The search for Earth 2.0 has begun in earnest. But there’s a huge variety of exoplanets out there, so narrowing down the search to focus valuable telescope time on only the best candidates is critical. One variable of a planet that will have a huge impact on its habitability is its size. A new paper, now available in pre-print on arXiv, by researchers at the University of California Riverside, looks into the impact of a planet’s size on one of its more critical features for habitability - whether it holds onto an atmosphere - and determines that slightly smaller than Earth is likely the smallest a planet can be and still be viable for life to develop. That magic number, specifically, is 0.8 Earth radii, according to the Smaller Than Earth Habitability Model (STEHM) that the researchers developed. And that limit seems to be determined by two specific hurdles that planets must jump in order to be viable for life. First is the obvious one - gravity.…