Free Astronomy Magazine November-December 2018

NOVEMBER-DECEMBER 2018 T his diagram represents Hubble Space Tele- scope photometric observations of the transit of Ke- pler-1625b. After the planet’s 19- hour-long transit was completed, astronomers noted a second, smaller dip in the light curve about three and a half hours later (panel 4). The second dip is inter- preted as the sig- nature of a moon trailing the planet. [NASA, ESA, D. Kipping (Columbia University), and A. Feild (STScI)] future transits. If these occur in noticeable advance or delay of prediction, it means that there is a non-negligible mass that al- ters the planet’s orbital velocity. That mass can be either another planet, probably placed on a more external orbit, or a moon orbiting the planet in transit. The first scenario is not easily verifiable if the perturbing planet does not appear to transit the stellar disk. The second scenario can in- stead be verified by highlighting the other type of fingerprint, that is, a very slight sec- ondary drop in starlight, which manifests it- self during the transit or in its proximity, but that is not due to the planet. Depending on where the moon is with re- spect to the observer, it can enter the disk be- fore the planet, it can follow it, or it can be overlapped with it. This means that the light curve can be quite complex for a planet- moon combination, but it is clear that the larger the moon, the easier it is to discover. Current instruments enable the discovery of much larger moons than those familiar to us. It follows that we must look for them around gigantic planets. Among the 284 worlds investigated by Kipping and Teachey, only one showed during the tran- sits a double fingerprint attributable to the presence of a moon. That planet is called Kepler-1625b and is the only known one in orbit around Kepler-1625, a star of mass comparable to that of the Sun but with a diameter 80% larger. These physical fea- tures suggest that Kepler-1625 is an old star, aged between 7 and 11 billion years. Its planet runs through an orbit that looks like the Earth’s and Kepler-1625b was de- termined by analysis to be within the star’s habitable zone in the first 5.4 billion years. Despite its favorable position, we can con- clude that Kepler-1625b is likely not a hos- pitable world, since it is a gas giant. Nev- ertheless, a possible moon, more or less the