Kepler-1625 is a 14th-

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

solar-mass

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

located in the constellation of Cygnus approximately 8,000 light years away. Its mass is within 5% of that of the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, but its radius is approximately 70% larger reflecting its more evolved state. A candidate

gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...

exoplanet was detected by the

Kepler Mission The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...

around the star in 2015, which was later validated as a likely real planet to >99% confidence in 2016. In 2018, the

Hunt for Exomoons with Kepler The Hunt for Exomoons with Kepler (HEK) is a project whose aim is to search for exomoons, natural satellites of exoplanets, using data collected by the Kepler (spacecraft), Kepler space telescope. Founded by British exomoonologist David Kipping ( ...

project reported that this exoplanet has evidence for a Neptune-sized

exomoon An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body. Exomoons are difficult to detect and confirm using current techniques, and to date there have been no confirmed exomoon detecti ...

around it, based on observations from NASA’s

. Subsequent observations by the larger

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

provided compounding evidence for a Neptune-sized satellite, with an on-going debate about the reality of this exomoon candidate.

Stellar characteristics

Kepler-1625 is an approximately solar-mass star and yet is 1.7 times larger in diameter. Its effective temperature is around 5,550 K, slightly lower than that of the Sun. These parameters suggest that Kepler-1625 may be a yellow

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

nearing the end of its life, with an age of approximately 8.7 billion years. The star has been observed to be photometrically quiet, with periodic variability below 0.02%. Kepler-1625 is located approximately 8,000 light-years away in the constellation Cygnus.

Planetary system

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.

Potential exomoon

The

recorded three planetary transits of Kepler-1625b from 2009 to 2013. From these, anomalous out-of-transit flux decrements indicated the possible existence of a

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

-sized

, as first reported by the

project in 2018. The Kepler data were inconclusive and so the planetary transit was re-observed by the

in October 2018. The light curve from Hubble exhibited evidence for both a moon-like transit and a

transit timing variation Transit-timing variation is a method for detecting exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confi ...

, both of which were consistent as being caused by the same Neptune-sized moon in orbit of Kepler-1625b. The transit timing variation has been independently recovered by two teams analyzing the same data. 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. The other team are unable to recover the moon-like transit and suggested it may be an artifact of the data reduction. 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.

References

{{Sky, 19, 41, 43.04, +, 39, 53, 11.6 Planetary systems with one confirmed planet Planetary transit variables 5084 J19414304+3953115 Cygnus (constellation)

Stellar characteristics

Planetary system

Potential exomoon

See also

References