The In Situ Formation of Giant Planets at Short Orbital Periods

TL;DR

This paper proposes a new model where hot Jupiters and close-in multi-planet systems form in situ through dynamical consolidation and gas accretion, challenging the traditional migration paradigm.

Contribution

It introduces a formation scenario where short-period giant planets develop locally after dynamical consolidation, without requiring inward migration from outer regions.

Findings

Most hot Jupiters form in situ rather than migrating inward.

Tightly-packed inner planet systems are metastable and can consolidate into larger planets.

Short-period giants can form with or without substantial gas accretion.

Abstract

We propose that two of the most surprising results so far among exoplanet discoveries are related: the existences of both hot Jupiters and the high frequency of multi-planet systems with periods $P\lesssim200$~days. In this paradigm, the vast majority of stars rapidly form along with multiple close-in planets in the mass range of Mars to super-Earths/mini-Neptunes. Such systems of tightly-packed inner planets (STIPs) are metastable, with the time scale of the dynamical instability having a major influence on final planet types. In most cases, the planets consolidate into a system of fewer, more massive planets, but long after the circumstellar gas disk has dissipated. This can yield planets with masses above the traditional critical core of $\sim$10 $M_\oplus$, yielding short-period giants that lack abundant gas. A rich variety of physical states are also possible given the range of collisional outcomes and formation time of the close-in planets. However, when dynamical consolidation occurs before gas dispersal, a critical core can form that then grows via gas capture into a short-period gas giant. In this picture the majority of Hot and Warm Jupiters formed locally, rather than migrating down from larger distances.