Kepler-90h

Artist impression of Kelper-90 h and its hypothetical exomoon.
Discovery
Discovered by	Kepler spacecraft
Discovery date	November 12, 2013[1]
Detection method	Transit[2]
Orbital characteristics
Semi-major axis	1.01 ± 0.11 AU (151,000,000 ± 16,000,000 km)[1]
Eccentricity	0.0 ≤ 0.001[1]
Orbital period (sidereal)	331.60 ± 0.00037[1] d
Inclination	89.6 ± 1.3[2]
Star	Kepler-90
Physical characteristics
Mean radius	1.01 (± 0.09)[3] RJ
Mass	0.639±0.016[4] MJ
Temperature	292 K (19 °C; 66 °F)[2]

Kepler-90h (also known by its Kepler Object of Interest designation KOI-351.01) is an exoplanet orbiting within the habitable zone of the early G-type main sequence star Kepler-90, the outermost of eight such planets discovered by NASA's Kepler spacecraft. It is located about 2,840 light-years (870 parsecs), from Earth in the constellation Draco. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.

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]

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 R_J, 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 period P_s around its primary and that of the primary around its star P_p 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 M_E.^[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 M_E.^[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]

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]

• Kepler-47c
• HD 69830 d