Kepler-90 System

How are Kepler-90’s eight planets arranged?

Kepler-90 is known to have eight planets, the same number as the solar system. However, the similarities only extend to the number of planets and the general trend of planets increasing in size from the inside out. The outermost planet, h, orbits its star in about 332 days, and inside it, there are three small planets, three medium-sized planets, and one giant planet. Even with the same number of planets, orbital distances, the energy each planet receives, size, and mass can vary greatly.

• 8 All planets confirmed to pass in front of the star
• About 7–332 days Orbital period from the innermost b to the outermost h
• Within about 1 AU Eight orbits within a range smaller than the distance between Earth and the Sun
• About 2,545 light-years Distance from Earth to Kepler-90

How much does Kepler-90 resemble our solar system?

Near the star, there are b, c, and i. The three planets with radii about 1.2–1.3 times that of Earth orbit the star once in 7 days, 8.7 days, and 14.4 days. The other planets d, e, and f are medium-sized planets with radii about 2.7–2.9 times that of Earth, and their orbital periods are about 60 days, 92 days, and 125 days. The outermost planets g and h are much larger, with radii about 8 times and 11 times that of Earth, respectively.

The fact that smaller planets are on the inside and larger planets are on the outside is similar to the solar system. However, while Mercury orbits the Sun in 88 days, Kepler-90 has five planets with shorter orbital periods than this. The orbit of h is similar in size to Earth's orbit, but Kepler-90 is larger and brighter than the Sun, so the energy h receives is different from what Earth receives. Therefore, calling it a 'miniature solar system' only compares the approximate arrangement of the planets and does not mean that the environment is the same.

Kepler-90 and the solar system are only similar in that they both have eight planets and the outer planets are generally larger. The distances between planets, their masses, and climates are not the same.

Why do the planet names differ from their actual order by distance?

If you read in order of proximity to the star, the planet names are b-c-i-d-e-f-g-h. The reason the third planet is i is that this planet was confirmed the latest in 2017. The lowercase letters attached to exoplanets indicate the order in which they were discovered or confirmed, not their distance from the star.

i was not discovered with a new telescope but was found in already stored Kepler data. The research team trained artificial intelligence on 15,000 signals that humans had judged as planetary, and then reanalyzed weak signals from 670 stars with multiple known planets. The AI selected candidates that repeated every 14.4 days, and the researchers verified them as planets by separately examining the influence of surrounding starlight, binaries, instrument noise, and transit shapes. The AI prioritized the candidates to be reviewed, and the final determination was made through astronomical observation and analysis.

How do Kepler-90 g and h differ?

Kepler observed g’s transit six times and h’s transit only three times. Because the two planets gravitationally pull on each other, significantly changing the timings as they pass in front of the star, it was difficult to accurately calculate the next transit time and mass with few observations. In 2025, the research team analyzed measurements of the star’s movement taken 34 times over 10 years together with existing transit timings. The transit of g in May 2024, predicted by this calculation, was also observed again with ground-based telescopes.

The masses calculated including new observations were 15.0±1.3 times that of Earth for g and 203±16 times that of Earth for h. The radius of g is similar to Saturn, but its mass is smaller than Neptune, resulting in a very low average density. h is about 14 times heavier than g. Even if both planets appear large in images, it cannot be assumed that their internal material and structure are the same.

1. 2013~2014 Several studies confirmed the transit signals and determined the orbits of seven planets gathered close to the star.
2. 2017 A neural network found candidate i in archived data, and the research team verified it as the eighth planet.
3. 2024 Ground-based telescopes observed the transit of g again at the predicted time calculated.
4. 2025 By analyzing the transit times and the star's motion together, the masses of g and h and the timings of their next transits were updated.

Could Kepler-90 have planets we have not found yet?

All eight known planets pass in front of their star as seen from Earth. This means that the orbits of the planets are almost aligned with our line of sight. A planet with a slightly more tilted orbit does not pass in front of the star, so it cannot be discovered using this method. Planets may also be missed if they are too small, causing the decrease in starlight to be hidden in the noise, or if their orbital period is long, so that passages did not repeat enough during Kepler's approximately four years of observation.

There is a larger difference between the orbital periods of planet i, which is about 14 days, and planet d, which is about 60 days, compared to other intervals. It is unknown whether this interval is actually empty or if there are planets that either do not pass in front of the star or are too small to have been discovered yet. The fact that eight planets are currently known only indicates the confirmed number and does not represent the final number of planets in this planetary system.

The planets shown on the page are illustrations based on size and representative color and are not photographs of the surfaces. While the transits of the eight planets have been confirmed, the masses, atmospheres, and internal materials of most of the inner planets are not yet well known.

Source

• NASA Exoplanet Archive — Kepler-90 System
• NASA Science — A Near-Twin of Our Solar System?
• Astronomy & Astrophysics — A Compact Analogue to the Solar System
• The Astrophysical Journal — Validation of Multiple Kepler Systems
• The Astronomical Journal — Identifying Exoplanets with Deep Learning
• The Astronomical Journal — Giant Transit-timing Variations
• The Astronomical Journal — Updated Masses for Kepler-90 g and h