The planet with a thick atmosphere of mainly hydrogen and helium due to conditions during the formation of planets is Jupiter.

Here's why:

* Early Solar System: During the early formation of the solar system, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed in the outer regions where temperatures were much colder. This allowed them to capture and retain large amounts of hydrogen and helium, the most abundant elements in the universe.

* Gas Giant: Jupiter, being the largest and most massive planet in our solar system, accumulated a vast amount of these gases, forming a thick atmosphere that is primarily composed of hydrogen (about 86%) and helium (about 13%).

While Saturn, Uranus, and Neptune also have atmospheres rich in hydrogen and helium, they are not as thick as Jupiter's. This is due to their lower gravity and smaller size, which allowed some of the lighter gases to escape over time.