Kepler-1625 is a 14th-magnitude solar-massstar located in the constellation ofCygnus approximately 7,200light-years (2,200parsecs) away. Its mass is within 5% of that of theSun, but its radius is approximately 70% larger reflecting its more evolved state. A candidategas giant exoplanet was detected by theKepler Mission around the star in 2015,[8] which was later validated as a real planet to >99% confidence in 2016.[9] In 2018, theHunt for Exomoons with Kepler project reported evidence for a Neptune-sizedexomoon around this planet, based on observations from NASA'sKepler mission and theHubble Space Telescope.[10][4] Subsequently, the evidence for and reality of this exomoon candidate has been subject to debate.[11][12][13][14]
Kepler-1625 is an approximately solar-mass star and yet is 1.7 times larger in diameter.[4] Its effective temperature is around 5,550 K, slightly lower than that of the Sun.[15][4] These parameters suggest that Kepler-1625 may be a yellowsubgiant nearing the end of its life, with an age of approximately 8.7 billion years.[4] The star has been observed to be photometrically quiet, with periodic variability below 0.02%.[13] Kepler-1625 is located approximately 7,200 light-years away[2] in the constellationCygnus.[15]
The star is known to have one validated planet. Designated Kepler-1625b, it is a Jovian-sized planet orbiting its star every 287.3 Earth days. No other candidate transiting planets have been found around the star.[13]
Artist's impression of the exoplanet Kepler-1625b and its candidate exomoon Kepler-1625b I.
TheKepler Mission recorded three planetary transits of Kepler-1625b from 2009 to 2013.[8] From these, anomalous out-of-transit flux decrements indicated the possible existence of aNeptune-sizedexomoon, as first reported by theHunt for Exomoons with Kepler project in 2018.[10] The Kepler data were inconclusive and so the planetary transit was re-observed by theHubble Space Telescope in October 2018. The light curve from Hubble exhibited evidence for both a moon-like transit and atransit timing variation, both of which were consistent as being caused by the same Neptune-sized moon in orbit of Kepler-1625b.[4] The transit timing variation has been independently recovered by two teams analyzing the same data.[11][12] One of these teams also independently recovered the moon-like transit, but suggest thatradial velocity measurements are needed to exclude the possibility of a close-in masquerading planet.[11] The other team are unable to recover the moon-like transit and suggested it may be an artifact of the data reduction.[12] This conclusion was challenged by the original team soon after, who showed that the other analysis exhibits larger systematics that may explain their differing conclusion.[13]
