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Kepler-90 is a G-type main-sequence star located 2,545 light years away from Earth in the constellation Draco. It is most notable for its planetary system of eight planets, equal to to the number of planets in the Solar System.

On December 14, 2017, NASA and Google announced the discovery of an eighth planet, Kepler-90i, in the Kepler-90 system: the discovery was made using a new machine learning method developed by Google.

Nomenclature and historyEdit

Prior to the Kepler observation, Kepler-90 had the 2MASS catalogue number "2MASS J18574403+4918185". It has the designation of KIC 11442793 in the Kepler Input Catalog, and given the Kepler object of interest number of KOI-351 when it was found to have a transiting planet candidate.

The star's planetary companion was discovered by NASA's Kepler Mission, a mission tasked with discovering planets in transit around their stars. The transit method that Kepler uses involves detecting dips in brightness in stars. These dips in brightness can be interpreted as planets whose orbits move in front of their stars from the perspective of Earth. The name Kepler-90 derives directly from the fact that the star is the catalogued 90th star discovered by Kepler to have confirmed planets.

The designation b, c, d, e, f, g, h, and i derives from the order of discovery. The designation of b is given to the first planet orbiting a given star, followed by the other lowercase letters of the alphabet. In the case of Kepler-90, there are eight planets discovered, so designations up to i are used.

Stellar characteristicsEdit

Kepler-90 is a G-type star that is approximately 120% the mass and radius of the Sun. It has a surface temperature of 6080 K, and an estimated age of around 2 billion years. In comparison, the Sun is about 4.6 billion years old and has a surface temperature of 5778 K.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 14, which makes it is too dim to be seen with the naked eye.

Planetary systemEdit

Kepler-90 is notable for similarity of its planetary system, which is very similar to that of the Solar System. For example, rocky planets are near the star, while gas giants are farther away. The six inner planets are either super-Earths or mini-Neptunes due to their size. Two of the outer planets are gas giants. The penultimate known planet orbits its host star at about the same distance as Earth from the Sun. The outermost planet has yet to be fully researched.

Kepler-90 was used to test the "validation by multiplicity" confirmation method for Kepler planets. Six inner planets met all the requirements for confirmation. The penultimate planet showed transit-timing variations, indicating that it is a real planet as well.

The Kepler-90 system is the only eight-planet candidate system from Kepler, and the second to be discovered, after the Solar System. It was also the only seven-planet candidate system from Kepler before the eighth was discovered in 2017, and one of two total seven-planet systems, along with TRAPPIST-1. Additionally, the inner six planets range in size from that of Earth to smaller than Neptune, and the outer two planets are the size of gas giants. All of the eight known planet candidates orbit within 1 AU from Kepler-90. A Hill stability test and an orbital integration of the system show that it is stable.

Planet Mass Semimajor axis Orbital period (days) Eccentricity Inclination Radius
b N/A 0.074 AU 7.008151 days N/A 89.4° 1.31 Earth radius
c N/A 0.089 AU 8.719375 days N/A 89.68° 1.18 Earth radius
d N/A 0.32 AU 59.73667 days N/A 89.71° 2.88 Earth radius
e N/A 0.42 AU 91.93913 days N/A 89.79° 2.67 Earth radius
f N/A 0.48 AU 124.9144 days 0.01 89.77° 2.89 Earth radius
g <0.8 Jupiter mass 0.71 AU 210.60697 days N/A 89.8° 8.13 Earth radius
h <1.2 Jupiter mass 1.01 AU 331.60059 days N/A 89.6° 11.32 Earth radius
i N/A 0.1234 AU 14.44912 days N/A 89.2° 1.32 Earth radius

Near resonancesEdit

Kepler-90's eight known planets all have periods that are close to being in integer ratio relationships with other planets' periods; that is, they are close to being in orbital resonance. The period ratios b:c, c:i and i:d are close to 4:5, 3:5 and 1:4, respectively (4:4.977, 3:4.97 and 1:4.13) and d, e, f, g and h are close to a 2:3:4:7:11 period ratio (2:3.078:4.182:7.051:11.102; also 7:11.021). f, g and h are also close to a 3:5:8 period ratio (3:5.058:7.964). Relevant to systems like this and that of Kepler-36, calculations suggest that the presence of an outer gas giant planet facilitates the formation of closely packed resonances among inner super-Earths