Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected].
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.
Which way is ‘down’?
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.
By convention, looking ‘down’ on the solar system you see the planets orbiting counterclockwise.
Andrzej Wojcicki/Science Photo Library via Getty Images
In the bigger picture, “down” could be defined as being below the plane of the solar system, which is known as the ecliptic. By convention, we say that above the plane is where the planets are seen to orbit counterclockwise around the Sun, and from below they are seen to orbit clockwise.
Even more flavors of ‘down’
Is there anything special about the direction of down relative to the ecliptic? To answer that, we need to zoom out even farther. Our solar system is centered on the Sun, which is just one of about 100 billion stars in our galaxy, the Milky Way.
Each of these stars, and their associated planets, are all orbiting around the center of the Milky Way, just like the planets orbit their stars, but on a much longer time scale. And just as the planets in our solar system are not in random orbits, stars in the Milky Way orbit the center of the galaxy close to a plane, which is called the galactic plane.
This plane is not oriented the same way as our solar system’s ecliptic. In fact, the angle between the two planes is about 60 degrees.
A side view of galaxy NGC 4217 taken by the Hubble Space Telescope shows how all the stars and their planetary systems lie on one plane.
NASA Goddard, CC BY
Going another step back, the Milky Way is part of a cluster of galaxies known the the Local Group, and – you can see where this is going – these galaxies mostly fall within another plane, called the…
