As planets begin to form around a young star, they build up a primordial atmosphere composed mostly of hydrogen and helium. Many planets end up losing this atmosphere as it is replaced by heavier gases such as oxygen and nitrogen, as did the rocky worlds in our solar system.
However, it is possible that some planets larger than the Earth were able to retain their original air. Christophe Mordasini and his team ran thousands of simulations to find out how long this effect could persist on other planets. Scientists have found that, depending on the mass of the planet and the distance from the star, this hydrogen greenhouse effect can keep planets temperate for up to eight billion years. This was true even for the so-called rogue planets, which had no stars.
Since the atmosphere under such conditions would be quite heavy, the pressure on the surface of these worlds would be high, and temperature fluctuations would be insignificant. As Mordasini summarizes, on Earth, day and night create temperature differences, but this will not happen on these planets.