| Discovery[1] | |
|---|---|
| Discovery date | 2019 (candidate)[2][3] 2021 (confirmed)[1][4] |
| Transit | |
| Designations | |
| KIC 7906827.01 | |
| Orbital characteristics[1] | |
| 1.64±0.10 AU | |
| Eccentricity | <0.40 |
| 737.1131+0.00146 −0.00770 d | |
| Inclination | 89.92°+0.03° −0.01° |
| Knownsatellites | Kepler-1708b I? |
| Star | Kepler-1708 |
| Physical characteristics[1] | |
| 0.8886+0.0535 −0.0526 RJ)[4] | |
| Mass | <4.6 MJ |
Kepler-1708b (previously known asKIC 7906827.01) is aJupiter-sizedexoplanet orbiting theSun-likestar Kepler-1708, located in the constellation ofCygnus approximately 5,600light years away from Earth.[1] It was first detected in 2011 byNASA'sKepler mission using thetransit method, but was not identified as a candidate planet until 2019.[2][3] In 2021, a candidateNeptune-sizedexomoon in orbit around Kepler-1708b was found by astronomer David Kipping and colleagues in an analysis usingKepler transit data.[1][4] However, subsequent research has raised discrepancies about the possible existence of an exomoon, similar to that ofKepler-1625b,[5] but even more recent research still find the existence of an exomoon likely.[6]
Kepler-1708b is agas giant planet slightly smaller than Jupiter in size, with a radius of 0.89 Jupiter radii.[7][4] The mass of the planet remains yet to be measured; precise analysis of its transit timings place a 2-sigma upper limit of <4.6 Jupiter masses. This mass upper limit predicts a maximumradial velocity amplitude of <98 m/s—although within reach of the most precisespectrographs available, the faintness of Kepler-1708b's host star would make observations difficult.[1][8]
Kepler-1708b orbits about 1.64 astronomical units from its host star and completes one revolution every 737.11 days or 2.02 years, comparable to the orbit ofMars in theSolar System.[9] At this distance, Kepler-1708b lies within thehabitable zone of its host star, where it receives aninsolation flux0.561+0.074
−0.068 times that of Earth at a relatively coolequilibrium temperature of 200–300kelvins (−73–27 °C; −100–80 °F).[1][10] Theeccentricity of its orbit is unmeasured and is given a 2-sigma upper limit of <0.40.[1]
Kepler-1708b orbits around the Sun-like star Kepler-1708, located in the constellation ofCygnus 5,580 ± 240 ly (1,712 ± 75 pc)light years away from Earth.[1] At anapparent magnitude of 16, this star is too faint to be seen by thenaked eye.[11] The star'scelestial coordinates based on theJ2000 epoch are:RA19h 47m 17.79s,Dec 43° 37′ 29.4″.[11] TheEuropean Space Agency'sGaia satellite has measured astellar parallax of0.5730±0.0340 milliarcseconds (mas) and directionalproper motion components of RA−0.770±0.057 mas/yr, Dec−5.005±0.059 mas/yr.[11] Kepler-1708 is known by other designations from variousstar catalogues including:UCAC4 669-077544,KIC 7906827,TIC 272716898,2MASS J19471778+4337295,WISE J194717.78+433729.2, andGaia DR2 2078801971283008128.[11]
Kepler-1708 is slightly larger and more massive than the Sun, with a mass of1.088±0.072 M☉ and radius of1.117±0.064 R☉. It is also hotter and more luminous than the Sun, with an effective temperature of6,157+231
−202 K and abolometric luminosity of1.521 L☉.[a] Based on these properties, Kepler-1708 is likely anF-type main sequence star with a Sun-likemetallicity of [Fe/H] =0.0±0.2 dex and an age of3.16±2.26 billion years.[b][7][1]
In 2021, David Kipping and colleagues performed a search forexomoons around cool, long-period gas giant exoplanets usingKepler photometric data. Out of a sample of 70 exoplanets analyzed, only Kepler-1708b exhibited signs of an orbiting exomoon manifesting as faint, secondary transits accompanying the planet's transits.[12] This possible exomoon, designated Kepler-1708b I, appears to measure below the size ofNeptune 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 moonEuropa, or twice theEarth–Moon distance.[1][13] The extraordinarily large size of Kepler-1708b I is reminiscent ofKepler-1625b I, another Neptune-sized exomoon candidate previously reported by Kipping et al. in 2017.[1]
Additional observations are necessary to confirm or refute the exomoon's existence—only two transits by Kepler-1708b and its possible exomoon have been observed, and notransit timing variations can be determined as of yet. Kipping et al. determine that the probability of detecting one false positive exomoon in the studied sample of 70 exoplanets was <50%. A follow-up study suggested Kepler-1708b I is likely undetectable with theHubble Space Telescope, but theJames Webb Space Telescope should be able to confirm or refute its existence.[14]
In 2024, a paper was published disputing both 1625b I and 1708b I’s existences,[5] but a reply to this paper refute most claims given, and conclude that the existence of an exomoon is likely but need additional observations.[6]
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