The Architecture of Exoplanets

Abstract

Prior to the discovery of exoplanets our expectations of their architecture were largely driven by the properties of our solar system. We expected giant planets to lie in the outer regions and rocky planets in the inner regions. Planets should probably only occupy orbital distances 0.3–30 AU from the star. Planetary orbits should be circular, prograde and in the same plane. The reality of exoplanets have shattered these expectations. Jupiter-mass, Neptune-mass, Superearths, and even Earth-mass planets can orbit within 0.05 AU of the stars, sometimes with orbital periods of less than one day. Exoplanetary orbits can be eccentric, misaligned, and even in retrograde orbits. Radial velocity surveys gave the first hints that the occurrence rate increases with decreasing mass. This was put on a firm statistical basis with the Kepler mission that clearly demonstrated that there were more Neptune- and Superearth-sized planets than Jupiter-sized planets. These are often in multiple, densely packed systems where the planets all orbit within 0.3 AU of the star, a result also suggested by radial velocity surveys. Exoplanets also exhibit diversity along the main sequence. Massive stars tend to have a higher frequency of planets ($\approx 20\mbox{--}25~\%$) that tend to be more massive ($M\approx 5\mbox{--}10~M_{\mathrm{Jup}}$). Giant planets around low mass stars are rare, but these stars show an abundance of small (Neptune and Superearth) planets in multiple systems. Planet formation is also not restricted to single stars as the Kepler mission has discovered several circumbinary planets. Although we have learned much about the architecture of planets over the past 20 years, we know little about the census of small planets at relatively large ($a>1~\mbox{AU}$) orbital distances. We have yet to find a planetary system that is analogous to our own solar system. The question of how unique are the properties of our own solar system remains unanswered. Advancements in the detection methods of small planets over a wide range of orbital distances is needed before we gain a complete understanding of the architecture of exoplanetary systems.