This is a difficult question to answer in much detail and with certainty because there are quite a few large uncertainties involved. A body such as Mars is smaller than Earth, so it is expected to cool much more quickly. read more
Very little is known either directly or indirectly about the core and interior structure of Venus: it's true that the planet has a weak magnetic field, but it is certainly thought by some at least that the core may be partly liquid and that it has a liquid mantle. read more
The layer outside the inner core is the outer core, with a thickness of approximately 1,430 miles. The outer core consists primarily of iron and nickel, two valuable metals found in abundant amounts on the planet. read more
Earth's inner core is solid even though the temperature is so high because the pressure is also very high. According to the Wikipedia article on the Earth's inner core, the temperature at the center is $5,700\ \text{K}$ and the pressure is estimated to be $330$ to $360\ \text{GPa}$ ($\sim3\cdot10^{6}\ \text{atm}$). read more