The light element found in most of the outer planets is hydrogen.

Here's why:

* Giant Gas Planets: The outer planets (Jupiter, Saturn, Uranus, Neptune) are classified as gas giants. This means their primary composition is gas, primarily hydrogen and helium.

* Abundance in the Universe: Hydrogen is the most abundant element in the universe.

* Low Mass: Hydrogen is very light. This contributes to the low density of these planets.

While helium is also present, hydrogen makes up the majority of their mass.