Kepler-90

Kepler-90, also designated 2MASS J18574403+4918185, is a G-type main sequence star located about from Earth in the constellation of Draco. It is notable for possessing a planetary system that has the same number of observed planets as the Solar System.
On 14 December 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 history.
Prior to 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 system 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 designations "b", "c", "d", "e", "f", "g", "h", and "i" derive 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 characteristics.
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. It is too dim to be seen with the naked eye, which typically can only see objects with a magnitude around 6 or less.
Planetary system.
Kepler-90 is notable for similarity of the configuration of its planetary system to that of the Solar System, in which rocky planets are nearer the star and gas giants farther away. The six inner planets range from super-Earths to mini-Neptunes in size. The two outermost planets are gas giants. The most distant known planet orbits its host star at about the same distance as Earth from the Sun.
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. All of the eight known planet candidates orbit within about 1 AU from Kepler-90. A Hill stability test and an orbital integration of the system show that it is stable.
The five innermost exoplanets, Kepler-90b, c, i, d, and e may be tidally locked, meaning that one side of the exoplanets permanently faces the star in eternal daylight and the other side permanently faces away in eternal darkness.
A 2020 analysis of transit-timing variations of the two outermost planets Kepler-90g and h found best-fit masses of and , respectively. Given a transit-derived radius of , Kepler-90g was found to have an extremely low density of , unusually inflated for its mass and insolation. Several possible explanations for its apparently low density include a puffy planet with a dusty atmosphere or a smaller planet surrounded by a tilted wide ring system (albeit the latter option is less likely due to the lack of evidence for rings in transit data).
Near resonances.
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 (as exemplified by g and h in this system) facilitates the formation of closely packed resonances among inner super-Earths. The semimajor axis of any additional nontransiting outer gas giant must be larger than 30 AU to keep from perturbing the observed planetary system out of the transiting plane.