A post from Reddit’s Ask Science community posed the head-scratching question: “How do we know that there is a Pluto-size ball of solid iron that makes up Earth’s inner core?”

Journeying to the center of the Earth, of course, is no easy task.

You’ll have to dig through solid rock to reach its core—more than 1,800 miles below our feet. To understand how far that is, the deepest human dive in an atmospheric diving suit was a mere 0.3 of a mile.

When you finally reach the inner core, a hot, burning hell of iron—roughly the size of Pluto—awaits. The pressure is equivalent to 43,000 elephants crushing your head.

Don Anderson/PNAS

Okay, so journeying to the center of the Earth is impossible. And our explorations are limited. So how are we so sure that a giant ball of iron lies beneath its crust?

To find out what’s going on under our feet, we have to do some math.

According to Reddit user Astromike23 and former astronomy professor named Mike (Editor’s Note: We’ve verified his academic standing but he’s chosen to remain anonymous), we know that rock has a density of about 3 grams/cm3. But Earth’s density is around 5.5 g/cm3. So we know that Earth is not entirely made of rock.



Reddit user Astromike23

“There must be quite a bit of something much denser for the total to average out to 5.5, and iron (with a density of 7.9) fits the bill nicely,” says Mike.

But how do we know that it’s made-up of solid iron rather than liquid, considering it’s about as hot as the sun at Earth’s core.

Earth is an unusual place because we’re protected by a magnetic field. “You can’t generate a magnetic field that strong without some kind of solid-liquid interaction,” he explains.

We know that Earth’s outer core is mostly made-up of liquid iron, so its inner core must be solid in order to produce a magnetic field.

And we’re able to determine the size of that solid ball by measuring earthquakes.

“Seismic waves generated by earthquakes can travel deep into the Earth’s core and back out again,” he describes.

By studying the waves’ vibrations, scientists concluded the iron ball is about 745 miles wide, approximately the same radius as Pluto.