Free Astronomy Magazine November-December 2018

44 NOVEMBER-DECEMBER 2018 the first contact. This was a wise choice, as the planet started to transit across the disc 77.8 minutes earlier than expected. This conspicuous advance was already indicative of the fact that Kepler-1625b is not the only body in orbit around its star. The most ex- citing part of the transit monitoring of Oc- tober 2017 was at the end, 3.5 hours after the planet came out of the disk, when Hub- ble recorded a secondary and very weak dimming of the starlight, consistent with the existence of a moon orbiting the planet. Unfortunately, the telescope time at the disposal of the two researchers exhausted before the candidate moon ended its tran- sit, and it was therefore not possible to complete a series of measurements perhaps decisive for the identification of the object. This is how the two authors of the study comment on these observations in their ar- ticle published on 3 October 2018 in Sci- ence Advances : “The most compelling piece of evidence for an exomoon would be an exomoon transit, in addition to the observed TTV [transit timing variation]. If Kepler-1625b’s early transit were indeed due to an exomoon, then we should expect the moon to transit late on the opposite side of the barycenter. The previously men- tioned existence of an apparent flux de- crease toward the end of our observations is therefore where we would expect it to be under this hypothesis. Although we A graphic rep- resentation of the gas giant Kepler-1625b with its Neptune- sized moon. In the box, the planet seen from the surface of a hypothetical smaller, farther and colder moon. have established that this dip is most likely astrophysical, we have yet to discuss its sig- nificance or its compatibility with a self- consistent moon model”. If, at the moment, it is not yet possible to confirm that the “mysterious” object tem- porarily named Kepler-1625b-I is a moon, the data collected during the transit still tell us that, if it were, it should have a mass of about 1.5% that of the planet, and it could, therefore, have a size comparable to that of Neptune. Such a big moon, not having been formed through the typical processes of our known satellite systems, would re- quire the review of the current theories