There are five large worlds that share names with chemical elements: the planets Mercury, Uranus, and Neptune, and the dwarf planets Ceres and Pluto.

What if, to have some fun, each world suddenly composed of its corresponding element?

Mercury and cerium are metals, so Mercury and Ceres would mostly just get slightly heavier and shinier.

From earth, they'd look a bit brighter in the night sky enough that Ceres would become visible to the naked eye. 

Unfortunately, the night sky and human eyes would get a little harder to find, thanks to the other planets.

Plutonium, uranium and neptunium are radioactive.

Plutonium and uranium do have non-fissile isotopes which decay slowly and mainly produce a bit of heat.

A small lump of Uranium's most common and stable isotope wouldn't even be hot to the touch.

But if you collected it into a planet-size ball, the tiny amount of heat produced by each part would add up to heat up the planet to thousands of degrees.

It might seem strange that something that's cool in small amounts would be hot when collected together in a big ball, but this is  just a consequence of geometry and the physics of radiating heat.

Since volume grows faster than surface area, and the volume is where the heat is produced, the interiors of large heat-producing objects produce more heat relative to their surface areas.

But thermodynamics doesn't allow objects to just radiate more heat, they have to get hotter to do so.

The hotter they are, the more heat they're allowed to radiate.

So, a large heat-producing object will produce more heat than it can radiate away until that heat builds up, and the object gets hot, hot enough that it can radiate away enough heat.

Really big objects can get extremely hot from just a tiny amount of heat production per unit of volume.

Like, the sun.

A cup of the sun's core produces about 60 milliwatts of thermal energy.

By volume, that's about the same heat production rate as the body of a lizard, and substantially less than that of a human.

In a sense, you are hotter than the sun–there's just not as much of you. 

But, we were talking about Uranus, which there is a lot of, and which would get really really hot if made from uranium.

The real Uranus, lit by the sun, is too dim to see with the naked eye.

But the super hot uranium Uranus would glow bright enough to be visible like an ordinary star in the night sky.

And plutonium Pluto would heat up and glow enough that from earth; it would also be visible to the naked eye, though just barely. 

Except you wouldn't be spending much time looking at the night sky anymore, thanks to neptunium Neptune.

Even the most stable neptunium isotope is fissile, so 237 Neptune would instantly undergo a runaway fission chain reaction, converting the planet into an expanding cloud of high-energy particles and X-rays.

Around four hours later, the shock wave would reach—and completely obliterate—the Earth, stripping away its surface and everything on it and leaving behind a molten blob.

We'd have gotten similar results for Uranus and Pluto if we'd instead used fissile isotopes of uranium or plutonium, though as a bonus, Uranus' shock wave would reach and destroy us about an hour faster than Neptune's,

There's a simple takeaway from all this: If you have a choice between isotopes and you're not sure which to pick, go for the most stable one. 

And just stay away from neptunium altogether. 

Ok, so I'll avoid neptunium.

But what if to be silly, we filled the solar system with soup out to Jupiter?

Or what if to pass the time we spun the earth up so that a day lasted a second?

Or what if to avoid getting in trouble you tried to read every single law that applies to you?

What if to answer these questions, you could just read a book called "What If? 2" that this video is based on and supported by?

What if two reasons to read "What If? 2" are that it was written by Randall Munroe and that it has over 60 answers to important "what if" questions, like "what if Japan disappeared?"

What if to read "What If 2", you just had to look at the information in the description of this video?

"What If 2", to be clear, is available wherever books are sold, and when you get yourself a copy, then you can ask "what if", too.