Dear Lucy,

When I go to the veterinarian, they measure how big I am. They use a scale to check my weight. They find my length and height with a measuring tape.

But nobody can put a planet on a scale. Or wrap a measuring tape around it.

I asked my friend Katie Cooper about it. She’s a geologist at Washington State University. She studies our planet and other planets, too.

She told me we send satellites to orbit our solar system. When a satellite swoops around a planet, it snaps pics and takes measurements. Those images and measurements help scientists estimate the … » More …

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