Warm Jupiters are less lonely than hot Jupiters: close neighbours

TL;DR

This study reveals that warm Jupiters often have small planetary companions, unlike hot Jupiters, suggesting different formation mechanisms, with warm Jupiters likely forming in-situ and hot Jupiters possibly arising from a different process.

Contribution

It provides the first statistical comparison of companion prevalence between hot and warm Jupiters using Kepler data, supporting in-situ formation for warm Jupiters.

Findings

Warm Jupiters frequently have small close companions.

Hot Jupiters generally lack detectable planetary companions.

Warm Jupiters with companions show low eccentricities and inclinations.

Abstract

Exploiting the Kepler transit data, we uncover a dramatic distinction in the prevalence of sub-Jovian companions, between systems that contain hot Jupiters (periods inward of 10 days) and those that host warm Jupiters (periods between 10 and 200 days). Hot Jupiters, with the singular exception of WASP-47b, do not have any detectable inner or outer planetary companions (with periods inward of 50 days and sizes down to $2 R_{\rm Earth}$). Restricting ourselves to inner companions, our limits reach down to $1 R_{\rm Earth}$. In stark contrast, half of the warm Jupiters are closely flanked by small companions. Statistically, the companion fractions for hot and warm Jupiters are mutually exclusive, particularly in regard to inner companions. The high companion fraction of warm Jupiters also yields clues to their formation. The warm Jupiters that have close-by siblings should have low orbital eccentricities and low mutual inclinations. The orbital configurations of these systems are reminiscent of those of the low-mass, close-in planetary systems abundantly discovered by the Kepler mission. This, and other arguments, lead us to propose that these warm Jupiters are formed in-situ. There are indications that there may be a second population of warm Jupiters with different characteristics. In this picture, WASP-47b could be regarded as the extending tail of the in-situ warm Jupiters into the hot Jupiter region, and does not represent the generic formation route for hot Jupiters.