Origin of the Misaligned Hot Jupiters: Nature or Nurture?

Abstract

What produces the spin–orbit misalignment of hot Jupiters? One natural speculation would be that it is related to the specific formation channel of hot Jupiters, i.e., their orbital migration. The “high-eccentricity migration” scenario, tidal migration following the eccentricity excitation through few-body dynamical processes, can naturally produce the spin–orbit misalignment along with the highly eccentric orbits as mentioned in Sect. 1.1.4. In Sect. 3.1, we describe this scenario in detail. The challenge to this (and actually to any other) scenario is the correlation between the obliquity and effective temperature of hot-Jupiter hosts discussed in Sect. 2.4.1. A possible explanation is that the trend is attributed to the different timescales for obliquity damping in cool and hot stars (Sect. 3.2). The subsequent studies, however, show that it is difficult to reproduce the trend at least with the current theory of tides. In addition, evidence against this “tidal realignment” scenario has recently been presented by new measurements of obliquities with the Kepler photometry. It has also been pointed out that it may be difficult to produce the most misaligned hot Jupiters within this framework of migration. Given the situation, another class of scenarios without resorting to the violent dynamical events has also been proposed; this is the topic of Sect. 3.3. These scenarios consider the misalignment to be of “primordial” origin, that is, the misalignment between the stellar spin and protoplanetary disk. They might consistently explain the obliquity dependence on the host star, as well as the presence of counter-orbiting hot Jupiters. In this chapter, we review both of these “nature” and “nurture” scenarios along with their strengths and weaknesses, and discuss how the trend could be explained in each class of scenarios. We also propose several important questions that need to be addressed to distinguish the two scenarios.