Kepler-1625b

Kepler-1625b is an extrasolar planet (exoplanet) orbiting the yellow star Kepler-1625 about 2,500 parsecs away.[2] A large gas giant, it is approximately the same radius as Jupiter[3] and orbits its star every 287.4 days.[4] In 2017, hints of a Neptune-sized exomoon in orbit of the planet was found using photometric observations collected by the Kepler Mission.[5][6] Further evidence for a Neptunian moon was found the following year using the Hubble Space Telescope, where two independent lines of evidence constrained the mass and radius to be Neptune-like.[3] The mass-signature has been independently recovered by two other teams.[7][8] However, the radius-signature was independently recovered by one of the teams[8] but not the other.[7] The original discovery team later showed that this latter study appears affected by systematic error sources which may be influencing their findings.[9]

Kepler-1625b
Discovery
Discovery siteKepler Space Observatory
Discovery dateMay 10th 2016
Transit (Kepler Mission)
Orbital characteristics
0.98±0.14 AU
Eccentricity-
287.378949 d
Inclination89.97±0.02
StarKepler-1625
Physical characteristics
Mean radius
11.4±1.6 R⊕
Mass≤11.60 MJ[1]

    Characteristics

    Mass and radius

    Kepler-1625b is a Jovian-sized gas giant, a type of planet several times greater in radius than Earth and mostly composed of hydrogen and helium. It is 11.4 times Earth's radius, approximately equal to that of the planet Jupiter. However, it is up to 11.6 times more massive (about 3,700 Earth masses), based on radial velocity observations.[1] This puts it just below the deuterium-fusing limit, which is around 13 Jupiter masses. Any more massive and Kepler-1625b would be a brown dwarf. However, this mass value only corresponds to an 3-sigma upper limit and the mass of the planet remains undetected at this time.[1]

    Orbit and temperature

    Unlike the gas giants in our Solar System, Kepler-1625b orbits much closer, slightly closer than the orbital radius as the Earth around the Sun.[3] The planet takes 287 days, or about 0.786 years, to orbit Kepler-1625, as a result of the star's slightly greater mass than the Sun. Kepler-1625b receives 2.6 times more insolation than the Earth,[3] meaning it lies at the inner edge of the habitable zone.[10] However, as the planet has likely no solid surface, bodies of liquid water are impossible.

    Candidate exomoon
    Main article: Kepler-1625b I
    Exomoon Kepler-1625b I orbiting Kepler-1625b (artist concept).[11]

    In July 2017, researchers found signs of a Neptune-sized exomoon (a moon in another solar system) orbiting Kepler-1625b using archival Kepler Mission data.[5][6]

    In October 2018, researchers using the Hubble Space Telescope published new observations of the star Kepler-1625 which revealed two independent lines of evidence indicative of a large exomoon Kepler-1625b I.[3][12] These were a 20-minute Transit Timing Variation signature that indicated an approximately Neptune-mass moon, and an additional photometric dip that indicated a Neptune-radius moon.[3] The relative phasing of the two signatures was also consistent with that which a real moon would cause, with the effects in anti-phase.[3] The study concluded that the exomoon hypothesis is the simplest and best explanation for the available observations, though warned that it is difficult to assign a precise probability to its reality and urged follow-up analyses.[11][3]

    In February 2019, a reanalysis of the combined Kepler and Hubble observations recovered both a moon-like dip and similar transit timing variation signal.[8] However, the authors suggested that the data could also be explained by an inclined hot-Jupiter in the same system that has gone previously undetected, which could be tested using future Doppler spectroscopy radial velocity measurements. A second independent reanalysis was published in April 2019 which recovered one of the two lines of evidence, the transit timing variation, but the not the second, the moon-like dip.[7] The original discovery team responded to this soon after, finding that this re-analysis exhibits stronger systematics in their reduction which may be responsible for their differing conclusion.[9]

    References
    1. Timmermann, Anina; Heller, Rene; Reiner, Ansgar; Zechmeister, Mathias (2020). "Radial velocity constraints on the long-period transiting planet Kepler-1625 b with CARMENES". Astronomy and Astrophysics. 635: 59. arXiv:2001.10867. Bibcode:2020A&A...635A..59T. doi:10.1051/0004-6361/201937325.
    2. Luri, X.; Brown, A.; Sarro, L. (2018). "Gaia Data Release 2. Using Gaia parallaxes". Astronomy and Astrophysics. 616: 19. arXiv:1804.09376. Bibcode:2018A&A...616A...9L. doi:10.1051/0004-6361/201832964.
    3. Teachey, Alex; et al. (2018). "Evidence for a large exomoon orbiting Kepler-1625b". Science Advances. 4 (10): eaav1784. arXiv:1810.02362. Bibcode:2018SciA....4.1784T. doi:10.1126/sciadv.aav1784. PMC 6170104. PMID 30306135.
    4. "Exoplanet Exploration: Planets Beyond our Solar System". Exoplanet Exploration: Planets Beyond our Solar System. Archived from the original on 2018-10-03.
    5. "First exomoon might have been spotted 4000 light years away".
    6. Teachey, Alex; et al. (2017). "HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I". The Astronomical Journal. 155 (1). 36. arXiv:1707.08563. Bibcode:2018AJ....155...36T. doi:10.3847/1538-3881/aa93f2.
    7. Kreidberg, Laura; Luger, Rodrigo; Bedell, Megan (2019-04-24). "No Evidence for Lunar Transit in New Analysis of HST Observations of the Kepler-1625 System". arXiv:1904.10618.
    8. Heller, Rene; Rodenbeck, Kai; Giovanni, Bruno (2019). "An alternative interpretation of the exomoon candidate signal in the combined Kepler and Hubble data of Kepler-1625". Astronomy and Astrophysics. 624: 95. arXiv:1902.06018. Bibcode:2019A&A...624A..95H. doi:10.1051/0004-6361/201834913.
    9. Teachey, Alex; Kipping, David M.; Burke, Christopher (2019). "Loose Ends for the Exomoon Candidate Host Kepler-1625b". The Astronomical Journal. 159 (4): 142. arXiv:1904.11896. Bibcode:2020AJ....159..142T. doi:10.3847/1538-3881/ab7001.
    10. Zsom, Andras; Seager, Sara; de Wit, Julien; Stamenković, Vlada (2013). "Towards the Minimum Inner Edge Distance of the Habitable Zone". The Astrophysical Journal. 778: 109. arXiv:1304.3714. Bibcode:2013ApJ...778..109Z. doi:10.1088/0004-637X/778/2/109.
    11. Drake, Nadia (3 October 2018). "Weird giant may be the first known alien moon - Evidence is mounting that a world the size of Neptune could be orbiting a giant planet far, far away". National Geographic Society. Retrieved 4 October 2018.
    12. "Kelpler-1625b-I". The Extrasolar Planets Encyclopaedia. 15 July 2019. Retrieved 14 December 2020.
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