Would the earth fit inside Pluto?
Asked by Moana, Age 6
Pluto is much smaller than any other planet in our solar system. Even our moon would be too big to fit inside Pluto! In fact, if you rolled Pluto out flat, it would only have slightly more surface area than Russia. So, the short answer is no, the Earth would not fit inside Pluto. But what if we tried anyway?
It turns out the science concerning what happens when you squeeze a planet's worth of rock and metal into a space less than 1% its former volume is not very well established. What we can say is that if we squeezed all 6*1024 kg of the Earth's mass into the volume of Pluto it would get pretty crazy. Our new travel-sized Earth would be very dense. A tennis ball-sized chunk of it would be about 135 kg or just shy of 300 pounds. Fortunately we'd still be a way off from creating a Black Hole, but we still have a lot of gravity to contend with. 
The force of gravity increases the closer you get to something, so now that we've packed Earth into a much smaller space we can get closer to it. The surface gravity would be approximately 28.7g, or 28.7 times normal Earth gravity, so a person weighing 150 pounds normally would now weigh over 4300 pounds. For comparison, Jupiter's gravity is only 2.5g, and we are just barely beating the Sun's 28.02g.
This would be very interesting for a very brief moment before everything exploded. The thing about compressing any material like this is that it tends to get very very hot, and with this much pressure we are looking at temperatures likely to exceed those at the surface of the Sun. The rock and metal that make up our planet will quickly liquefy, vaporize, and likely turn into a plasma. Then it would explode. We don't have enough gravity to maintain our compact Earth, so it won't stay compact for much more than a fraction of a second before it went from a very hot ball of metal to a very large cloud of rapidly cooling gas. Essentially a tiny supernova.
If Moana is really set on making this happen, I'd suggest trying it with Mars first.