Why Does Nature Form Exoplanets Easily?
The ubiquity of worlds beyond our Solar System confounds us
Rocks are Hard to See
The hardest thing to observe with a telescope is a rock. Astronomers can see disks of gas and dust swirling around nascent stars, as well as fully formed exoplanets orbiting mature stars. But it is terribly difficult—if not impossible—to detect the presence of planetesimals, at least outside of our Solar System. An entire cottage industry of astronomers is engaged in the study of protostellar or protoplanetary disks, the purported birthplaces of exoplanets. They find a rich cacophony of features in these disks: asymmetries, gaps, warps, dust made from exotic substances. Sometimes, they even find exoplanets embedded in the disks (which often cause the features themselves). A decisive test of planet formation hypotheses is to study the properties of planetesimals within these disks, but these objects are practically invisible. Planetesimals are neither numerous (compared to dust grains) nor large enough (compared to exoplanets) to re-emit significant radiation that can be detected by telescopes. Any link to them is indirect and uncertain at best, made through assumptions of their relationship to the observed dust grains. Even estimating the masses of these disks remains a crude exercise at best and requires Solar System–centric assumptions on the opacity of the dust grains and the relative amounts of dust and gas present in the disk. We find ourselves bombarded with information, but knowledge eludes us.
The prevalence of rocky exoplanets in nature lends credence to the paradigm of core accretion. Astrophysicists hoping to model formation by this mechanism appear to have their work cut out for them: They must start with micrometer-sized dust grains and congregate them to produce larger particles, either on paper, in computer simulations or in the laboratory. The task is daunting, because there is conceivably a factor of a billion in size between dust grains and planetesimals. The protoplanetary disk conspires to exacerbate matters—gas orbiting a star tends to move at slower speeds than the dust grains do. The gas molecules, mostly hydrogen, in the disk are so numerous that they act like a fluid, which has an internal pressure. This self-exerted pressure support results in the gas orbiting the star at a speed slower than would be predicted. Dust grains or rocks, present in much fewer numbers, do not exhibit this property, and thus in their reference frame the gas appears to be coming towards them, much like what one encounters when one runs in the rain. This relative motion between the dust grain and the gas is azimuthal (around the star) instead of radial (toward or away from the star), which then leads to a loss of angular momentum and the radial motion of the dust grains towards the star. The time scale on which this “gas drag” phenomenon occurs is, uncomfortably, shorter than any conceivable time scale for grain growth. This conundrum is commonly called the “meter-size problem,” because it afflicts meter-sized objects the most when they are located at the same distance from the Sun as the Earth.
Proposed solutions to the meter-size problem are plentiful. Some astrophysicists devote their careers to re-creating in their computers the conditions for encounters between dust grains. These simulations capture the intricate details of collisions, both constructive and destructive: coagulating, chipping, bouncing, deflecting, shattering. Dust grains of all sizes and shapes are studied. Again, we are awash in information, but knowledge is slower to come. Nature is hinting to us that planet formation is a robust phenomenon—surely, the mechanism involved cannot be privy to all of the micro-details of the dust grains and must be related somehow to the global properties of the protoplanetary disk. Although it cannot serve as a proof, we are guided by principles such as Occam’s razor—when faced with many explanations, we pick the simplest one.