What have we measured for Kepler-90 b, and what has only been estimated?
Kepler-90 b passes in front of its star approximately every 7 days. During the roughly 4 years Kepler observed this star, there were more than 200 opportunities for transits. Repeated observations allow precise determination of the orbital period and the planet's size, but the mass cannot be known. The currently listed mass of about 2.27 times that of Earth is not a value measured directly from b's gravity. It is an estimate applying statistics from other planets of similar size and known mass.
- Approximately 7.008 days Orbital period measured from repeated transits
- About 1.31 Earth radii Radius derived from transit depth and the star's size
- About 2.27 Earth masses Mass estimated from statistics for similarly sized planets
- About 316 times Earth's Calculated stellar energy received by b
What can repeated transits tell us precisely?
The amount of starlight blocked by b is about 1/10,000 of the star's brightness. If such a decrease in brightness were observed only once, it could be confused with instrumental noise, changes on the star's surface, or brightness changes of a background binary. However, if a signal of the same depth and duration repeats every 7.008 days, the explanation that a planet is passing in front of the star becomes far more certain.
Not all transits were recorded. Observations were sometimes interrupted, overlapped with the transit of another planet, or data was missing due to instrument conditions. Still, because the orbital period is short and many transits are repeated, the next transit time and the average amount of starlight decrease can be calculated accurately. By applying the radius of the central star to this, the radius of b comes out to be about 1.31 times that of Earth.
Repeated transits allow us to know when the planet passes in front of the star and how much starlight it blocks. To determine the planet's mass, the gravitational effects it has on the star or other planets must be measured separately.
Can the starlight blocked by a planet also reveal its mass?
Even planets of the same size can have very different masses depending on how much iron and rock they contain, or whether they have water or a thick layer of gas. Transit observations indicate the planet's size across its disk but do not reveal how much material is inside. To measure mass, astronomers must either measure how fast the star moves back and forth because of the planet's gravity, or analyze how strongly neighboring planets pull on one another and change their transit times.
The mass of Kepler-90 b has not yet been definitively measured by these methods. The approximately 2.27 Earth masses listed in the NASA Exoplanet Archive is an estimated value obtained by applying the radius of b to the size-mass relationship derived from several planets whose masses are already known. It is not the result of measuring b's own gravity, but rather a value that applies the average trend observed in planets of similar size.
What does a density calculated from an estimated mass mean?
Using an estimated mass of 2.27 Earth masses and a radius of 1.31 Earth radii, the average density comes out similar to Earth's. However, since the mass itself was estimated based on the radius, this density cannot be considered a separate measurement result. This is because the density was recalculated from the two values obtained by estimating the mass using the radius.
Statistics from various planets suggest that b may be rock-based, but the proportion of iron cores, the amount of material that evaporates easily like water, and the presence of an atmosphere are unknown. If the actual mass is measured and found to be larger or smaller than the estimate, interpretations of density and internal structure also change. Therefore, you need to check not only the numbers on the list but also how those numbers were obtained.
- Repeated transits Measure an orbital period of approximately 7.008 days and the average decrease in starlight.
- Applying the radius of the star The radius of b is calculated to be about 1.31 times that of Earth.
- Comparison with other planets Statistics for planets of similar size estimate about 2.27 Earth masses.
- Measurement needed for the future The actual mass must be determined by observing the effect of b's gravity on stars or neighboring planets.
Can we know Kepler-90 b’s actual surface temperature?
b is about 0.074 AU, approximately 11.1 million km, away from its star. The energy received by b is calculated to be about 316 times the amount that Earth receives from the Sun. The temperature, calculated with simple assumptions about how much starlight the planet reflects and how heat spreads across the entire planet, is about 1,062 K, or roughly 790 degrees Celsius. It is clear that it is a planet receiving extremely intense heat, but this temperature is not a measurement of the actual surface.
If it has an atmosphere, it can transfer heat from the dayside to the nightside or trap energy at certain wavelengths. If the surface reflects a lot of starlight, the energy absorbed will also decrease. Based on the transit data currently available, we cannot know the state of b's atmosphere and surface. The fact that multiple transits have been observed means that the orbital period and size are known accurately, but it does not mean that the mass, atmosphere, and surface are fully known.
The hot rocky surface shown on the page is not an observed photo but an illustration to aid understanding. The orbital period, the amount of starlight dimming, and the radius are based on b's transit observations. The mass, which is 2.27 times that of Earth, is an estimated value compared to other planets, and the temperature of roughly 790 degrees Celsius is a calculation based on simple assumptions about the atmosphere and reflectivity.
Sources
- NASA Exoplanet Archive — Kepler-90 b Parameters and Mass Provenance
- Astronomy & Astrophysics — Kepler-90 System Analysis
- The Astrophysical Journal — Validation of Multiple Kepler Systems
- The Astrophysical Journal — Probabilistic Mass–Radius Relation
- The Astrophysical Journal — Forecasting Masses from Radii
- NASA Science — Kepler Transit Method
- NASA Exoplanet Archive — Kepler Data Products