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From Kikkiwedi in San Francisco, I'm Alexis Madrigal.

We live on the surface of a sphere that's almost unimaginably vast.

Beneath our feet, there are thousands of miles of rock subject to unimaginable pressures

and heat surrounding an iron core.

Unlike the oceans or even outer space, we basically cannot directly explore the interior of our

planet or any other.

So how do we know what our planet is made of, when it was formed, how the moon got hung in the sky.

That's what Johns Hopkins Planetary Scientist Sabina Stanley's new book is about, ranging from

Mercury to Neptune.

It's called What's Hidden Inside Planets, and she'll take us on a tour of the solar system

after this news. Welcome to Forum. I'm Alexis Madrigal, and it is so good to be back live with you this morning.

Today, let's start with the moon. We know the rocky body that orbits our own planet,

but how did it form? In her new book, Planetary Scientist Sabina Stanley narrates the story. In the

distant past, a Mars-sized object crashed into the Earth, tossing up an enormous mass of

material into space. Over what scientists estimate was just a few weeks, something like 40 days, the proto-moon

...