HD139139, The System that Disconcerts Astronomers

In October 2018, the Kepler Space Telescope retired after discovering more than 2600 exoplanets through the transit method. This detection technique is quite simple to understand. Telescopes monitor a number of stars for slight variations in brightness. A small drop in brightness may indicate that an object has just passed between the star and the telescope. If this decline is periodic, it is likely that it is because of a planet orbiting around the star.

We can then use this information to roughly estimate the diameter of the planet. The method does not work for all the systems because it is necessary that the orbital plane of the planets passes by the observer. But by watching a sufficient number of stars, it is possible to accumulate discoveries. That’s exactly what Kepler has done for almost ten years.

The astronomers detected a strange phenomenon

By analysing the data of the telescope, a team of astronomers has just discovered a star with very strange behavior. HD 139139 is a double star that was monitored by Kepler for 87 days, a period during which it experienced a drop in brightness. This is strongly reminiscent of transits, with the difference that their transit period appears completely random.

The orbit of a planet is normally always the same. The Earth is circling the sun in 365 days, and that does not vary from one orbit to another. How then to explain a system whose transits seem completely random ? Of course we can explore several scenarios, but they all seem more unlikely than the others.

The Sun with circling planet. Elements of this image furnished by NASA

Several scenarios are studied

The 139139 HD system may be incredibly complex with a very large number of planets. But it is to suppose that Kepler was extremely lucky in being able to see them transit over a very short period. An almost perfect alignment that probably would only happen several hundred or even thousands of years apart.

We can also imagine a planet emitting dust or particles in a very irregular way. Alone, it would be too small to be detected by transit, but surrounded by its cloud of dust it would sometimes manage to hide a sufficient amount of light to be detected by Kepler. This supposes that it is a planet very close to its star and that it orbits it in five hours. But it was detected only 28 times when it would have made 400 transit during the period of observation of the telescope. This scenario is just as unlikely as the first one.

In a similar way, an asteroid disk emitting enough dust would almost make a good candidate. It is not surprising that asteroids are very numerous. But the bright variations observed by the Kepler Space Telescope have very similar profiles. All these asteroids should therefore be able to emit fairly similar amounts of dust despite probably very different orbits and thermal stresses.

The mystery of the HD 139139 system persists

The team behind this discovery explored many other scenarios, without finding any that fit with the observations. There remains the possibility that Kepler is witnessing an extremely rare alignment or simply that we are facing a phenomenon that we do not yet understand. As often in these cases, we can be tempted to see the work of an extraterrestrial civilization. But it’s a safe bet that a more natural explanation will eventually emerge, perhaps thanks to new observations by other telescopes.

Pictures courtesy of Vadim Sadovski.

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