Kepler-90 d

Could another planet lie between Kepler-90 i and d?

The orbital period of Kepler-90 i is about 14.45 days, and the next confirmed period of d is about 59.74 days. In this region, where the orbital period increases more than four times at once, no planets have yet been discovered. However, the fact that they have not been discovered is not evidence that there are no planets. A 2025 study calculated that a planet 1.3 times the size of Earth, in the middle orbit of this region, passing in front of the star, might have less than a 50% chance of being detected in the Kepler data.

• About 59.74 days The orbital period of d measured by repeated transits
• About 4.13 times Orbital period increase from i to d
• About 2.87 times the Earth Radius of d calculated from transit depth and star size
• Less than 50% The rate of detecting the signal of a planet 1.3 times the size of Earth placed in the middle orbit

Why are transits of small planets easy to miss?

If the currently known planets are arranged in order of proximity to the star, they are b, c, i, d. The difference in orbital periods between i and d is about 45.3 days, and the midpoint based on the period is about 29.4 days. If there is a planet orbiting with this period, it could pass in front of the star roughly 50 times during the approximately 4 years that Kepler observed. Although the number seems high, if the amount of starlight blocked by the planet is very small, it is difficult to detect amid noise.

When a planet 1.3 times the size of Earth passes in front of Kepler-90, the starlight decreases by about one ten-thousandth. If observation interruptions, intrinsic stellar brightness variations, instrument noise, and signals from several already known planets overlap, such small changes can be buried. In fact, i, which is similar in size, was present in the same data but was missed in several analyses. In 2017, artificial intelligence selected this signal as a priority candidate for review, and after separate statistical verification, it was confirmed as a planet.

Even planets that pass in front of the star multiple times can be missed in the data if the signal is small. The fact that a signal has not yet been found cannot alone lead to the conclusion that a planet does not exist.

How does the chance that a planet exists differ from the chance of detecting it?

In 2025, the research team placed a hypothetical planet between i and d and tested two things. First, they calculated whether this planet could maintain its orbit for a long time without colliding with surrounding planets. In several calculations with masses ranging from a few times to tens of times that of Earth, the hypothetical planet maintained a stable orbit. This means that there is space for a planet to exist between i and d, but it does not mean that an actual planet has been discovered.

Next, the researchers calculated how often such a hypothetical signal would be detected, reflecting the noise of real Kepler data and the planet search process. When a planet 1.3 times the size of Earth was placed in a middle orbit, even when set to pass in front of the star, more than half of the occurrences were missed. Therefore, there is a possibility that the transit of a small planet remains in the actual data. The explanation that the gravity of the outer giant planet tilts the hypothetical planet's orbit so it does not pass in front of the star did not show a sufficient effect in the calculations.

The calculation that a stable orbit is possible does not mean a hidden planet has been discovered. The result that the influence of the outer giant planet was weak does not imply that all planets that do not pass in front of the star are absent. The current confirmed fact is not that there is no planet between i and d, but that there is not yet a reliable additional signal.

Can Kepler-90 d’s size explain the apparent gap in the orbits?

The radius of d is about 2.87 times that of Earth, making it much larger than the inner planets b, c, and i. This size is commonly observed in planets with materials that evaporate easily like water over a rocky core or in planets with significant hydrogen and helium atmospheres. However, size alone cannot determine the internal composition. The approximately 8.6 Earth masses listed in the NASA Exoplanet Archive for d is not a direct measurement of its gravity but an estimate based on statistics of other planets of similar size.

The sudden increase in planetary size at d following i provides clues for studying how planetary systems form and lose their atmospheres. However, because there is a large gap between i and d, there is no evidence that d grew or that it pushed out surrounding smaller planets. To test such explanations, the actual mass and orbital shape of d, as well as the existence of intermediate planets, need to be determined.

1. Current observations No transit signals have been detected between i's 14.45-day period and d's 59.74-day period.
2. Searching for hypothetical signals Signals of a hypothetical planet with a set size and period are inserted into the data, and the frequency of their detection is calculated.
3. Orbital stability calculations Check whether a hypothetical planet can maintain its orbit without colliding with surrounding planets.
4. Additional Observations Reanalyze the star's brightness data and look for the influence of additional planets from the transit times and the star's movement.

How are d’s environment and a possible hidden planet related?

d is about 0.32 AU, approximately 47.9 million km, away from the star. The energy received by d is calculated to be about 18 times what Earth receives from the Sun. Assuming a simple level of starlight reflection and heat transfer, the temperature is about 520 K, around 247 degrees Celsius. This value is not the temperature measured in the actual atmosphere or on the surface, and d's clouds, color, and internal features have also not been directly observed.

To find additional planets, Kepler's star brightness data must be reanalyzed using more sensitive methods, and it must be checked whether suspected signals repeat with the same period and pattern. There are also methods to detect changes in the star's motion speed or the transit times of existing planets caused by planetary gravity, but it is difficult to measure in distant and faint stars like Kepler-90. Currently, observational data is insufficient to determine whether there is an additional planet between i and d.

Sources

• NASA Exoplanet Archive — Kepler-90 d Parameters and Mass Provenance
• The Astronomical Journal — The Gap–Giant Association: Are Planets Hiding in the Gaps?
• The Astronomical Journal — Identifying Exoplanets with Deep Learning
• The Astronomical Journal — Final Kepler Mission Transiting Planet Search
• NASA Exoplanet Archive — Kepler Completeness and Reliability
• Astronomy & Astrophysics — The Planetary System to KIC 11442793
• NASA Science — Kepler Transit Method