Question: How is the size of a solar system determined?

  1. Great question! We only have an approximate idea of the size of the Solar System really.

    First of all, we have to decide what defines the “edge” of the Solar System. Is it the orbit of the outermost planet, Neptune? Is it the edge of the Kuiper Belt, which is where Pluto and many asteroids and comets live?

    If it’s the orbit of Neptune, then we know how far away the planet is based on how long it takes to orbit the Sun: it takes about 165 years, so is approximately 30 times further from the Sun than Earth. We find this using some physical laws determined by Johannes Kepler, where all you need to know is how long it takes something to orbit the Sun.

    If it’s the edge of the Kuiper Belt, then we have to figure out what the furthest asteroid is and then work it out from that and Kepler’s laws. The rough answer is about 50 times the distance between the Earth and the Sun.