It solidified in the heat, like an egg

Guess the sun is so hot that Mercury gets hard.

Relativistic quantum chemistry must be the reason.

Actually, it's very cold in space. Mercury isn't that close to the sun. Although it's surface does heat up a lot when it's directly facing the sun, on the whole the planet is frozen cold (judging by its polar regions):

Having almost no atmosphere to retain heat, surface temperatures varies diurnally more than any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions. The polar regions are constantly below 180 K (−93 °C; −136 °F).

https://en.wikipedia.org/wiki/Mercury_(planet)

Mercury thermometers work because heat causes the liquid mercury to expand. The sun is very hot, so that is why Mercury is so large near the sun.
And how do you know its solid? Have you landed there? It could be a big floating glob of liquid! :P

Maybe it has a cold heart?

The sun makes it rock hard *insert lenny face here*

Neither, Freddie Mercury is dead since 1991.

Yes