Kepler-90h is a gas giant with no solid surface. Its equilibrium temperature is 292 K (19 °C; 66 °F).[3] It is around 0.64 times as massive and around 1.01 times as large as Jupiter.[3] This makes it very similar to Jupiter, in terms of mass and radius.[3]
Illustration of the Kepler-90 system compared to the inner solar system. Kepler-90h is the outermost planet of the Kepler-90 system.
Orbit
Kepler-90h orbits its host star about every 331.6 days at a distance of 1.01 astronomical units, very similar to Earth's orbital distance from the Sun (which is 1 AU).[3]
Kepler-90h resides in the circumstellar habitable zone of the parent star. The exoplanet, with a radius of 1.01 RJ, is too large to be rocky, and because of this the planet itself may not be habitable. Hypothetically, large enough moons, with a sufficient atmosphere and pressure, may be able to support liquid water and potentially life.
For a stable orbit the ratio between the moon's orbital periodPs around its primary and that of the primary around its star Pp must be < 1/9, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days.[5][6] Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1 AU from a Sun-like star.[7] In the case of Kepler-90h, this would be practically the same to have a stable orbit.
Tidal effects could also allow the moon to sustain plate tectonics, which would cause volcanic activity to regulate the moon's temperature[8][9] and create a geodynamo effect which would give the satellite a strong magnetic field.[10]
To support an Earth-like atmosphere for about 4.6 billion years (the age of the Earth), the moon would have to have a Mars-like density and at least a mass of 0.07 ME.[11] One way to decrease loss from sputtering is for the moon to have a strong magnetic field that can deflect stellar wind and radiation belts. NASA's Galileo's measurements hints large moons can have magnetic fields; it found that Jupiter's moon Ganymede has its own magnetosphere, even though its mass is only 0.025 ME.[7]
The planet orbits a F-type star named Kepler-90, its host star. The star is 1.2 times as massive as the Sun and is 1.2 times as large as the Sun. It is estimated to be 2 billion years old, with a surface temperature of 6080 K. In comparison, the Sun is about 4.6 billion years old[12] and has a surface temperature of 5778 K.[13]
The star's apparent magnitude, or how bright it appears from Earth's perspective, is 14.[14] It is too dim to be seen with the naked eye, which typically can only see objects with a magnitude around 6 or less.[15]
Discovery
In 2009, NASA's Kepler spacecraft was completing observing stars on its photometer, the instrument it uses to detect transit events, in which a planet crosses in front of and dims its host star for a brief and roughly regular period of time. In this last test, Kepler observed 50000 stars in the Kepler Input Catalog, including Kepler-90; the preliminary light curves were sent to the Kepler science team for analysis, who chose obvious planetary companions from the bunch for follow-up at observatories. Observations for the potential exoplanet candidates took place between 13 May 2009 and 17 March 2012. After observing the respective transits, which for Kepler-90h occurred roughly every 331 days (its orbital period), it was eventually concluded that a planetary body was responsible for the periodic 331-day transits. The discovery, was announced on November 12, 2013.[16]
Artist's impression of the planets of Kepler-90 exoplanetary system compared to the eight planets of the Solar System. Kepler-90h is depicted on the far right, being the largest and outermost planet of the Kepler-90 system.
