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Earth has a density of about ${\rm 5.513 ~g/cm^3}$ with one Earth radius and one $g$. Venus, a very similar planet to Earth and likely with about the same bulk composition, has a density of about ${\rm 5.24 ~g/cm^3}$ with 0.95 Earth radii and $0.9 \cdot g$. How does this scale? If a planet has 0.5 Earth radii, how would the lower core pressure, lower temperature, and lower gravity impact the density based on the same bulk composition?

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If you know for sure and can prove it: write a paper, submit it to Nature or Science. They will accept it and will give you the cover image.

We can only look at the planetary interiors superficially. We know the bulk mass, we can do a bit tomography with seismics to determine boundaries in the speed-of-sound, with magnetotelurics in the magnetic and electric permeability and maybe a bit with gravity and rotational information on the distribution of masses inside and their surface compositions.

We can make reasonable assumptions on the chemistry and our knowledge how these materials behave. We can even make experiments where we try to re-create the conditions within the interior - but that's where the difficulty starts: temperatures of a few 1000°C and especially pressures over long duration are something which concurrently are hard to re-create in a laboratory (e.g. Dorogokupets et al) , thus many information are from shock-experiments where the pressures only persist very shortly.

In essence the equations-of-state for the materials inside the planets cores are only approximately known, deduced as well as possible, interpreting available information as consistently as possible. This level of detail, in getting all the internal structure right is of course tremendously easier on Earth with all our seismic, magentotelluric and gravity stations and plethora of satellites measuring all things also from the exterior. Data on other planets is MUCH more sparse, so information on the internal structure must be taken with many more grains of salt than the ones of Earth. Many is assumed by analogy, taking into account the few information available while matching the physical models we can make of the chemistry and their assumed equations of state.

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