How does a revised size for Kepler-90 change what we know about its planets?
The Kepler space telescope did not directly see the diameters of the planets in Kepler-90, but measured how much light each planet blocked from the star. To convert this ratio into the actual size of the planet, the radius of the host star must be known. For example, if the star is 5% larger than previously estimated, the radii of all eight planets blocking the same amount of starlight will also be about 5% larger. Therefore, the more accurately the host star is measured, the more accurate the planetary sizes become.
- About 6,000K The surface temperature of the star calculated in various studies
- About 1.2 times the Sun The mass of the star used to calculate planetary values in this document
- About 1.2 times the Sun The radius of the star used to calculate planetary values in this document
- About 848 parsecs The distance from Earth corresponding to about 2,770 light-years
How can blocked starlight reveal a planet’s size?
If we simply assume that a planet passes through the center of a star, the proportion of starlight that dims is approximately equal to the square of the ratio of the planet's radius to the star's radius. If the starlight decreases by one ten-thousandth, it means the planet's radius is about 1/100 of the star's radius. At this stage, what can be known is not the actual length but the size relative to the star.
To convert that 1/100 into kilometers or Earth radii, we need to set Kepler-90's radius to 1.2 times that of the Sun. In actual analysis, calculations also account for which part of the star the planet passes over, how much dimmer the star's edge is compared to the center, and whether light from other stars is mixed in. Even after such corrections, the fact that the planet's actual radius depends on the central star's radius remains the same.
What Kepler directly measured is the proportion of starlight blocked by the planet. The planet's actual radius is calculated using this proportion along with Kepler-90's radius.
How is distance from the star calculated from orbital period?
The fact that the transit of the planet repeats every 14.4 days can be directly confirmed from the star's brightness data. On the other hand, the distance of the planet from the star is calculated using the orbital period and the mass of the central star. If the star's mass differs by 20%, the calculated orbital distance changes by about 6%. This change affects the distance calculations of all eight planets orbiting the same star.
To determine the energy received by the planet, the star's radius and temperature are also needed. The larger and hotter the star, the more energy it emits, and the farther the planet, the less energy it receives. Therefore, even though h's orbital period of 332 days is similar to Earth's 1 year, h and Earth do not receive the same amount of energy.
How does Gaia measure the distance to a star?
While Earth orbits the Sun, nearby stars appear to move slightly against the background of very distant stars. Gaia measured these small angular changes observed in Kepler-90, and a smaller angle indicates that the star is farther away. The current listed distance of about 848 parsecs corresponds to approximately 2,770 light-years. The difference from the approximately 2,545 light-years announced by NASA in 2017 is not because the star moved that much, but because Gaia's data and calculation methods have been updated.
By combining the distance and brightness measured from various types of light, the total energy a star actually emits can be estimated. Spectroscopic observations that divide starlight in detail determine the ratio of surface temperature to heavy elements like iron. The researchers compare these values with the stellar evolution model to calculate mass, radius, and age. Each study used different data and methods, resulting in differences such as approximately 5,970K and 6,238K. About 6,000 K, roughly 1.2 times that of the Sun, is a representative value from various studies and is not a measurement without error.
- Changes in star brightness It measures the ratio of transit periods to the radii of planets and stars.
- Spectral observation It estimates the star's effective temperature, metal content, and surface gravity.
- Gaia's distance measurement It calculates the star's distance and actual brightness more accurately.
- Recalculating Planetary Values The newly obtained star values update the radii, orbital distances, and incoming energy of the eight planets.
How do we distinguish uncertainties in the star from those in its planets?
If the radius of the star is modified, the radii of all eight planets also change in the same proportion. However, this does not mean that the measurement errors of each planet are exactly the same. Smaller planets block less starlight and are more affected by noise. Planets like g and h, which have long orbital periods and few observed transits, have greater errors in determining their periods and orbits. The extent to which planets mutually attract each other and alter transit times, as well as the degree to which the star moves due to planetary gravity, also creates separate errors in calculating masses.
Therefore, if the host star is measured accurately, the sizes and orbits of the eight planets can be calculated more precisely, but this does not mean we can know all information about each planet. The temperature, radius, and mass of Kepler-90 are the basic values needed to interpret the planets. Additional observations are required for each planet to determine their masses, atmospheres, and internal materials.
Kepler-90 is too far away to distinguish a round surface in photos and appears as a single point. Its temperature, mass, radius, and distance are values calculated after measuring starlight and positional changes, along with stellar evolution models. Representative values vary slightly depending on the data used and the research methods.
Source
- NASA Exoplanet Archive — Kepler-90 Stellar Parameters
- Astronomy & Astrophysics — KIC 11442793 Stellar and Planet Analysis
- The Astronomical Journal — Kepler-90 i Transit Validation
- The Astronomical Journal — Precise Planet Radii with Gaia
- The Astronomical Journal — Gaia–Kepler Stellar Properties Catalog
- The Astronomical Journal — Kepler-90 Radial Velocities and Transit Timing
- NASA Science — Kepler Transit Method