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Kepler-1625b is an extrasolar planet orbiting the Sun-like star Kepler-1625, located around 8,023 light years (2,460 parsecs) away from Earth in the constellation Cygnus. The planet is around 5.9 to 11.67 times the size of Earth, and orbits its host star every 287 Earth days. This places it near or in the star's habitable zone, depending on the exact size of the star. In 2017, hints of a Neptune-sized extrasolar moon orbiting at a distance of 20 planetary radii was announced.

CharacteristicsEdit

Mass and radiusEdit

Kepler-1625b is a Jovian-sized gas giant. The radius of the planet is around 12 times that of Earth, slightly larger than the planet Jupiter. However, certain models have it placed at around 6 Earth radii. The planet is mostly composed of hydrogen and helium. The planet is about 10 times the mass of Jupiter, based on observations of its candidate moon. This puts it just below the deuterium-fusing limit, which is about 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 temperatureEdit

Unlike the gas giants in the Solar System, Kepler-1625b orbits near the habitable zone of its star. The planet takes around 287 days, or 0.786 years, to complete an orbit around 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 350 K (77 °C; 170 °F), which is slightly lower than the boiling point of water. However, because the planet has no solid surface, bodies of liquid water are impossible.

HabitabilityEdit

It is currently highly debated and unknown whether a gas giant could sustain extraterrestrial life. Since the planet is located near the "habitable zone" of its star, it is possible that any exomoons orbiting Kepler-1625b could hypothetically sustain life, as moons orbiting gas giants have been shown to be capable of holding atmospheres - i.e. Titan, which has an atmosphere thicker than Earth's, and others have been shown to be able to have magnetic fields - i.e. Ganymede. Certain scientists, such as Stephen Hawking and Carl Sagan, have also theorized that lifeforms could evolve in the atmospheres of gas giants.

Differences in size modelsEdit

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 0.94 solar radii and 0.96 solar masses, with a temperature of 5677 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 253 K (−20 °C; −4 °F), which is roughly the same as Earth's.

Possible exomoonEdit

Main article: Kepler-1625b I

In July 2017, researchers found signs of a possible 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 suggests that the host planet is likely several Jupiter masses, while the exomoon has a mass and radius similar to that of 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. Some have argued that this may be an instance of a double planet rather than an extrasolar moon.