The Life of Super-Earths by Dimitar Sasselov Read Free Book Online Page A

directions. They are just far enough apart not to bother each other too much.
    The picture changes dramatically in the world of the stars, and then again in the world of the planets. You can’t build a scaled model of the Sun’s stellar neighborhood in your living room—the stars are minuscule compared to the distances between them. While for the galaxies, the ratio between size and distance would be about 1:50 to 1:10 (like comparing M&Ms to dinner plates), that same ratio for stars would be 1:100,000,000 and more (like comparing humans to atoms). With planets, it is similarly huge, albeit less so than for stars. Such is the world!
    Is this relevant to life?

    One answer could be that it is not. These different scales just happened to be what they are, and that’s all. Or perhaps not. Life is a system—a chemical system—that, at least as we know it, seems to work only on small scales. We do not know what life is, but we do know what some of its basic functions are. There is something special about the scale occupied by life to ensure a stable environment that allows such functions to develop. Let us try to understand this by returning to the big picture.
    Galaxies in the Universe move with respect to each other with speeds of about 500 kilometers per second. Stars in the galaxy move with similar speeds on their orbits and slightly slower (say, 50 to 200 kilometers per second) with respect to each other. Such speeds are mind-boggling for our everyday experience; for example, a bullet is about 100 times slower.
    Here is the problem: such speeds are still minuscule for the distances between galaxies. The Andromeda galaxy is approaching ours at 400 kilometers per second but will require 3 billion years to come close (and may in fact collide with us). 2 Not so for stars! At such speeds, if stars had sizes comparable to the distances between them, they would be running into each other all the time—not to speak of the fate of any orbiting planets. Fortunately, stars don’t exist on such scales, so collisions between them are exceedingly rare. Even if the Andromeda galaxy smashes into the Milky Way in 3 billion years, the stars will not collide. Andromeda stars will just glide past Milky Way stars, and then all will mix and merge their orbits around a common new galaxy.

    So, on a galactic scale, there is a relative stability, which is important for life. But how much stability is enough? After all, what is stable enough for a microbe might be chaos and doom for a dinosaur.
    This issue is similar to the famous question Erwin Schroedinger asked in 1944, Why is life so big compared with an atom? 3 I ask the question in reverse: Why is life so tiny compared to a planet? To answer his question Schroedinger first pointed out how the basic units of life are large chemical complexes of atoms—large molecules. Large molecules and chemical reactions between them are at the heart of every process associated with life. They store and release energy, carry information that can be inherited, and assemble into filaments, walls, structures, and more.
    Schroedinger also pointed out that the small scale of the atoms—a world described by the rules of quantum mechanics—is ever changing and not strictly predictable (quite chaotic, indeed). 4
    He should know, being one of the giants of science who helped develop quantum mechanics, demonstrating how very different it is from the classical mechanics developed three centuries earlier by Isaac Newton. Classical mechanics provides the rules for the large scale and large objects—stars and planets, their orbits, bridges, car engines, and so on. Life is large enough to fit in the realm of classical mechanics, and so too are its essential basic units, the large molecules—but only just so.
    In answering his question, Schroedinger suggested that the molecules of life and the cells they build are just large enough
to avoid the unpredictable and destructive