Do all of the planets in our solar system orbit the sun on the same plane?
SUPER COOL QUESTION.
Short answer: Almost, but not quite.
Long answer: When our solar system formed out of clouds of gas and dust, it collapsed into a relatively flat disk out of which all the planets were born.
Imagine that as each of the planets orbit, they trace out the perimeter a solid disk: these are called their orbital planes. We call the earth’s orbital plane the ecliptic and it’s defined as having an inclination (an angle) of zero. There’s nothing special about our orbit, but it makes comparison easy, because we can describe the inclinations of other planets with respect to this—we just imagine another disk cutting through at an angle, like this:
The Moon orbits at an inclination of about 5 degrees to the ecliptic. This is actually the reason we don’t have lunar eclipses every time there’s a full moon, or solar eclipses every time there’s a new moon. Most of the time, the Moon doesn’t exactly line up with the Sun because it’s orbiting at a tilted angle.
The other planets also orbit at various inclinations, from Uranus at 0.77 degrees to Mercury at 7 degrees. Smaller bodies, however, can have vastly different inclinations, because they’re more likely to be influenced by a larger planet’s influence and thrown out into different orbits. Pluto’s inclination is 17 degrees, the inclination of the large asteroid Pallas is 44 degrees, and comets are just all over the place. They originate on the Oort cloud on the very fringes of the solar system, so the fact they have all kinds of inclinations suggests that the Oort cloud itself is roughly spherical in shape.
But in general, the larger bodies—the planets—in our solar system definitely orbit in a pretty flat plane. Recent evidence hints that planets in some distant solar systems might have huge differences between their inclinations, and this may mean they’ve had a much more violent history than our own solar system, filled with collisions and close calls.