Kepler-1625b is a super-Jupiter exoplanet orbiting the Sun-like star Kepler-1625 about 2,500 parsecs (8,200 light-years) away in the constellation of Cygnus.^[4] The large gas giant is approximately the same radius as Jupiter,^[1] and orbits its star every 287.4 days.^[5] In 2017, hints of a Neptune-sized exomoon in orbit of the planet were found using photometric observations collected by the Kepler Mission.^[6][7] Further evidence for a Neptunian moon was found the following year using the Hubble Space Telescope, where two independent lines of evidence constrained the mass and radius to be Neptune-like.^[1] The mass-signature has been independently recovered by two other teams.^[8][9] However, the radius-signature was independently recovered by one of the teams^[9] but not the other.^[8] The original discovery team later showed that this latter study appears affected by systematic error sources that may have influenced its findings.^[10]

Kepler-1625b is a Jovian-sized gas giant, a type of planet several times greater in radius than Earth and mostly composed of hydrogen and helium. It has been estimated to be 11.4±1.5 times Earth's radius, approximately equal to that of the planet Jupiter.^[1] Its mass is unknown, but is constrained at 3-sigma confidence to be less than 11.6 times the mass of Jupiter (about 3,700 Earth masses), based on non-detection in radial velocity observations.^[3] This indicates that it is below the deuterium-fusing limit, which is around 13 Jupiter masses, and so it is not a brown dwarf.^[3]

Unlike the gas giants in our Solar System, Kepler-1625b orbits much closer, slightly closer than the orbital radius as the Earth around the Sun.^[1] The planet takes 287 days (0.786 years; 9.43 months) to orbit Kepler-1625, as a result of the star's slightly greater mass than the Sun. Kepler-1625b receives 2.6 times more insolation than the Earth,^[1] meaning it lies at the inner edge of the habitable zone.^[11] However, as the planet has likely no solid surface, bodies of liquid water are impossible.

In July 2017, researchers found signs of a Neptune-sized exomoon (a moon in another solar system) orbiting Kepler-1625b using archival Kepler Mission data.^[6][7]

In October 2018, researchers using the Hubble Space Telescope published new observations of the star Kepler-1625 which revealed two independent lines of evidence indicative of a large exomoon Kepler-1625b I.^[1][12] These were a 20-minute Transit Timing Variation signature that indicated an approximately Neptune-mass moon, and an additional photometric dip that indicated a Neptune-radius moon.^[1] The relative phasing of the two signatures was also consistent with that which a real moon would cause, with the effects in anti-phase.^[1] The study concluded that the exomoon hypothesis is the simplest and best explanation for the available observations, though warned that it is difficult to assign a precise probability to its reality and urged follow-up analyses.^[13][1]

In February 2019, a reanalysis of the combined Kepler and Hubble observations recovered both a moon-like dip and similar transit timing variation signal.^[9] However, the authors suggested that the data could also be explained by an inclined hot-Jupiter in the same system that has gone previously undetected, which could be tested using future Doppler spectroscopy radial velocity measurements. A second independent reanalysis was published in April 2019, which recovered one of the two lines of evidence, the transit timing variation, but the not the second, the moon-like dip.^[8] The original discovery team responded to this soon after, finding that this re-analysis exhibits stronger systematics in their reduction which may be responsible for their differing conclusion.^[10]