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What is below Earth, since space is present in every direction? – Purvi, age 17, India

If you’ve seen illustrations or models of the solar system, maybe you noticed that all the planets orbit the Sun in more or less the same plane, traveling in the same direction.

But what is above and below that plane? And why are the planets’ orbits aligned like this, in a flat pancake, rather than each one traveling in a completely different plane?

I’m a planetary scientist who works with robotic spacecraft, such as rovers and orbiters. When my colleagues and I send them out to explore our solar system, it’s important for us to understand the 3D map of our space neighborhood.

Earth’s gravity has a lot to do with what people think is up and what is down. Things fall down toward the ground, but that direction depends on where you are.

Imagine you’re standing somewhere in North America and point downward. If you extend a line from your fingertip all the way through the Earth, that line would point in the direction of “up” to someone on a boat in the southern Indian Ocean.

In the bigger picture, “down” could be defined as being below the plane of the solar system,........

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