Kepler-454b: Rocky or Not?
Planets with radii less than 1.6 Earth radii have rocky, Earth-like compositions, following a single relation between their mass and radius. Planets between 2 and 2.7 Earth radii, however, have lower densities and don’t follow a single mass-radius relation. Their low densities suggest they contain a significant fraction of volatiles, likely in the form of a thick gas envelope of water, hydrogen, and/or helium.
The planet Kepler-454b, discovered transiting a Sun-like star, was initially estimated to have a radius of 1.86 Earth radii — placing it in between these two categories. A team of astronomers led by Sara Gettel (Harvard-Smithsonian Center for Astrophysics) have since followed up on the initial Kepler detection, hoping to determine the planet’s composition.
Kepler-454b’s newly measured size and mass place it firmly in the category of non-rocky, larger, less dense planets (the authors calculate a density of ~2.76 g/cm3, or roughly half that of Earth). This seems to reinforce the idea that rocky planets don’t grow larger than ~1.6 Earth radii, and planets with mass greater than about 6 Earth masses are typically low-density and/or swathed in an envelope of gas.
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