Transit of Kepler-1625b and Suspected Moon
This diagram represents Hubble Space Telescope photometric observations of the planet Kepler-1625b passing in front of its parent star — called a transit. The planet blocks a small fraction of the star's light and this is recorded on a light curve (bottom green line) as a slight dip in the star’s brightness. 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). (Due to observing constraints Hubble was not able to record the full event.) The second dip is interpreted as the signature of a moon trailing the planet. The moon is estimated to be as big as the planet Neptune. The inclination of the candidate moon's orbit is just one of a broad range of possible inclinations that are consistent with the data. Astronomers hope to repeat this observation to confirm the moon's existence. If follow-up observations are successful, this would be the first moon discovered outside of our solar system.