detect Jupiter or any planet beyond it even if observed with perfect edge-on alignment.  Again, astronomers try to correct for this selection bias when estimating the population of planets.

    It is only after correcting for these effects (which involves a bit of guesswork) that one arrives at the estimate that there are around as many planets in the galaxy as there are stars.

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  4. The purpose of this paper is largely to bring this enigmatic object to the attention of the larger astrophysics community in the hope that (i) some time on larger telescopes, or ones with high photometric precision, might be devoted to its study, and (ii) some new ideas might be generated to explain the mysterious dips in flux.

    P. 11.

    This will be an interesting test of the power of large numbers (of people looking at the problem). I assume we sill shortly see detailed analyses of alternate explanations.

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