What defines the first atmosphere on a rocky exoplanet?

The first atmosphere on a rocky exoplanet is like the blanket that wraps around a planet when it’s born, it depends on what's nearby and how hot things get.

Imagine you're in a kitchen with a big pot of soup boiling. The steam rising from the pot is like gases escaping from the planet. On a rocky exoplanet, if there are volcanoes, they can act like big pots, when they erupt, they send out gas and vapor, creating the first atmosphere.

If the planet is close to its star, it might get really hot. This heat can cause gases to escape into space, making the atmosphere thin. But if the planet has lots of volcanic activity, like a kitchen with many pots boiling at once, it can keep adding more gas and vapor, keeping that blanket thick and cozy.

So, the first atmosphere is made by what’s inside the planet and how close it is to its star, like how much steam comes out of your soup pot and whether you cover it with a lid or not!