Astronomy Colloquium

The data-rich Kepler mission provided an unprecedented view of the demographics of planetary systems. Close to the star (orbital periods shorter than ~100 days), super-Earths (~1--4 Rearth) and Earth-sized planets dominate. These small planets are evenly distributed in log orbital period down to ~10 days, but dwindle in number at shorter periods. I will demonstrate that both the break at ~10 days and the slope of the occurrence rate down to ~1 day can be reproduced if planets form in situ in disks that are truncated by their host star magnetospheres at co-rotation. Planets can be brought from disk edges to ultra-short (<1 day) periods by asynchronous equilibrium tides raised on their stars. Close-in super-Earths are massive enough to trigger runaway gas accretion, yet they accreted atmospheres that weigh only a few percent of their total mass, keeping their size below that of the Neptune. This puzzle is solved if super-Earths formed late, in the inner cavities of transitional disks. Over a wide range of nebular depletion histories, super-Earths can robustly build their ~1% by mass envelopes. Super-puffs present the inverse problem of being too voluminous for their small masses. I will show that super-puffs most easily acquire their thick atmospheres as dust-free, rapidly cooling worlds outside 1 AU, and then migrate in just after super-Earths appear. Small planets may remain ubiquitous out to large orbital distances. The variety of debris disk morphologies revealed by scattered light images can be explained by viewing an eccentric disk, secularly forced by a planet of just a few Earth masses, from different observing angles. The farthest reaches of planetary systems may be perturbed by eccentric super-Earths.