The heavy isotopes sink down to the core of planets. If hot then the densest metals transfer heat to the lighter elements. I've given this some thought. If the horse head nebula is second generation stars then there isn't enough plutonium and uranium present to maintain heat for billions of years. The Sol system is at least a fifth generation star having components from early star collisions and nova events. The gold on earth is from neutron star collisions billions of years before this system existed. Any planets in the horse head nebula are facing the exact same problem as Mars. Too few heavy metals.
Too many and you wind up with Mercury. Plutonium core surrounded by lead. Venus had enough heat internally to resurface itself, erasing nearly all meteorite craters. Venus's proximity to the sun doomed it to a run away greenhouse effect.
The Earth has the right mix of distance from the sun... heavy metal mixtures... lunar mass(asteroid buffer)... Amino acid levels... and we shouldn't discount theories that the outer star system was in a liquid state four billion years ago. Life may have started near Jupiter and made its way inwards as the young star cooled down.
Once life started out there it changed each habitat to suit itself. A lack of sunlight in the outer system means methane breathing bacterium. Same as Earth's diverse genomes. RNA or DNA doesn't really matter. There is also circular MtDNA. If we explore the galaxy we will probably find right handed DNA and RNA.