Kepler-1625 is a 14th-magnitude solar-mass star located in the constellation of Cygnus approximately 7,200 light-years (2,200 parsecs) away. Its mass is within 5% of that of the Sun, but its radius is approximately 70% larger reflecting its more evolved state. A candidate gas giant exoplanet was detected by the Kepler Mission around the star in 2015,[5] which was later validated as a real planet to >99% confidence in 2016.[6] In 2018, the Hunt for Exomoons with Kepler project reported evidence for a Neptune-sized exomoon around this planet, based on observations from NASA’s Kepler mission and the Hubble Space Telescope.[7][3] Subsequently, the evidence for and reality of this exomoon candidate has been subject to debate.[8][9][10][11]
Stellar characteristics
Kepler-1625 is an approximately solar-mass star and yet is 1.7 times larger in diameter.[3] Its effective temperature is around 5,550 K, slightly lower than that of the Sun.[12][3] These parameters suggest that Kepler-1625 may be a yellow subgiant nearing the end of its life, with an age of approximately 8.7 billion years.[3] The star has been observed to be photometrically quiet, with periodic variability below 0.02%.[10] Kepler-1625 is located approximately 7,200 light-years away[1] in the constellation Cygnus.[12]
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.[10]
The Kepler Mission recorded three planetary transits of Kepler-1625b from 2009 to 2013.[5] From these, anomalous out-of-transit flux decrements indicated the possible existence of a Neptune-sized exomoon, as first reported by the Hunt for Exomoons with Kepler project in 2018.[7] The Kepler data were inconclusive and so the planetary transit was re-observed by the Hubble Space Telescope in October 2018. The light curve from Hubble exhibited evidence for both a moon-like transit and a transit timing variation, both of which were consistent as being caused by the same Neptune-sized moon in orbit of Kepler-1625b.[3] The transit timing variation has been independently recovered by two teams analyzing the same data.[8][9] One of these teams also independently recovered the moon-like transit, but suggest that radial velocity measurements are needed to exclude the possibility of a close-in masquerading planet.[8] The other team are unable to recover the moon-like transit and suggested it may be an artifact of the data reduction.[9] 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.[10]
