 Given any arbitrary criteria for what constitutes a Boltzmann brain (or planet, or universe), smaller structures that minimally and barely meet the criteria are vastly and exponentially more common than larger structures; a rough analogy is how the odds of a real English word showing up when you shake a box of "Scrabble" letters are greater than the odds that a whole English sentence or paragraph will form.The average timescale required for formation of a Boltzmann brain is vastly greater than the current age of the Universe.In modern physics, Boltzmann brains can be formed either by quantum fluctuation, or by a thermal fluctuation generally involving nucleation.