Kepler-1708b, previously named KIC 7906827.01, is a Jupiter sized extrasolar planet orbiting the F-type main-sequence star Kepler-1708, located around 5,600 light years away from Earth in the Cygnus constellation.
Discovery[]
The planet was first discovered in 2011 by the Kepler space mission, using the transmit method. However, it was not identified as a candidate planet until 2019. In 2021, a candidate Neptune-sized exomoon in orbit around Kepler-1708b was discovered by David Kipping.
Characteristics[]
The planet is a gas giant, slightly smaller than the planet Jupiter in size, and has a radius of 0.89 that of Jupiter; or about 9.960 times that of Earth. The mass of the planet has yet to be measured; precise analysis of its transit timings place a 2-sigma upper limit of about <4.6 Jupiter masses. The mass upper limit predicts a maximum radial velocity amplitude of <98 m/s - talthough within reach of the most precise spectrographs available, the faintness of Kepler-1708b's host star would make observations difficult.
Orbit and temperature[]
The planet orbits around 1.64 astronomical units from its host star, Kepler-1708, and completes a full orbit every 737 days, or 2.02 years, similar to that of Mars in the Solar System. At this distance, Kepler-1708b would lie within the habitable zone of the star; an area in which it would be possible for liquid water to exist on a planet's surface. It receives an insolation flux of 0.561 times that of Earth at a relatively cool equilibrium temperature of 200-300 kelvins (-73-27 °C; -100-80 °F). The eccentricity of its orbit is currently unmeasured, and is given a 2-sigma upper limit of <0.40.
Possible exomoon[]
- See article: Kepler-1708b I
In 2021, David Kipping and his colleagues performed a search for extrasolar moons around cool, long-period gas giant extrasolar planets, using photometric data from Kepler. Out of a sample of around 70 exoplanets analyzed, only Kepler-1708b showed signs of an orbiting exomoon, manifesting as faint, secondary transits accompanying the planet's transits. This possible exomoon, designated Kepler-1708b I, appears to measure below the size of Neptune at 2.6 times Earth's radius. It likely orbits coplanar to its host planet from a distance up to 12 planetary radii - comparable to the distance between Jupiter and its moon Europa, or twice the Earth-Moon distance. The huge size of Kepler-1708b I is similar to Kepler-1625b I, another Neptune-sized exomoon reported by Kipping.
Additional observations are necessary to confirm or disprove the exomoon's existence - only two transits by Kepler-1708b and its possible exomoon have been observed, and no transit timing variations can be determined as of yet. Possible external sources of contamination in the host star's light curve have been ruled out in Kipping et al.'s analysis, and the exomoon candidate was rated with a false positive probability of 1% - mostly due to the not-yet-ruled-out possibility of a second transiting planet in the Kepler-1708 system.