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 Neptuneand 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 (≈ 20-25 %) that tend to be more massive (M ≈ 5-10 M 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 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.

Rocky Worlds Limited to ∼1.8 Earth Radii by Atmospheric Escape during a Star’s Extreme UV Saturation

“…2012;Lopez & Rice 2016;Owen & Jackson 2012;Owen & Morton 2016;Owen & Wu 2013, 2016Wolfgang & Lopez 2015).…”

“…Hatzes 2016). The variety of characteristics observed among these planets has raised many questions about planetary formation and evolution.…”

K2-111 b − a short period super-Earth transiting a metal poor, evolved old star

“…Very high precision transit photometry, preferably carried out from space, gives us access to the orbital parameters, which combined with either radial velocity (RV) data and/or transit timing variations (TTVs) enables the measurement of the planetary fundamental parameters, most notable, the planet's radius, mass, and mean density (Charbonneau et al 2000;Henry et al 2000;Mayor & Queloz 1995;Marcy & Butler 1996;Ford et al 2011). Determination of the fundamental parameters of exoplanets, and their host stars, are necessary in order to study the internal structure, composition, dynamical evolution, tidal interactions, architecture of systems, and the atmosphere of exoplanets (Madhusudhan et al 2014;Winn & Fabrycky 2015;Hatzes 2016).…”

Anti-correlation between multiplicity and orbital properties in exoplanetary systems as a possible record of their dynamical histories

“…Hatzes 2016, for a recent review). Albeit it is likely that our picture of exoplanetary systems could be incomplete because of the limitations of our present observational capabilities, this nevertheless reveals the large variety of exosystems across the Galaxy and the diversity of the outcomes of the processes shaping the formation and evolution of planetary systems (see Hatzes 2016, and references therein).…”