The Life of Super-Earths by Dimitar Sasselov

All these particles have wave properties, as do all elementary particles, including electrons, protons, and neutrons.
    Now consider the scales that are much larger—galaxies, stars, and planetary systems. This is a world governed solely by the force of gravity. There is no friction or any other manifestation of electromagnetic force strong enough to deflect or slow down stars and planets from their paths, and the chemical bonding essential to life doesn’t happen. On the other extreme is the scale that is much smaller than our own—the quantum scale. The force of gravity has no influence here; the individual particles have such little mass that only electromagnetic forces rule. On both the very large and the very small scales, there is no shelter for life. The cosmic scale is awash with radiation and beyond freezing cold, pummeling anything that depends on electromagnetism for cohesion. On the smallest scale, things move so fast and unpredictably that nothing as ordered as life stands a chance. Life on Earth exists
at a comfortable scale in between those two—let’s call it the large-molecule scale . Its range starts a few steps above the quantum scale (10 —9 m) and ends closer to home (10 —5 m).
    The large-molecule scale is the true scale of life as we know it. All essential life processes, information-carrying molecules, and most organisms (the microbes) fit nicely in it (see Figure 7.1 ). The scale of life itself fits nicely on a planet. And never mind the big plants and animals that have outgrown the large-molecule scale—they are a recent development.
    What is special about the large-molecule scale? Gravity is still weak, but the mass of large molecules is no longer negligible, so they respond to the force in measurable ways. An important effect is that the water solutions in which such molecules and their structures exist and function are likely to be affected by gravity. Gravity acts as a stabilizing agent at these scales—counteracting electromagnetic forces and providing equilibrium. Planet Earth is a good example of such balance: it is massive enough to shrink and compress under its own gravity. Its rocks, as we’ve seen, are squeezed under immense pressure, packing their oxygen, silicon, and iron atoms close together. The electromagnetic forces between these atoms, being repulsive, put a limit on how compressed gravity can make the matter; thus Earth has been stable in this state for billions of years and will continue to be for a long time to come. The same balance keeps our Sun stable and shining over billions of years too.
    Smaller things—our bodies, for example—are held likewise together in a pressure balance, but it is not the balance between gravity and electromagnetic forces. (Our bodies have
too little mass for our own gravity to help hold us together.) Instead, we are pulled down by the gravity of planet Earth and simultaneously we are under pressure by the air above our heads—about fifteen pounds of it on each square inch. The air, of course, is also pulled down by gravity. As we go to even smaller scales, electromagnetism gets more and more dominant and gravity all but drops out of the competition. Electromagnetism, as manifested by the chemical bond, makes our tissues structurally sound, keeping cells and larger multicell bodies together.
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    FIGURE 7.1 . Comparative sizes: the molecules of life are huge when compared to the common molecule of water (H 2 O); they define the large-molecule scale.

    To everything, then, it seems there is the right size, as J. B. S. Haldane—an early geneticist and famous science popularizer—mused in a 1928 essay. He was answering the question we always ask as children: How are insects built to be able to remain unscathed after a fall from a height that is multiple times their size? Why do some insects walk on water while others drown in it?
    Haldane gave the right answers, but only in a general sense. A