It's basically explained in the article: it's probably just very hot—the atmospheric scale height[0] is proportional to temperature. The open question (per the

It's basically explained in the article: it's probably just very hot—the atmospheric scale height[0] is proportional to temperature. The open question (per the article) is why it got so hot, because they don't have a model for that. ("It certainly requires a significant deposit of energy deep into the planet’s interior, but the details of the mechanism are not yet understood.“") [0] https://en.wikipedia.org/wiki/Scale_height https://en.wikipedia.org/wiki/Scale_height