Here's why:

* Universe: The universe encompasses everything that exists, including all galaxies, stars, planets, and all matter and energy. So, all planets are inherently part of the universe.

* Galaxy: Our solar system, which includes the Earth and other planets, is located within the Milky Way galaxy. Galaxies are massive collections of stars, gas, dust, and planets, so planets are also part of the galaxy they reside in.

* Solar System: A solar system is a collection of planets and other celestial bodies orbiting a star. Planets are specifically defined as celestial bodies that orbit a star and are large enough to have cleared their neighborhood of other objects.

Therefore, planets exist within the universe, the galaxy, and their specific solar system.