# Why are the cores of planets melted?

We all know that the core of Earth is in liquid form, as are most other planets as recently read. Why is this the case? Obviously, high temperatures are the cause, but what causes the high temperatures?

• en.wikipedia.org/wiki/Planetary_core – user83548 Nov 19 '15 at 21:46
• Indeed the core core of Earth is solid. And for small telluric objects (e.g. the Moon) the interior is totally solidified. – Fabrice NEYRET Nov 19 '15 at 23:30
• Effective duplicate: physics.stackexchange.com/q/66169 Thought the best answer to the several similar questions is (IMHO, of course): physics.stackexchange.com/a/154514. – dmckee --- ex-moderator kitten Nov 20 '15 at 0:58
• @FabriceNEYRET - Contrary to pop sci nonsense, the Moon's core almost certainly is not totally solidified (nor is Mars' for that matter). The Moon's observed $k_2$ Love number is too high for a solid object made of anything short of some weird species of unobtanium; the same goes for Mars. – David Hammen Nov 20 '15 at 5:46
• @dmckee - Thanks! I spent a good deal of time researching that answer, a lot more time compared to the paltry number of points I received for that answer. – David Hammen Nov 20 '15 at 5:50