astronomy

Kepler-90

Kepler-90 is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Kepler-90 rather than just read about it. In short: Kepler-90, also designated 2MASS J18574403+4918185, is an F-type star located about 2,790 light-years (855 pc) from Earth in the constellation of Draco. It is notable for being the only confirmed planetary system with the same number of observed planets as the Solar System.

Key takeaways

Kepler-90 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kepler-90 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kepler-90 from memory before moving on to harder problems.

Reference excerpt

Kepler-90, also designated 2MASS J18574403+4918185, is an F-type star located about 2,790 light-years (855 pc) from Earth in the constellation of Draco. It is notable for being the only confirmed planetary system with the same number of observed planets as the Solar System.

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 was 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 whole star and planet system is designated by just "Kepler-90", without a postfix, with Kepler-90A specifically referring only to the star, if needed for clarity. The first planet discovered is Kepler-90b, with subsequently discovered planets given subsequent lowercase letters in order of discovery, up to Kepler-90i, for the last planet found to date.

Stellar characteristics Kepler-90 is an F-type star that is approximately 120% the mass and radius of the Sun. It has a surface temperature of around 6,100 K. In comparison, the Sun has a surface temperature of 5,772 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.

Planetary system

Kepler-90 is notable for sharing similarities with the planetary system 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. On 14 December 2017, NASA and Google announced the discovery of an eighth exoplanet, Kepler-90i, in the Kepler-90 system. The discovery was made using a new machine learning method developed by Google. The Kepler-90 system is the only eight-planet 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 of 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 are likely 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 15+0.9−0.8 M🜨 and 203±5 M🜨, respectively. Given a transit-derived radius of 8.13 R🜨, Kepler-90g was found to have an extremely low density of 0.15±0.05 g/cm3, unusually inflated for its mass and insolation. Several proposed 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). A 2024 study updated the mass of Kepler-90g and h to 15.0±1.3 M🜨 and 203±16 M🜨 respectively with the inner planets' masses being estimated using the measured radius and an empirical mass–radius relation.

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 non-transiting outer gas giant must be larger than 30 AU to keep from perturbing the observed planetary system out of the transiting plane.

See also TRAPPIST-1, star with seven known exoplanets HD 10180, star with at least six known exoplanets, and three exoplanet candidates Kepler-385, star with at least three known exoplanets, and four more candidates HD 219134, star with six exoplanets 55 Cancri, star with multiple planets Tau Ceti, star with at least four exoplanets and four more candidates

Footnotes

References

External links NASA Astronomy Picture of the Day: The Kepler 90 Planetary System (18 December 2017)

Illustrations

Worked examples

Example 1 — a first encounter with Kepler-90

Start with the simplest possible case. Write down what Kepler-90 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Kepler-90 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Kepler-90 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Kepler-90

In research: Kepler-90 appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Kepler-90 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Kepler-90 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Draco (constellation), F-type main-sequence stars, Kepler-90, so understanding it makes those chapters shorter.
In everyday life: Look for Kepler-90 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Kepler-90 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kepler-90 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Kepler-90 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kepler-90 in simple terms?

Why does Kepler-90 matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Kepler-90?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Kepler-90.