Do all planets have a molten core? - Astronomy Stack Exchange

The Earth's inner core is the Earth's innermost part and according to seismological studies, it is primarily a solid ball with a radius of about 1220 kilometers, or 760 miles (about 70% of the Moon's radius). It is believed to consist primarily of an iron–nickel alloy and to be approximately the same temperature as the surface of the Sun: approximately 5700 K (5400 °C).
$\begingroup$I think Earth's outer core layer is still molten but the inner core is solid (due to pressure, etc). Also the solid part is growing as the molten part around it crystallises. So the question may benefit from editing to make it more clear. (Still a good question in my opinion.)$\endgroup$
$\begingroup$@AustinPhilips, your own reference provides a counterexample to your question: The Earth. The Earth has a solid inner core. If you ask whether planets have a partially liquid core, that's a different question.$\endgroup$
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The short answer is no. Take Mercury for example in this comparison of Earth Mercury core. Mercury is thought to have a liquid outer core and solid inner core. The gas giants like Jupiter are thought to have a relatively tiny rocky core but the convective motion in the metallic hydrogen is what gives them the strong magnetic fields.
A lot of the data we have on the inner structure of the Earth comes from seismology - the study of earthquake waves travelling through the ground. This requires sensors placed at different points around the planet to pick up the waves and infer the structures they passed through. I haven't looked into it but I would think that this would be very difficult to do on other planets in terms of getting lots of sensors there and positioning them correctly, so maybe we just don't know this sort of thing about other planets yet!
$\begingroup$Adding to this: Seismometers have been placed on the moon, from which we know that the moon has no liquid core. The Insight-mission that arrived at Mars in late 2018 placed the second seismometer on another planet (Vikings did it before, but results were confusing). Insight follows a new concept which allows to determine some structure and molten/solidified state of the core with a new single-seismometer approach, so in 2021 we'll maybe know if Mars has a molten core.$\endgroup$
Depends on what you call "planet." For example, in comparison to Earth, the Earth and Moon is a "binary planet system." And the moon doesn't have any molten core. Also, you said inner. Inner core of Earth (and many other planets) are solid because of (what I think is pressure). Finally, if it is outer core you meant, and you didn't count the moon as a "planet", would you count extrasolar planets as "planets". Please clarify, but to your current question, no.
Saturn has a rock or ice core thirty times the mass of earth while jupiter has ice core ten-thirty times the mass of earth.Mars has a liquid core,moon however doesn't have a core .Composition of venus core varies significantly (Iron-88%,nickel-5%,sulphur-5%).Mercury Core occupies 84% of planets radius making it the largest core relative to the size of the planet in our solar system .While Gilese 1214 b is an exoplanet 42 light years from the sun has ice core.