How much atmosphere is too much for life? As scientists discover more super-Earths and mini-Neptunes, the question becomes more relevant. Often, the rocky cores of these planets are believed to be about the same size, while the distinguishing difference is the size of the atmosphere. Mini-Neptunes look more like gas giants, with a thicker atmosphere that would create too much pressure at the surface, and super-Earths have a much thinner layer. A recent research study considered what would happen if a mini-Neptune migrated close to a dwarf star. M-class stars, as this type are known, have a volatile first billion years. The energy production from the stars can range drastically, with x-rays and extreme ultraviolet rays hitting planets with as much as 100 to 10,000 times more radiation than what the Earth experiences today.
For habitability, this is a huge challenge. Because the star is smaller, rocky planets need to huddle in closer to be within the star's habitable zone. The radiation emanating from the star in its youth slams into the atmosphere, stripping away molecules until there is little left. What if, however, a mini-Neptune got in close because the gravity of its star or other planets influenced its orbit? There appears to be a small set of situations where the planet could hold on to just enough atmosphere to become a "super-Earth," a planet that is a little larger than Earth but still small enough to have a reasonable-sized atmosphere. In such a scenario life forms would still deal with radiation bombardment, but less so as over time as the star moves out of its active phase, although solar flares could remain a hazard.
Kemo D. 7