Kepler-138
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==Planetary system== |
==Planetary system== |
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The three inner known planets of Kepler-138 [[Astronomical transit|transit]] the star; this means that all three planets' orbits appear to cross in front of their star as viewed from the Earth's perspective. Their [[inclination]]s relative to Earth's line of sight, or how far above or below the plane of sight they are, vary by less than one degree. This allows direct measurements of the planets' orbital periods and relative diameters (compared to the host star) by monitoring each planet's transit of the star. There is also a likely fourth non-transiting planet, Kepler-138e, detected through [[transit-timing variation]]s.namenasa |
The three inner known planets of Kepler-138 [[Astronomical transit|transit]] the star; this means that all three planets' orbits appear to cross in front of their star as viewed from the Earth's perspective. Their [[inclination]]s relative to Earth's line of sight, or how far above or below the plane of sight they are, vary by less than one degree. This allows direct measurements of the planets' orbital periods and relative diameters (compared to the host star) by monitoring each planet's transit of the star. There is also a likely fourth non-transiting planet, Kepler-138e, detected through [[transit-timing variation]]s.name="NASA2022"/> |
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Although the innermost planet has a size similar to [[Mars]], Kepler-138c and d both have a radius of about 1.5 [[Earth radius|Earth radii]] (revised from earlier estimates of 1.2 Earth radii). Although Kepler-138c and d have similar radii, their masses and densities were initially thought to vary greatly. Of these two, the inner planet was thought to be consistent with a rocky [[super-Earth]], whereas the outer planet's low density implies it may have a substantial proportion of water ice or a significant gas envelope, resembling a miniaturized [[gas giant]] (a [[gas dwarf]]). The striking differences between these two planets have been hypothesized to be due to [[photoevaporation]]. However, more recent observations as of 2022 have found similarly low densities for both planets c and d, suggesting that they are likely to be [[ocean world]]s. The mass of candidate Kepler-138e would be intermediate of Mars and [[Venus]]. While a radius could not be estimated for planet e, it is likely smaller than c and d and larger than b, which is consistent with an Earth-like composition. |
Although the innermost planet has a size similar to [[Mars]], Kepler-138c and d both have a radius of about 1.5 [[Earth radius|Earth radii]] (revised from earlier estimates of 1.2 Earth radii). Although Kepler-138c and d have similar radii, their masses and densities were initially thought to vary greatly. Of these two, the inner planet was thought to be consistent with a rocky [[super-Earth]], whereas the outer planet's low density implies it may have a substantial proportion of water ice or a significant gas envelope, resembling a miniaturized [[gas giant]] (a [[gas dwarf]]). The striking differences between these two planets have been hypothesized to be due to [[photoevaporation]]. However, more recent observations as of 2022 have found similarly low densities for both planets c and d, suggesting that they are likely to be [[ocean world]]s. The mass of candidate Kepler-138e would be intermediate of Mars and [[Venus]]. While a radius could not be estimated for planet e, it is likely smaller than c and d and larger than b, which is consistent with an Earth-like composition. |
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