Kepler-1625b I is a possiblemoon ofexoplanetKepler-1625b, which may be the firstexomoon ever discovered (pending confirmation), and was first indicated after preliminary observations by theKepler Space Telescope.[4] A more thorough observing campaign by theHubble Space Telescope took place in October 2017, ultimately leading to a discovery paper published inScience Advances in early October 2018. Studies related to the discovery of this moon suggest that the host exoplanet is up to severalJupiter masses in size, and the moon is thought to be approximately the mass ofNeptune. Like several moons in the Solar System,[5] the large exomoon would theoretically be able to host its own moon, called asubsatellite, in a stable orbit, although no evidence for such a subsatellite has been found.[6]
Relative size and distance ofKepler-1625b and its moonKepler-1625b-I, using images of Jupiter and Neptune
The original paper[2] presented two independent lines of evidence for the exomoon, atransit timing variation indicating a Neptune-mass moon, and a photometric dip indicating a Neptune-radius moon. An independent re-analysis of the observations published in February 2019[7] recovered both but suggested that an inclined and hidden hot-Jupiter could also be responsible, which could be tested with futureDoppler spectroscopy radial velocity observations. A third study analyzing this data set recovered the transit timing variation signature but not the photometric dip, and thus questioned the exomoon hypothesis.[8] The original discovery team later addressed this paper, finding that their re-reduction exhibits higher systematics that may explain their differing conclusions.[9]
