How different is Kepler-90 h from Earth?
Kepler-90 h is about 0.97 AU away from its host star and completes one orbit every 331.603 days. The data also lists a calculated temperature of about 294K, roughly 21 degrees Celsius. While this evokes Earth's orbit and familiar temperatures, it is not an actual measured weather value. h is a Jupiter-sized gas planet that receives about 1.8 times as much energy from its star as Earth does. Its radius is about 11.25 times Earth's and its mass is 203±16 times Earth's, while its atmospheric composition and actual temperature have not been confirmed.
- 331.603 days The time it takes for h to orbit its star
- About 0.97 AU Average distance between the star and h
- About 1.8 times that of Earth The calculated energy h receives from its star
- 203±16 times that of Earth Mass analyzed using both transit timing and radial velocity
Can distance from the star alone reveal a planet’s climate?
1 AU refers to the average distance between the Earth and the Sun, about 150 million km. Being at this distance from any star does not mean there will be liquid water or a temperate climate. The energy a planet receives depends not only on distance but also on how bright the central star is. Kepler-90 is slightly larger and brighter than the Sun, so planet h receives about 1.7 to 1.85 times the amount of energy that Earth receives from the Sun, even at a similar distance from its star.
A 331.603-day orbital period does not signify an environment like Earth's 365.26-day year. Orbital time is determined by both the distance and the mass of the central star. Kepler-90 is heavier than the Sun, causing a planet at a similar distance to orbit faster. Distance and period describe the orbit but do not provide information about clouds, wind, pressure, or the atmosphere's greenhouse effect.
0.97 AU is the distance between the star and h, and 331.603 days is the orbital period. These two values alone cannot reveal h's actual temperature and atmospheric conditions.
Which assumptions affect the calculated temperature?
The equilibrium temperature is a value calculated under the assumption that the energy a planet receives from its star is equal to the heat it radiates into space. The representative calculation used in Kepler data assumes that the planet reflects 30% of the incoming light and that the absorbed heat is evenly distributed between the day side and the night side. The 292–294K for h is a value calculated under these conditions and is not a temperature directly measured in the atmosphere or on the surface.
Converting 294K to Celsius gives about 21 degrees. However, how much starlight h reflects and how much heat from the day side moves to the night side have not been measured. Clouds, greenhouse effects, and heat coming from inside the planet can also change the actual temperature. On Earth, for instance, the average surface is more than 30 degrees warmer than the calculated temperature assuming no atmosphere due to the atmospheric greenhouse effect. Therefore, the climate of h cannot be considered similar to Earth's just because the calculated value is similar to familiar temperatures.
Does Kepler-90 h have a solid surface?
The radius of h, determined from the starlight blocked during transit, is about 11.25 times that of Earth, almost the same as Jupiter. The mass from the joint analysis in 2025, 203±16 Earth masses, is about 64% of Jupiter's mass. Combining these two values as the average for a sphere gives a density of about 0.79 g/cm³. This is lower than Jupiter and close to the density of Saturn, characteristic of a gas giant.
The radius of a gas planet is determined based on the height of the atmosphere where starlight can no longer easily pass through. As you go further down, the gas becomes increasingly dense, and both pressure and temperature rise, but unlike Earth's seas and continents, there is no distinct solid boundary. Therefore, saying that h has a 'surface at 21 degrees Celsius' is inconsistent with observational results and does not fit the structure of a gas planet.
How was Kepler-90 h’s mass measured?
The main mission of Kepler recorded three transits of the long-period h. The transit depth indicates its size but does not directly give its mass. Clues about mass came from changes in the transit timing caused by the mutual gravitational pull between the inner g and h. Since h is much heavier than g, it causes significant changes in g's transit timing, but with few observations, the mass and orbital shape can be explained in multiple ways.
The 2020 study calculated the mass of h to be about 203 times that of Earth based on the variations in transit timing observed by Kepler. The 2025 study analyzed the movement of the host star measured 34 times between 2011 and 2022, along with additional transit timings and the transit of g observed from the ground in 2024. In this calculation as well, the mass of h was found to be 203 ± 16 times that of Earth. It is important that even when different data are combined, the result of about 203 times came out again. The errors of ±5 in 2020 and ±16 in 2025 cannot be compared to determine which study is more accurate, as the data used and the calculation methods differ.
- 2009~2013 Kepler records three long transits of h and the transit timing variations of the neighboring g.
- 2020 A study analyzing only the transit timing variations of g and h estimates the mass of h to be about 203 times that of Earth.
- 2011~2022 The HIRES spectrograph measures the speed at which the host star moves due to the gravity of the two outer planets 34 times.
- 2024~2025 A joint analysis of a new transit, additional transit times, and radial velocities again finds a mass of 203±16 Earth masses for h.
What must we observe to understand Kepler-90 h’s actual climate?
What is currently known relatively accurately about h is its orbital period, radius obtained from transits, mass derived from analyzing various data together, and the amount of energy calculated from starlight. It has not been measured which molecules are in the atmosphere, how high the clouds are, how much starlight is reflected, or how heat from the bright side moves to the dark side. No satellites orbiting h have been discovered, so one should not speak as if the possibility of a temperate satellite is an observational fact.
h passes in front of its star only about once every 332 days, so opportunities to observe the atmosphere are rare. Transit times can also continue to vary due to the gravity of neighboring g, so the exact timing of the next transit must be predicted. In the future, the absorption of various types of light by h's atmosphere must be measured to determine its composition and temperature. Until then, 294K is only a calculated value used in preparation for observations, not evidence that h is temperate like Earth.
The planet colors and clouds on this page are not from actual photographs. Kepler-90 h has not been photographed as a separate disk from its star, and its current appearance is a visualization based on its radius, mass, and received light.
Sources
- NASA Exoplanet Archive — KOI-351 h Parameters and Multiple Solutions
- NASA Exoplanet Archive — Equilibrium Temperature and Insolation Definitions
- NASA Kepler — Q1–Q17 Transit Signal and Equilibrium-Temperature Assumptions
- The Astronomical Journal — Updated g and h Masses from Transit Timing and Radial Velocities
- The Astrophysical Journal — Giant Transit-Timing Variations of Kepler-90 g and h
- The Astrophysical Journal — Discovery and Analysis of the KIC 11442793 Planetary System
- NASA Science — Gas Giants and the Absence of a Hard Surface
- NASA Earth Observatory — Atmosphere and Surface Temperature