Kepler-1625b is an extrasolar planet orbiting the yellow starKepler-1625 about 8,000 light-years away. A large gas giant, it is between 5.9 and 11.67 times the radius of Earth and orbits every 287.4 days. This places it within or near the system's habitable zone, depending on the exact size of the star. In 2017, hints of a Neptune-sized exomoon orbiting at a distance of 20 planetary radii was announced. However, a reanalysis of the data published in April 2019 concluded that the exomoon was probably the result of an artifact in the data reduction and likely does not exist.
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 about 12 times Earth's radius, slightly larger than that of the planet Jupiter. However, it is up to 10 times more massive, based on observations of its candidate moon. 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. Because of this high mass and radius, the planet likely has a very strong gravitational pull, with a surface gravity calculated to be up to 22.08 times that of Earth. It is also very dense at up to 10.15 g/cm3.
Orbit and temperature
Unlike the gas giants in our Solar System, Kepler-1625b orbits near the habitable zone of its host star. The planet takes 287 days, or about 0.786 years, to orbit Kepler-1625. This places it at 0.811 AU, similar to Venus' distance from the Sun. At this distance Kepler-1625b has an equilibrium temperature of, which is slightly lower than the boiling point of water. However, as the planet has no solid surface, bodies of liquid water are impossible.
Differences in size models
The exact size of the host star Kepler-1625 is not fully known. The most accepted model puts it at 1.79 solar radii and 1.08 solar masses with a temperature of 5548 K. However, there is a second result for the size of Kepler-1625, which puts it at 84% the Sun's radius and 86% its mass, with a slightly higher temperature at 5645 K. With this second model, Kepler-1625b is only about 6 times the radius of Earth and is in the middle of the conservative habitable zone, with an equilibrium temperature of, about the same as Earth's.
Candidate exomoon
In July 2017, researchers found signs of a Neptune-sized exomoon orbiting Kepler-1625b at a distance of about 20 planetary radii. In October 2018, researchers using the Hubble Space Telescope published observations of the candidate exomoon Kepler-1625b I, which suggest that the host planet is likely several Jupiter masses, while the exomoon has a mass and radius similar to Neptune. 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. However, a reanalysis of the data published in April 2019 concluded that the data was fit better by a planet-only model. According to this study, the discrepancy was an artifact of the data reduction, and Kepler-1625b I likely does not exist.
