| Here's the paper. http://arxiv.org/pdf/1210.2720.pdf It's reasoned guess. They know three things about the planet: its mass, its size (radius/volume), and its orbital distance. They also know some of the chemistry of the star it orbits. From the mass and radius, they inferred the density of the planet is a lower than other rock planets. They can explain low density in several ways. One is a thick layer of low-density supercritical water on the surface. Very large, extremely hot "oceans". An alternative is low-density graphite or diamond inside the mantle. What they show is that a carbon interior is plausible given the data. They model several things. For one, the planet's formation from a primordial dust disk -- given the star's chemistry, which is unusually carbon-rich, and physical models of how elements separate out in accreting to planets. And they model the planet's current interior, based on known physics. They can't directly observe the planet chemistry; they can only model it. Where are the measurements from? The planet's radius/size was measured photometrically when it passed between us and its star (an occlusion, like an eclipse). They don't have the resolution to see the planet, but they can measure the reduction in the star's brightness when it occludes, and hence how big of a cross-section it has. http://en.wikipedia.org/wiki/Transiting_extrasolar_planets#T... The mass was discovered by measuring the gravitational effect on its star. It is a very tiny effect -- it's too small to see any difference in the star's position. But small changes in velocity shift the frequency of its emitted light, and that can be detected. This is a very small effect: the star's velocity variation due to this planet is just 6.3 meters/second, and it was measured with 0.2 meter/second precision! That's how they know the mass to within 5% accuracy. http://en.wikipedia.org/wiki/Radial_velocity_method http://arxiv.org/pdf/1208.5709v1.pdf The star's element composition is known by spectrometry -- measuring how much of an atom or ion is at its surface, by how much that atom absorbs light of a specific color. http://spiff.rit.edu/classes/phys230/lectures/spec_interp/sp... |