Introduction

Since the discovery, a decade ago, of the first planet orbiting a Sun-like star (Mayor & Queloz 1995), radial velocity surveys have taught us much about the nature of other solar systems. In particular, the distribution of masses, orbital semi-major axes, eccentricities, and parent star metallicities have driven home the importance of migration processes in protoplanetary disks, and strongly suggested an important role for metallicity in planet formation (e.g., Marcy et al. 2003; Santos et al. 2003; Thommes & Lissauer 2005; Fischer & Valenti 2005). Similarly, the relative abundance of multi-planet systems (with components often lying in resonant orbits) raises intriguing questions about the interactions between larger bodies in such circumstances (e.g., Kley et al. 2005; Laughlin et al. 2005).

But radial velocity measurements alone do not provide any direct information about the planets' composition, structure, or internal dynamics. To learn these things, we need to spatially resolve the planet from its host star so that we can perform spectroscopy on it alone, or we need some surrogate of this process. A useful surrogate is available for the small fraction of extrasolar planets whose orbits are aligned so that we can see them transit the disks of their parent stars. During such primary transits, when the planet passes between us and the star, the overwhelming light from the parent star can be used to the observer's advantage; it allows high S/N measurements to be made so that the geometry of the transit can be tightly specified, and so that small differences between in- and out-of-transit spectra can be detected. During secondary transits, the planet passes from sight behind the star within the span of a dozen or so minutes; this rapid disappearance allows accurate time-differential flux measurements to be performed, giving the ratio of fluxes between the planet and the star.

f The National Center for Atmospheric Research is supported by the National Science Foundation

These methods are beginning to answer some of the basic questions concerning the hot, close-in extrasolar planets. Are they giant, dense, red-hot balls of nickel and iron? (No.) Are they round, or do they have square corners? (Round planets fit the data better.) Do they have clouds, and weather? (Very likely, though details are obscure.) In what follows, we will try to explain what we think we know about such things, and how we know it.

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